Abstract:
An apparatus and method for distributing the SS7 Message Transfer Part Level 2 (MTPL2) between a Media Gateway and a Signalling Gateway in a Voice over IP Switch is disclosed. The distribution of the MTP Level 2 stack allows data links in a fully associated SS7 network to be terminated on the Media Gateway and SS7 messages to be exchanged between the Media Gateway and the Signalling Gateway over an IP backbone.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to SS7 signalling and more particularly to a method and apparatus for distributing the Message Transfer Part (MTP) Level 2 functionality between a Signalling Gateway and a Media Gateway. 
     BACKGROUND TO THE INVENTION 
     The following acronyms will be used throughout this document: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 AERM 
                 Alignment Error Rate Monitor 
               
               
                 ACK 
                 Acknowledge 
               
               
                 ACM 
                 Address Complete Message 
               
               
                 AM 
                 AERM Monitoring 
               
               
                 ANSI 
                 American National Standards Institute 
               
               
                 ATM 
                 Asynchronous Transfer Mode 
               
               
                 BERT 
                 Bit Error Rate Testing 
               
               
                 CC 
                 Congestion Control 
               
               
                 CCS 
                 Common Channel Signalling 
               
               
                 CRC 
                 Cyclic Redundancy Check 
               
               
                 DAEDR 
                 Delimitation, Alignment, Error Detection Rcceive 
               
               
                 DAEDT 
                 Delimitation, Alignment, Error Detection Transmit 
               
               
                 DL2P 
                 Distributed L2 Protocol 
               
               
                 DPS 
                 Distributed Protocol Stack 
               
               
                 EM 
                 Emergency Monitoring 
               
               
                 FISU 
                 Fill-In Signal Unit, a signalling unit complying with 
               
               
                   
                 MTPL2 
               
               
                 HDLC 
                 High-Level Data Link Control 
               
               
                 IAC 
                 Initial Alignment Control 
               
               
                 IAM 
                 Initial Address Message 
               
               
                 IN 
                 Intelligent Network 
               
               
                 INH 
                 Inhibit 
               
               
                 ISUP 
                 ISDN User Part 
               
               
                 ITU 
                 International Telecommunication Union 
               
               
                 LSC 
                 Link State Control 
               
               
                 LNP 
                 Local Number Portability 
               
               
                 LSSU 
                 Link Status Signal Unit, a signalling unit complying 
               
               
                   
                 with MTPL2 
               
               
                 LL-MTPL2 
                 Lower Level - Message Transfer Part Level 2, a portion 
               
               
                   
                 of the Distributed MTP Level 2 Architecture 
               
               
                 Mb/s 
                 Megabit-per-second 
               
               
                 MG 
                 Media Gateway 
               
               
                 MSU 
                 Message Signal Unit, a signalling unit complying 
               
               
                   
                 with MTPL2 
               
               
                 MTP 
                 Message Transfer Part 
               
               
                 MTPL1 
                 MTP Level 1 
               
               
                 MTPL2 
                 MTP Level 2 
               
               
                 MTPL3 
                 MTP Level 3 
               
               
                 OM 
                 Operational Measurement (OM-MG Operational 
               
               
                   
                 Measurement located at the Media Gateway, OM-SG 
               
               
                   
                 Operational Measurement located at the 
               
               
                   
                 Signalling Gateway) 
               
               
                 OSI 
                 Open Standards Interconnect 
               
               
                 PSTN 
                 Public Switched Telephone Network 
               
               
                 RC 
                 Receive Control (RC-SG Receive Control located at 
               
               
                   
                 Signalling Gateway, RC-MG Receive Control located 
               
               
                   
                 at Media Gateway) 
               
               
                 REM 
                 Remote 
               
               
                 RTB 
                 Re-Transmission Buffer 
               
               
                 RTP 
                 Reliable Transport Protocol 
               
               
                 SCPs 
                 Service Control Points 
               
               
                 SG 
                 Signalling Gateway 
               
               
                 SIB 
                 Status Indicator Busy (used by LSSUs) 
               
               
                 SIE 
                 Status Indicator Emergency (used by LSSUs) 
               
               
                 SIF 
                 Signalling Information Field 
               
               
                 SIN 
                 Status Indicator Normal (used by LSSUs) 
               
               
                 SIO 
                 Status Indicator Out of Alignment (used by LSSUs) 
               
               
                 SIOS 
                 Status Indicator Out of Service (used by LSSUs) 
               
               
                 SIPO 
                 Status Indicator Processor Outage (used by LSSUs) 
               
               
                 SS7 
                 Signalling System 7 
               
               
                 SSPs 
                 Service Switching Points 
               
               
                 STPs 
                 Signalling Transfer Points 
               
               
                 SU 
                 Signalling Unit 
               
               
                 SUERM 
                 Signalling Unit Error Rate Monitor 
               
               
                 TB 
                 Transmission Buffer 
               
               
                 TDM 
                 Time Division Multiplexed 
               
               
                 TXC 
                 Transmit Control (TCX-SG Transmit Control located at 
               
               
                   
                 Signalling Gateway; TCX-MG Transmit Control 
               
               
                   
                 located at Media Gateway) 
               
               
                 UL-MTPL2 
                 Upper Level - Message Transfer Part Level 2, a portion 
               
               
                   
                 of the Distributed MTP Level 2 Architecture 
               
               
                 UDP 
                 User Datagram Protocol 
               
               
                 UNI 
                 Un-Inhibit 
               
               
                 VoIP 
                 Voice over IP 
               
               
                   
               
             
          
         
       
     
     A Public Switched Telephone Network (PSTN) is, conventionally, comprised of two networks—a voice/data network and a signalling network. The signalling network carries information for call set-up and tear down. The Common Channel Signalling System #7 (CC-SS7) protocol is used for call set-up and tear down. 
     As is known in the art, the PSTN provides users with a dedicated, end-to-end circuit connection for the duration of each call. The circuits are reserved between the originating call, tandem switches (if any), and the terminating switch based on the called party number. In a typical Intelligent Network (IN), such as the North American SS7 network, to set-up a basic call circuit between a calling party at a first Service Switching Point (SSP), and a called party, having an out-of-switch number (i.e., the called party is at a second SSP), SS7 messages are exchanged between the two SSPs using the SS7 network. 
     Recent developments, notably the popularity of the Internet, have increased the use of Voice over IP (VoIP) communication. Voice over IP switches need to interface to the existing PSTN networks (voice and signalling) to allow PSTN phones to use the VoIP network. A VoIP switch would interface to the PSTN Voice network using a Media Gateway. The Media Gateway provides the translation between Time Division Multiplexed (TDM) data and IP packet data. This VoIP switch would interface to the PSTN Signalling network using a Signalling Gateway. The Signalling Gateway provides the translation between SS7 and IP. 
     In a quasi-associated SS7 signalling network, the Signalling Gateway would connect to the local STP mated pair using A-links. In a fully associated SS7 Signalling network, the signalling gateway would connect directly with the SSPs using F-links. In the fully associated network signalling case, the SS7 signalling link is usually carried over the same facilities as the voice trunks. Since these voice trunks terminate on the media gateway, external circuitry (e.g. channel banks) are required to extract the signalling channel and route it to the signalling gateway. Moreover, since the media and signalling gateways are typically geographically quite dispersed, the equipment associated with transmitted the extracted signalling channel to the signalling gateway is expensive. 
     Accordingly, it is desired to provide a system which addresses some of these shortcomings. 
     SUMMARY OF THE INVENTION 
     This invention distributes the MTP Level 2 functionality between the Signaling Gateway and the Media Gateway. It allows F-links in a fully associated SS7 signaling network to be terminated on the Media Gateway and the SS7 messages to be passed between the Media Gateway and the Signaling Gateway over IP. 
     According to one aspect of the invention, there is provided a method providing distributed Message Transfer Part (MTP) functionality over an Internet Protocol (IP) network, said method comprising: at a first media gateway: receiving conventional MTP signalling units from a first network element; removing repeated MTP signalling units from said MTP signalling units received; either before or after said removing, encapsulating received MTP signalling units into data packets to form a reduced signalling unit packet stream; transmitting said reduced signalling unit packet stream to a signalling gateway; receiving at said signalling gateway said reduced packet stream at said signalling gateway; transmitting packets encapsulating MTP signalling units to one of said first media gateway and a second media gateway, said transmitted packets being responsive to said received reduced signalling unit packet stream; receiving at said one of said first media gateway and said second media gateway said packets transmitted by said signalling gateway; and at said one of said first media gateway and said second media gateway, re-creating conventional MTP signalling units based on said routed packets and transmitting said re-created conventional MTP signalling units to one of said first network element and a second network element. 
     According to a further aspect of the invention there is provided a Voice over Internet Protocol (VoIP) switch providing a distributed Message Transfer Part Level 2 (MTPL2) protocol, said VoIP switch comprising: a plurality of media gateways, each of said media gateways in communication with a conventional SS7 physical link, said conventional SS7 physical link in communication with a network element; a signalling gateway; an IP network providing IP communication between said plurality of media gateways and said signalling gateway. 
     According to a still further aspect of the invention there is provided a media gateway comprising: a receiver for receiving a contiguous stream of signalling units from a network element; a filter for discarding repeated signalling units in said received stream; an encapsulated data transmitter adapted to transmit data encapsulating said signalling units over a packet switched network; an encapsulated data receiver adapted to receive encapsulated data encapsulating signalling units from a packet switched network; a processor adapted to: encapsulate signalling units forming said stream of signalling units either before or after said discarding by said filter; and de-encapsulate signalling units received by said encapsulated data receiver and generate signalling units to re-create a contiguous stream of signalling units; and a signalling unit transmitter transmitting a re-created contiguous stream of signalling units to a network element. 
     According to a still further aspect of the invention there is provided a signalling gateway comprising: an encapsulated data receiver for receiving encapsulated signalling units from a packet switched network; an encapsulated data transmitter adapted to transmit data encapsulating signalling units over a packet switched network; a processor adapted to: de-encapsulate said received encapsulated signalling units and generate signalling units compliant with the Message Transfer Part Level 2 protocol; transmit said generated signalling units to a Message Transfer Part Level 3 processor; receive conventional signalling units from said Message Transfer Part Level 3 processor; and encapsulate said received conventional signalling units for transmission by said encapsulated data transmitter. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood after reference to the following detailed specification read in conjunction with the drawings wherein: 
     FIG. 1 illustrates a conventional quasi-associated SS7 network in communication with a conventional a VoIP switch; 
     FIG. 2 illustrates a fully associated SS7 network in communication with a conventional VoIP switch; 
     FIG. 3 illustrates a fully associated network in communication with a VoIP switch incorporating an embodiment of the invention; 
     FIG. 4 detailed schematic of an enlarged portion of FIG. 3; 
     FIG. 5 is a schematic illustrating the distribution of a conventional MTP stack between portions of FIG. 4; 
     FIG. 6A is a schematic of a protocol stack embodying one aspect of the invention; 
     FIG. 6B is a detailed schematic of a first portion of FIG. 6A; 
     FIG. 6C is a detailed schematic of a second portion of FIG. 6A; 
     FIG. 7 is a detailed schematic of a portion of FIG. 4; 
     FIG. 8 is a first signal flow diagram of the operation of the SS7 network portion of FIG. 4; and 
     FIG. 9 is a second signal flow diagram of the operation of the SS7 network portion of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referencing FIG. 1, a conventional quasi-associated signaling network  100  having dedicated facilities  124  (e.g. T 1  or E 1 ) carrying signaling traffic between SG  102  of VoIP  120  and SS7 Signaling network  108  is illustrated. Separate facilities  122  are used to connect MG  106  with the PSTN voice network  110 . MGC  104  controls SG  102  an MG  106 . SSP  112  is operatively connected to SS7 Signaling network  108  and PSTN voice network  110 . 
     Referencing FIG. 2, in a fully associated signaling network  200  facilities  206  carry both the signaling channel and the voice trunks from the Voice/TDM Network  202 . Since the same facilities  206  carry both voice trunks and signalling channels an external MUX  204  is required to extract the signaling channel from the facility  206  and route it to the SG  102  of VoIP switch  120  over dedicated signaling facilities  210 . The voice trunks on facilities  206  are transmitted by MUX  204  to MG  106  of VoIP  120  over dedicated facilities  208 . 
     Generally, FIG. 3 shows a fully associated signaling network comprising SSPs  12  and voice/signaling links  14  in communication with VoIP switch  20  embodying one aspect of the invention. As illustrated, both the voice trunks and signaling channels carried on links  14 A,  14 B are terminated at MG  22 . The signaling channel information is then routed via IP from the MGs  22  to the SG  24 . Advantageously external MUXes are no longer required to extract the signaling channel. 
     Links  14 B may be any conventional physical links such as, for example, an E- 1 , DS- 1 , DS- 3 , or other similar links. Although a single link  14  is shown terminating at each Media Gateway (MG)  22 , the present invention, an embodiment of which is illustrated in FIG. 3, would likely have a plurality of links terminating at each MG  22  when actually deployed. 
     In communication with SSP  12 A is calling party telecommunications device  11  (hereinafter calling party  11 ). Similarly, called party telecommunications device  13  (hereinafter called party  13 ) is in communication with SSP  12 B. Calling party  11  and called party  13  may be, for example, a telephone handset, a computer, a wireless voice or data terminal, or the like. 
     VoIP switch  20  comprises media gateways (MGs)  22 A,  22 B (collectively MGs  22 ) in communication with SSPs  12 A,  12 B via links  14 A,  14 B, respectively, and each other. Also in communication with MGs  22  is Signalling Gateway (SG)  24  and Media Gateway Controller (MGC)  26  via communications links  28 . Communications links  28  represent logical, transient routing paths over an IP network such, as for example, the Internet. Moreover, it should be noted that VoIP switch  20 , while represented as a single entity physically located at a single physical site, it may not always be so configured. In fact, MGs  22 , SG  24  and MGC  26  are likely to be physically separate but inter-operate to provide similar MTP functionality over an IP network that is conventionally provided by an Intelligent Network (IN) such as, for example, an SS7 network. 
     Although VoIP switch  20  is illustrated with two MGs  22 , a single SG  24  and a single MGC  26 , it may be desirable to implement the present invention with a different number of the elements forming VoIP switch  20 . For example, a plurality of MGs  22 , a plurality of SGs  24  and a plurality of MGCs  26  may be in communication with each other over network  30 , all forming VoIP switch  20 . 
     Generally, and in summary, in operation of voice and data network  10 , SSP  12 A, through a conventional request made by a calling party  11 , requires communication with SSP  12 B so that communication between calling party  11  and called party  13  can be established. Voice and SS7 signals, in the MTPL2 format corresponding to conventional SS7 signaling units, are transmitted by SSP  12 A to MG  22 A. The signaling data is extracted by MG  22 A and transmitted to SG  24  in IP packets over the network formed by connections  28 . The IP packets received by SG  24  are further processed and transmitted to MGC  26  as IP packets over communications links  28 . Responsive to the received IP packets, MGC  26  determines the MG  22  associated with the called party  13 , which in this example is MG  22 B. MGC  26  communicates with MG  22 B and SSP  12 B via SG  24  and a voice connection from calling party  11  to called party  13  is created via SSP  12 A, link  14 A, MGs  22 A and  22 B, link  14  and SSP  12 B. via SG  24  and a voice connection from calling party  11  to called party  13  is created via SSP  12 A, link  14 A, MGs  22 A and  22 B, link  14  and SSP  12 B. 
     Referencing FIG. 4, a more detailed and functional view of VoIP switch  20  is illustrated. Since there are no fixed connections between the elements of VoIP switch  20  (i.e., network  30  provides connectionless communications between a pair of these elements), each element (i.e. each MG  22 , SG  24  and MGC  26 ) can communicate directly with every other element of VoIP switch  20 . 
     Further, and as described in greater detail below, MGs  22  and SG  24  interact to provide MTPL2 functionality that is distributed between these two components of VoIP switch  20 . That is, when viewed from the perspective of a network element, such as, for example, an SSP  12 , signalling units (SUs) complying with the conventional MTPL2 standard are transmitted to and received from VoIP switch  20  at the MGs  22 . 
     Referencing FIG. 5, conventional MTPL3 comprises MTPL3 network management functions  40  and MTPL3 message handling functions  42 . Conventional MTPL2 comprises MTPL2 LSSU and MSU handling functions  44 , MTPL2 FISU handling functions  46  and MTPL2 framing  48 . In contrast, the distributed MTPL2 protocol described herein maintains MTPL3 functions which are performed by MTPL3 functional block  36 . However, the MTPL2 functions are distributed between LL-MTPL2 functional block  32  (which performs MTPL2 framing and MTPL2 FISU handling functions  48  and  46 , respectively) and UL-MTPL2 block  34  which performs MTPL2 LSSU and MSU handling functions  44 . 
     As indicated above, both MGs  22  and SG  24  provide a portion of the MTPL2 functionality. Specifically, MGs  22  incorporate Lower Level MTPL2 (LL-MTPL2) functional block  32  while SG  24  includes Upper Level MTPL2 (UL-MTPL2) functional block  34 . As described above, MGs  22  and SG  24  communicate via communications links  28  over IP network  30 . Network  30  is, for example, the internet, an intranet, or other IP network. SG  24  also includes MTPL3 functional block  36  which is in communication with UL-MTPL2 block  34  and is described in greater detail below. 
     In operation, and in overview, VoIP switch  20  will receive at MG  22 A, for example, conventional MTPL2 compliant SUs over link  14 A. As is known in the art in a conventional MTPL2 environment, a data link, such as link  14 A transmits a continuous stream of MTPL2 compliant data in both directions. This continuous stream of data will include Message Signalling Units (MSUs), Link Status Signalling Units (LSSUs) and Fill-In Signalling Units (FISUs). Typically, the continuous data stream will include repetitive and identical LSSUs and FISUs. Moreover, packetizing (i.e., encapsulating the SUs into IP compliant data packets), transmitting all of these IP data packets over an IP network, and de-encapsulating (i.e., stripping off the IP encapsulation) would require a prohibitive amount of overhead. The present invention, an embodiment illustrated in FIG. 4, receives at a VoIP switch and transmits from the VoIP switch conventional MTPL2 compliant SUs in conventional numbers (i.e., a continuous data stream corresponding to MSUs, LSSUs and FISUs are transmitted and received on links  14 ). However, internally VoIP switch  20  MGs  22  filters extraneous and duplicate FISUs and LSSUs. Accordingly, the number of packets that must be transmitted over IP network is significantly reduced. From the view of a conventional MTPL2 compliant network element such as SSPs  12 , VoIP switch  20  appears as simply another MTPL2 network element. 
     FIGS. 6A,  6 B and  6 C illustrate the data structures—which incorporate conventional Internet Protocol (IP) data structures—used to transmit data conforming to the distributed MTPL2 protocol embodying one aspect of this invention. The distributed MTPL2 protocol is used for communication between the SG  24  and MGs  22  (FIGS. 4 and 7) and, specifically, between the UL-MTPL2  34  and LL-MTPL2  32  components. Referencing FIG. 6A, Distributed Protocol Stack (DPS)  300  is comprised of an IP layer  314 , a User Datagram Protocol (UDP) layer  312 , Distributed L2 Protocol (DL 2 P) Header layer  308 , MG Control Information layer  306 , MTPL2 protocol layer  304  and MTPL3 protocol layer  302 . Appended to DPS  300  is payload data  301  which represents the SS7 signal payload (e.g., ISUP). IP layer  314  and UDP layer  312  are illustrated in greater detail in FIG.  6 B and described and discussed below. DL 2 P header  308 , which is described below and illustrated in greater detail in FIG. 6C, is positioned directly above the UDP header  312  of the DPS  300 . MG Control Information header layer  306  contains control information used to control the resources of the MGs  22  from SG  24 . For example, SG  24  can request that BERT (Bit Error Rate Testing be performed on the SS7 links by sending a message containing the appropriate information in the MG Control Information header  306 . Finally, MTPL2  304  conforms to the SS7 standards (i.e. ANSI T1.111.3 or ITU-T Q.703, the contents of which are hereby incorporated herein). That is, the MTPL2 header  304  contains the data necessary to form the three standard signalling units under the conventional MTPL2 protocol, namely the Fill-In Signal Units (FISUs), Link Status Signal Units (LSSUs) and the Message Signal Units (MSUs). Conventionally, data corresponding to FISUs is transmitted continuously on a signalling link in both directions between the two network elements unless other signalling units (i.e., LSSUs or MSUs) are present. FISUs carry basic level two information (e.g., acknowledgement of signalling unit receipt by a remote network element). Because a CRC checksum is calculated for each FISU, signalling link quality between the two network elements is checked continuously by both network elements at either end of the link. LSSUs carry one or two octets (8-bit bytes) of link status information between network elements at either end of a signalling link. The link status is used to control link alignment and to indicate the status of a network element (e.g., local processor outage) to another network element. MSUs carry all call control, database query, database response, network management and network maintenance data. 
     Continuing with the description of FIG. 6A, MTPL2 header  304  is populated with conventional data known to those skilled in the art. This data in part, used in sequencing of received SUs. Similarly, MTPL3 header  302  contains the header information for MTPL3 which is conventionally stored in each MSUs&#39; Signaling Information Field (SIF). 
     Referring to FIG. 6B, the IP header  312  and UDP header  314  are illustrated in greater detail. Other fields found in a conventional IP header are not illustrated but may also be used. The Source IP Address  330  and Destination IP Address  332  are standard 32-bit addresses. 
     UDP header  312 , which conforms to RFC  768 , the contents of which are hereby incorporated herein, comprises an eight bit Zero field  334 ; an eight bit Protocol field  336 ; an eight bit UDP length field  338 , identifying the length of the UDP datagram; a sixteen bit Source Port field  340 , identifying the port in the source host (associated with IP address  330 ) to which a reply may be sent; a sixteen bit Destination port field  342 , identifying the point of termination in destination host; a sixteen bit Length field  344 , indicating the total length of the IP packet, and a sixteen bit Checksum field  346 , for performing an error check on the IP header  314  and UDP header  312 , and the remaining portions of DPS  300  and payload data  301 . Accordingly, a checksum field in the remaining portions of DPS  300  may be unnecessary in some applications since Checksum  346  is calculated for the entire packet. 
     Generally, there will be a single UDP port on the SG  24  for each SS7 link  14  serviced by the SG  24 . To correlate messages received on a specific UDP port, a mapping operation will be performed based on the source IP address  330  and the source port  340  stored within the IP header  314  and UDP header  312 , respectively. Messages with an unknown source IP address  330  or unknown source port  340  will be discarded by SG  24 . 
     Accordingly, for each MG  22  serviced by SG  24 , SG  24  provides service for each signalling link terminating on a serviced MG  22 . To accomplish this a mapping or routing table is generated and stored at SG  24  which identifies the: IP address for each MG  22  serviced by the SG  24 ; and the UDP source ports associated with each link  14  terminating on a serviced MG  22 . A similar mapping or routing table will be generated and stored at each MG  22 . The MG routing table will include the UDP port associated with each link  14  terminating at MG  22 , the IP address of any SG  24  which services the MG  22  and the UDP port associated with the IP address of the SG  24  associated with the MG  22 . 
     Based on the foregoing, it should be apparent that whenever an IP message is transmitted on VoIP switch  20  (FIG. 3) the UDP and HP headers  312  and  314 , respectively, will be populated with the IP addresses and UDP ports retrieved from the routing table stored at the transmitting element (e.g., MG  22 , SG  24 , MGC  26 ). 
     Referring to FIG. 6C, the thirty-two bit DL 2 P Header  308  four eight bit fields, namely the version field  356 , the filler field  354 , the type field  352  and the sub-type field  350 . 
     The Version field  356  identifies the release version of the Distributed MTPL2 protocol that is being used. If a message identifies a version of the distributed MTPL2 architecture that is unknown to the receiving node (such as the MGs  22  or the SG  24 ), the receiving node will respond with a signal indicating a supporting version of the distributed MTPL2 architecture. This signal will inform the sender of the message using an unknown version of the Distributed MTPL2 protocol that a message with an unsupported version has been received. Moreover, the signal message indicates to the receiver of this message (i.e., the sender of the message of unknown protocol version) the version of the protocol supported by the sender of the signal message. 
     Filler field  354  is set to zero and is used to ensure that DL 2 P Header  308  is long word aligned (i.e., DL 2 P header  308  is a multiple of thirty-two bits). 
     While specific data structures, including number of fields, bit lengths for those fields, and general organization embodying one aspect of the invention are illustrated in FIGS. 6A,  6 B and  6 C, a person skilled in the art will appreciate that other data structures including a differing number of fields of various bit lengths, different bit lengths of the fields disclosed, and other organizations can be used to implement the invention disclosed herein. 
     With reference to the foregoing acronym definitions, the components of VoIP switch  20  (FIG. 3) will support the values in Type field  352  of DL 2 P Header  308  identified in Table 1. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Type Field 352 Entries for MGs 22, SG 24 and MGC 26 
               
             
          
           
               
                 Value 
                 Description 
               
               
                   
               
               
                 01 
                 TXC-MG 
               
               
                 02 
                 RC-MG 
               
               
                 03 
                 AERM 
               
               
                 04 
                 SUERM 
               
               
                 05 
                 BERT 
               
               
                 06 
                 OM-MG 
               
               
                   
               
             
          
         
       
     
     MGs  22  and SG  24  will populate sub-type field  350  of DL 2 P Header  308  with one of many possible values corresponding to a value in Type field  352 . Table 2 lists the values that can be entered into Sub-type field  350  by the components of VoIP switch  20  and the message represented by those values. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Sub-Type Field 350 Entries of VoIP Switch 20 
               
             
          
           
               
                   
                 Type 
                 Sub-Type 
                 Message 
               
               
                   
                   
               
               
                   
                 TXC-MG 
                 01 
                 START-TXC 
               
               
                   
                   
                 02 
                 ACK-START-TXC 
               
               
                   
                   
                 03 
                 STOP-TXG 
               
               
                   
                   
                 04 
                 ACK-STOP-TXC 
               
               
                   
                   
                 05 
                 INH-MSU 
               
               
                   
                   
                 06 
                 ACK-INH-MSU 
               
               
                   
                   
                 07 
                 UNI-MSU 
               
               
                   
                   
                 08 
                 ACK-UNI-MSU 
               
               
                   
                   
                 09 
                 MSU (no acks) 
               
               
                   
                   
                 11 
                 FISU 
               
               
                   
                   
                 12 
                 ACK-FISU 
               
               
                   
                   
                 13 
                 SEND-LSSU 
               
               
                   
                   
                 14 
                 ACK-SEND-LSSU 
               
               
                   
                 RC-MG 
                 01 
                 START-RC 
               
               
                   
                   
                 02 
                 ACK-START-RC 
               
               
                   
                   
                 03 
                 STOP-RC 
               
               
                   
                   
                 04 
                 ACK-STOP-RC 
               
               
                   
                   
                 05 
                 FISU 
               
               
                   
                   
                 06 
                 ACK-FISU 
               
               
                   
                   
                 07 
                 LSSU 
               
               
                   
                   
                 08 
                 ACK-LSSU 
               
               
                   
                   
                 09 
                 MSU (no acks) 
               
               
                   
                 AERM 
                 01 
                 START-AM 
               
               
                   
                   
                 02 
                 ACK-START-AM 
               
               
                   
                   
                 03 
                 STOP-AM 
               
               
                   
                   
                 04 
                 ACK-STOP-AM 
               
               
                   
                   
                 05 
                 EMERGENCY 
               
               
                   
                   
                 06 
                 ACK-EMERGENCY 
               
               
                   
                   
                 07 
                 NORMAL 
               
               
                   
                   
                 08 
                 ACK-NORMAL 
               
               
                   
                   
                 09 
                 ABORT 
               
               
                   
                   
                 10 
                 ACK-ABORT 
               
               
                   
                   
                 11 
                 CORRECT-SU 
               
               
                   
                   
                 12 
                 ACK-CORRECT-SU 
               
               
                   
                 SUERM 
                 01 
                 START-EM 
               
               
                   
                   
                 02 
                 ACK-START-EM 
               
               
                   
                   
                 03 
                 STOP-EM 
               
               
                   
                   
                 04 
                 ACK-STOP-EM 
               
               
                   
                   
                 05 
                 LINK-FAIL 
               
               
                   
                   
                 06 
                 ACK-LINK-FAIL 
               
               
                   
                 BERT 
                 01 
                 START-BERT 
               
               
                   
                   
                 02 
                 ACK-START-BERT 
               
               
                   
                   
                 03 
                 STOP-BERT 
               
               
                   
                   
                 04 
                 ACK-STOP-BERT 
               
               
                   
                   
                 05 
                 STATUS-BERT 
               
               
                   
                   
                 06 
                 ACK-STATUS-BERT 
               
               
                   
                   
                 07 
                 LOCAL-LOOP 
               
               
                   
                   
                 08 
                 ACK-LOCAL-LOOP 
               
               
                   
                   
                 09 
                 REM-LOOP 
               
               
                   
                   
                 10 
                 ACK-REM-LOOP 
               
               
                   
                 OM-MG 
                 01 
                 OM-START 
               
               
                   
                   
                 02 
                 ACK-OM-START 
               
               
                   
                   
                 03 
                 OM-DATA 
               
               
                   
                   
                 04 
                 ACK-OM-DATA 
               
               
                   
                   
               
             
          
         
       
     
     Addition information may be required for some DL 2 P messages. This information would be passed in the MG Control information field  306 . Table 3 identifies the extra information that may be used in the DL 2 P messages. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Type Field 352 and Sub-Type Field 350 Entries for MGC 22 
               
             
          
           
               
                 Type 
                 Sub-Type 
                 Message 
                 Additional Entries 
               
               
                   
               
               
                 TXC-MG 
                 13 
                 SEND-LSSU 
                 Octet indicating which LSSU 
               
               
                   
                   
                   
                 to send: 
               
               
                   
                   
                   
                 00 - Status Indicator Out of 
               
               
                   
                   
                   
                 Alignment (SIO) 
               
               
                   
                   
                   
                 01 - Status Indicator 
               
               
                   
                   
                   
                 Normal (SIN) 
               
               
                   
                   
                   
                 02 - Status Indicator 
               
               
                   
                   
                   
                 Emergency (SIE) 
               
               
                   
                   
                   
                 03 - Status Indicator Out of 
               
               
                   
                   
                   
                 Service (SIOS) 
               
               
                   
                   
                   
                 04 - Status Indicator Processor 
               
               
                   
                   
                   
                 Outage (SIPO) 
               
               
                   
                   
                   
                 05 - Status Indicator 
               
               
                   
                   
                   
                 Busy (SIB) 
               
               
                   
                 14 
                 ACK-SEND- 
                 Also contains octet indicating 
               
               
                   
                   
                 LSSU 
                 which LSSU command is 
               
               
                   
                   
                   
                 acknowledged. 
               
               
                 BERT 
                 01 
                 START-BERT 
                 Octet indicating which BERT 
               
               
                   
                   
                   
                 pattern to send: 
               
               
                   
                   
                   
                 00 - 511 bit pattern 
               
               
                   
                   
                   
                 01 - 2047 bit pattern 
               
               
                   
                   
                   
                 02 - Repeated pattern of 100 
               
               
                   
                   
                   
                 octets of FFh followed by 100 
               
               
                   
                   
                   
                 octets of 00h. 
               
               
                   
                   
                   
                 03 - Repeated patterns of 100 
               
               
                   
                   
                   
                 octets of 7Eh followed by 100 
               
               
                   
                   
                   
                 octets of 00h. 
               
               
                   
                   
                   
                 04 - Continuous 32h 
               
               
                   
                   
                   
                 05 - Continuous 40h 
               
               
                   
                   
                   
                 Only pattern 00 will be used. 
               
               
                   
                 02 
                 ACK-START- 
                 Returns octet specifying 
               
               
                   
                   
                 BERT 
                 pattern actually being sent. 
               
               
                   
                 04 
                 ACK-STOP- 
                 Returns status of BERT test, 
               
               
                   
                   
                 BERT 
                 which includes: 
               
               
                   
                   
                   
                 Octet 
               
               
                   
                   
                   
                 1-4 Bits Sent 
               
               
                   
                   
                   
                 5-8 Bits in error 
               
               
                   
                 05 
                 STATUS-BERT 
                 Returns same information as 
               
               
                   
                   
                   
                 ACK-STOP-BERT message. 
               
               
                 OM-MG 
                 03 
                 OM-DATA 
                 Returns the OMs kept 
               
               
                   
                   
                   
                 on the MG. 
               
               
                   
                   
                   
                 Octet 
               
               
                   
                   
                   
                 1-2 = Number of 
               
               
                   
                   
                   
                 SU errors (SUERM). 
               
               
                   
               
             
          
         
       
     
     FIG. 7 shows the split in MTP Level 2 functionality between the SG  24  and the MG  22 . The UL-MTPL2 block  34  (Upper Layer of MTP Level 2) resides on SG  24 , while the LL-MTPL2 block  32  (Lower Layer of MTP Level 2) resides on MG  22 . Examples of the signal flows between LL-MTPL2 and UL-MTPL2 functional blocks  32  and  34  are illustrated in FIGS. 8 and 9. Signals transmitted between LL-MTPL2 functional block  32 , which forms part of MG  22 , and UL-MTPL2 functional block  34 , forming part of SG  24 , are transmitted across network  30 , which is illustrated as a dotted line boundary between UL-MTPL2 block  34  and LL-MTPL2 block  32  in FIGS. 7,  8  and  9 . 
     MG  22  comprises LL-MTPL2 functional block  32  which operates to perform: the reception of conventional data signals on data links  14 ; the transmission of data complying with the distributed MTPL2 architecture described herein to UL-MTPL2 functional block  34  of SG  24 ; the reception of data from SG  24 ; and the transmission of conventional data on links  14 . LL-MTPL2 functional block  32  comprises: Delimitation, Alignment, Error Detection Receive (DAEDR) block  402 ; Receive Control located at Media Gateway (RC-MG) block  406 ; Transmit Control located at Media Gateway (TXC-MG) block  408 ; and Delimitation, Alignment, Error Detection Transmit (DAEDT) block  410 . Also forming part of LL-MTPL2 functional block  32  are Bit Error Rate Testing (BERT) block  404 , Signalling Unit Error Rate Monitor (SUERM) block  412 , Alignment Error Rate Monitor (AERM) block  414 , and Operational Measurement at the MG (OM-MG) block  416 . 
     SG  24  comprises UL-MTPL2 functional block  34  which operates to perform: the reception of MTPL3 data from MTPL3  36 ; the transmission of data complying to with the distributed MTPL2 architecture to LL-MTPL2 functional block  32  of MG  22 ; the reception of data from MG  22 ; and the transmission of MTPL3 data to MTPL3  36 . UL-MTPL2 functional block  34  comprises: Reception Control at the SG (RC-SG) block  420  and Transmission Control at the SG (TCX-SG) block  426 . TCX-SG block  426  comprises Transmission Buffer (TB) block  434  and Re-Transmission Buffer (RTB) block  436 . TCX-SG  426  transmits sequenced data packets and stores a copy in RTB  436 . If one or more of the transmitted packets are not received, the missing packets are retrieved from RTB  436  an retransmitted. Otherwise, the copies of received packets are deleted from RTB  436 . UL-MTPL2 functional block  34  further comprises Congestion Control (CC) block  422 ; Operational Measurements at the SG (OM-SG) block  424 ; Link State Control (LSC) block  428 ; and Initial Alignment Control (IAC) block  430 . 
     Generally and in overview, DAEDR block  402  receives conventional MTPL2 data from link  14 . The data is processed (i.e., delimited and error detection is performed) and transferred to RC-MG block  406  which filters the received data to remove repeated LSSUs and FISUs. The filtered data is then transmitted to RC-SG  420  of SG  24 . RC-SG  420  performs management of the sequence numbers of the received data based on the information stored in MTPL2 header  304 . The data received by SG  24  is processed and MSUs are transmitted to MTPL3  36  while FISUs and LSSUs are processed by the RC-SG block  420 . Transmitted MTPL3 data is transferred to TCX-SG block  426  where, as described above, sequence number management is performed. The TCX-SG block  426  transmits, over IP network  30  (FIG.  7 ), the MTPL3 data to the TXC-MG block  408  where repetitive tasks (e.g., generation of repetitive LSSUs and FISUs) are performed. Data received and generated by TXC-MG block  408  is transmitted to DAEDT block, processed (i.e., delimited, aligned—into conventional SS7 signals) and then transmitted by MG  22  over link  14  to an SS7 network element such as, for example, an SSP  12 . 
     Also in overview, SUERM block  412 , AERM block  414  and OM-MG block  416  also form part of MG  22 . Generally, the SUERM and AERM blocks,  412  and  414 , respectively, communicate with DAEDR block  402  and monitor the number of signalling unit (SU) errors detected by the DAEDR block  402 . The OM-MG block  416  records operational measurements (OMs) of LL-MTPL2 functional block  32 . These OMs are transmitted, when requested, to SG  24 . The OMs are cleared from OM-MG  416  when uploaded to SG  24 . 
     Also forming part of SG  24  are CC block  422 , OM-SG block  424 , LSC block  428  and IAC block  430 . CC Block  422  operates to control congestion on SG  24  by monitoring the MTPL2 receive buffer. OM-SG block  424  records a number of operational measurements of UL-MTPL2 functional block  34 . The OM-SG block  424  is also in communication with OM-MG block  416  of MG  22 , and may send a request to the OM-MG block  416  to transmit its OMs records to the OM-SG block  424 . OM-SG block  424  will then store the OM data uploading to OM-SG  424  in response to this request. LSC block  428  provides general management of the MTPL2 link  14 . IAC block  430 , which is in communication with AERM block  414  of MG  22 , performs initial alignment of links  14 , monitors LSSUs received from link  14  and communicates with AERM block  414  of MG  22  to determine the quality of the link  14  during the proving period. 
     Each of the functional blocks that form UL-MTPL2 functional block  34  and LL-MTPL2 functional block  32 , and their interactions, are described in detail below. It should be noted that many aspects of LL-MTPL2  32 , UL-MTPL2  34  and MTPL3  36  operate in accordance with ANSI T1.111.4 or BellCore GR-246, the contents of both of which are hereby incorporated herein. 
     DAEDR functional block  402  receives a data stream from voice/signaling link  14  and extracts the signaling data therefrom. DAEDR  402  then, as is conventional, performs delimitation, alignment, error detection on the received signalling data forming conventional MTPL2 signalling units. 
     The RC-MG block  406 , in co-operation with RC-SG block  420  of SG  24 , provides signal reception control. RC-MG block  404  is tasked with performing repetitive functions in order to reduce IP traffic between MG  22  and SG  24 . As is known in the art, FISUs are conventionally transmitted, in both directions, continuously over a signaling link, such as links  14 , unless LSSUs or MSUs are present. This process would, if performed over an IP network, such as network  30  (FIG.  4 ), waste network resources. Accordingly, RC-MG  406  performs operations to reduce IP traffic between MG  22  and SG  24  while simultaneously providing the benefits of the MTP protocol. 
     RC-MG block  406  transitions from IDLE to IN-SERVICE upon receipt of a START-RC message from RC-SG block  420  of SG  24 . Upon receipt of the START-RC message (Type: 01, Sub-Type: 02 are entered in fields  352  and  350 , respectively, of DL 2 P Header  308 —FIG.  6 C), RC-MG  406  will transmit an acknowledgement signal (ACK-START-RC) to RC-SG block  406  of SG  24 . As indicated previously, messages transmitted between LL-MTPL2 functional block  32  of MG  22  and UL-MTPL2 functional block  34  of SG  24  are transmitted over network  30 . RC-MG block  406  transitions from IN-SERVICE to IDLE upon receipt of a STOP-RC message from RC-SG block  420 . A STOP-RC message received by RC-MG block  406  is acknowledged by the transmission of an ACK-STOP-RC message from RC-MG  406  to RC-SG  420 . 
     While in IN-SERVICE mode RC-MG  406  will, upon receipt of an FISU, LSSU or MSU from DAEDR block  402 , transmit a corresponding FISU, LSSU or MSU message complying with the distributed standard described herein (i.e. sub-type field  350 —FIG.  6 C—having the value of 05, 07 or 09), to RC-SG block  420 . RC-MG  406  will receive, in response to an FISU or LSSU message, an ACK-FISU or ACK-LSSU, respectively, from RC-SG block  420 . To reduce IP traffic between RC-MG  406  and RC-SG  420 , RC-MG  406  will filter out duplicate FISUs and LSSUs received from DAEDR  402 . This filtering involves transmitting only one FISU (or LSSU) in a continuous stream of FISUs (or LSSUs) in a pre-determined period (i.e. only one FISU—or LSSU—every T sanity  period). Further, RC-MG  406  will not receive an acknowledgement of MSUs transmitted to RC-SG  420  thereby further reducing traffic on IP network  30 . A summary of the substantive message (i.e., non-ACK style messages) are summarised in Table 4 shown below. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 RC-MG Block 406 Message Table 
               
             
          
           
               
                   
                   
                 Signal 
                 Signal 
                   
               
               
                 Message 
                 MSG ID 
                 Source 
                 Destination 
                 Comments 
               
               
                   
               
               
                 Start RC 
                 START-RC 
                 RC-SG 
                 — 
                 RC-SG 420 sends this 
               
               
                   
                   
                   
                   
                 signal when start 
               
               
                   
                   
                   
                   
                 issued by LSC 428. 
               
               
                 Stop RC 
                 STOP-RC 
                 RC-SG 
                 — 
                 Stops RC-MG 406. 
               
               
                 FISU 
                 FISU 
                 — 
                 RC-SG 
                 FISUs received from 
               
               
                   
                   
                   
                   
                 MTPL3 block 36 
               
               
                   
                   
                   
                   
                 transmitted to RC-MG 
               
               
                   
                   
                   
                   
                 406 by RC-SG 420 
               
               
                   
                   
                   
                   
                 (these FISUs have 
               
               
                   
                   
                   
                   
                 been filtered by 
               
               
                   
                   
                   
                   
                 MG 22, but are 
               
               
                   
                   
                   
                   
                 repeated every 
               
               
                   
                   
                   
                   
                 T sanity  - i.e., only 
               
               
                   
                   
                   
                   
                 one FISU in a stream 
               
               
                   
                   
                   
                   
                 of identical FISUs is 
               
               
                   
                   
                   
                   
                 transmitted every 
               
               
                   
                   
                   
                   
                 T sanity  seconds). 
               
               
                 LSSU 
                 LSSU 
                 — 
                 RC-SG 
                 LSSUs received from 
               
               
                   
                   
                   
                   
                 MTPL3 block 36 
               
               
                   
                   
                   
                   
                 transmitted to RC-MG 
               
               
                   
                   
                   
                   
                 406 by RC-SG 420 
               
               
                   
                   
                   
                   
                 (these have also 
               
               
                   
                   
                   
                   
                 filtered by MG 22 but 
               
               
                   
                   
                   
                   
                 are repeated every 
               
               
                   
                   
                   
                   
                 T sanity ). 
               
               
                 MSU 
                 MSU 
                 — 
                 RC-SG 
                 MSUs received from 
               
               
                   
                   
                   
                   
                 MTPL3 block 36 
               
               
                   
                   
                   
                   
                 transmitted to RC-MG 
               
               
                   
                   
                   
                   
                 406 by RC-SG 420. 
               
               
                   
               
             
          
         
       
     
     RC-MG block  406  transmits to and receives IP messages from RC-SG block  420  of SG  24 . These IP messages transmitted between RC-SG  420  and RC-MG  406  are transmitted over network  30 . RC-SG  420  sends IP packets to RC-MG  406  that initiate and cease operation of RC-MG  402 . Accordingly, RC-MG  406  operates under partial control of RC-SG  420 . In response to the control messages (sent by RC-SG  420 ) received by RC-MG  406 , RC-MG  406  transmits IP packets incorporating an acknowledgement portion in the sub-type field  350  (FIG.  6 C). Other IP packets sent by RC-MG  420  to RC-SG  420  include FISU, LSSU and MSU messages. In response to the FISUs and LSSUs (only) transmitted by RC-MG  402 , RC-SG  420  sends acknowledgement messages upon receipt of these messages. 
     As described briefly above LSC block  428  provides some of the control of SG  24  and MG  22 . Generally, LSC  428  interfaces with MTPL3 block  36  and controls the distributed MTPL2 functions of MG  22  and SG  24 . LSC  428  issues messages to RC-SG  420  to start and stop the receipt and transmission of IP packets between SG  24  and MG  22 . As a result of the messages issued by LSC  428 , RC-SG  420  transmits the start and stop messages to RC-MG  406  described above. Generally, LSC  428  has the following states: POWER OFF, OUT OF SERVICE, INITIAL ALIGNMENT, ALIGNED/READY, ALIGNED/NOT READY, IN SERVICE and PROCESSOR OUTAGE. These state conditions are transmitted to MTPL3 block  36  (FIG. 4) in accordance with the MTPL3 protocol. 
     IAC block  430  also provides some of the operational control of SG  24 . IAC  420  performs initial alignment between SG  24  and a link  14 , when an alignment request is received from a link  14 . In addition to providing alignment functions, IAC  430  interacts with AERM block  414  of MG  22  to verify the quality of data received by MG  22  over link  14  during a proving or testing period. Generally, IAC  430  has four states: IDLE, NOT ALIGNED, ALIGNED and PROVING. IAC  430  will transmit IP packets (via the transmission facilities provided by TXC-SG  426 ) to AERM block  414  to start and stop error monitoring and to indicate an emergency status. IAC  430  will receive from AERM  414  messages indicating that the signaling units received during the monitoring phase are acceptable (CORRECT SU) or that the link should be aborted (ABORT). Table 6 includes of a summary of the messages transmitted between IAC  430  and AERM  414 . 
     TXC-SG  426 , which includes transmission (TB) and re-transmission (RTB) buffers  434 ,  436 , respectively, provides general transmission facilities to SG  24 . These two buffers operate to manage the forward and backward sequence numbers in a manner known by those skilled in the art. 
     Messages transmitted to an MG  22  from TXC-SG  426  of SG  24 , via network  30  (FIG.  4 ), are received by the TXC-MG block  408  of a receiving MG  22 . TXC-MG  408  has two states IDLE and IN-SERVICE. The transition from IDLE to TN-SERVICE is initiated by the receipt of a START TXC message transmitted by TXC-SG  426  and received by TXC-MG  408 . Upon receipt of a START TXC message, TXC-MG  408  will request DEADT block  410  to commence operation. The reverse transition, from IN-SERVICE to IDLE, results from the transmission of a STOP TXC message transmitted from TXC-SG  426  to TXC-MG  408 . Generally, TXC-MG  408  and TXC-SG  426  interact to minimize the number of messages that are transmitted over network  30 . The quantity of SS7 messages (in IP packets) that need to be transmitted over network  30  are lower than the quantity of conventional SS7 signals that are ultimately transmitted over link  14  by MG  24 . For example, TXC-MG  408  will only receive a single message (FISU—see Table 5), transmitted over network  30 , that indicates that FISUs are to be generated and transmitted to a desired network element, such as, for example, SS7  12 A. Upon receipt of this message, MG  22  will commence generation and transmission of a stream of identical of FISUs to SSP  12 A until indicated otherwise. 
     TXC-SG  426  also transmits to TXC-MG  408  an INH-MSU message which informs TXC-MG  408  to only send FISUs (that is inhibit the transmission of LSSUs and MSUs). A UNI-MSU (Uninhibit MSU) message received by TXC-MG  408  indicates that MSU and LSSU transmission can re-commence. An MSU message received by TXC-MG  408  indicates that an MSU is being transmitted to TXC-MG  408  and is stored in payload  301  (FIG.  6 A). A FISU message transmitted to TXC-MG  408  indicates that FISUs are to generated and transmitted from TXC-MG  408  and, eventually, to link  14  (after operations are performed by DAEDT  410 ) until a message indicating otherwise is transmitted from TXC-SG  426  and received by TXC-MG  408 . The FISU message is generally used to check for quality of the transmission media between the two signaling points (in this case, MG  22  and SG  24 . An LSSU message received from TXC-SG  426  will indicate that an LSSU message is to be generated with the status field of the standard LSSU set to one of: SIB (which indicates that SG  24  is busy); SIOS (out of service indicator); SIPO (processor outage); SIO (out of alignment indicator); SIE (emergency indicator); and SIN (system normal indicator). A summary of the messages, the source or destination of the message, (i.e., an entry in the Signal Source column indicates that the block entered in the cell is the transmitter of the associated signal message to TXC-MG Block  408  while an entry in the Signal Destination column indicates that TXC-MG block  408  is transmitting the associated signal message to the block entered in the Signal Destination cell) and an associated comment for each message is found in Table 5. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 TXC-MG Block 408 Message Table 
               
             
          
           
               
                   
                   
                 Signal 
                 Signal 
                   
               
               
                 Messages 
                 MSG ID 
                 Source 
                 Destination 
                 Comments 
               
               
                   
               
               
                 Start TXC 
                 START- 
                 TXC-SG 
                 — 
                 TXC-SG sends this signal when 
               
               
                   
                 TXC 
                   
                   
                 start issued by LSC. TXC-MG 
               
               
                   
                   
                   
                   
                 will inform DAEDT to also start. 
               
               
                 Stop TXC 
                 STOP-TXC 
                 TXC-SG 
                 — 
                 Stop TXC-MG. 
               
               
                 Inhibit MSU 
                 INH-MSU 
                 TXC-SG 
                 — 
                 Indicates that the TXC-MG 
               
               
                 Transmission 
                   
                   
                   
                 should send FISU and cancel 
               
               
                   
                   
                   
                   
                 LSSU and MSU transmission. 
               
               
                 Uninhibit 
                 UM-MSU 
                 TXC-SG 
                 — 
                 Indicates that MSU transmission 
               
               
                 MSU 
                   
                   
                   
                 should be resumed. 
               
               
                 Indication 
               
               
                 MSU 
                 MSU 
                 TXC-SG 
                 — 
                 MSU transmitted 
               
               
                 FISU 
                 FISU 
                 TXC-SG 
                 — 
                 FISU transmitted. This is really 
               
               
                   
                   
                   
                   
                 used as a heartbeat between the 
               
               
                   
                   
                   
                   
                 SG and MG. 
               
               
                 Send LSSU 
                 SEND-LSSU 
                 TXC-SG 
                 — 
                 Indication to send one of the 
               
               
                   
                   
                   
                   
                 following LSSU on link: 
               
               
                   
                   
                   
                   
                 (SIB - Busy) 
               
               
                   
                   
                   
                   
                 (SIOS - Out-of-service) 
               
               
                   
                   
                   
                   
                 (SIPO - Processor Outage) 
               
               
                   
                   
                   
                   
                 (SIO - Out of alignment) 
               
               
                   
                   
                   
                   
                 (SIE - Emergency) 
               
               
                   
                   
                   
                   
                 (SIN - Normal) 
               
               
                   
               
             
          
         
       
     
     DAEDT (Delimitation, Alignment, Error Detection Transmit) block  410  provides functionality necessary for the transmission of conventional MTPL2 compliant messages from an MG  22  to an SSP  12  over link  14 . Similar to many of the other blocks, DAEDT  410  has two states: IDLE and IN-SERVICE. When in the IDLE state, DAEDT  410  generates and transmits  7 Eh flags onto link  14 . When IN-SERVICE, that is when a signal is transmitted to DAEDT  410  from TXC-MG  408 , DAEDT  410  will, after generating check-bits, inserting zeros, and generating and inserting flags between signalling units, transmit an SS7 signalling unit complying with conventional MTPL2 which has been generated from the FISU, LSSU or MSU received from TXC-MG  408 , to a network element, such as an SSP  12 . 
     CC (Congestion Control) block  422 , as is known by those skilled in the art, monitors the number of signaling units waiting to be processed by MTPL3  36 . 
     AERM (Alignment Error Rate Monitor) block  414  of MG  22  generally provides error monitoring tasks (using, for example, a “leaky bucket” algorithm), in co-operation with DAEDR  402 , under control of IAC block  430  of SG  24 . A signal flow diagram for AERM block  414  is illustrated in FIG.  8 . AERM  414  has two states: IDLE and MONITORING. In the IDLE state AERM  414  lies dormant and provides no functions. In the MONITORING state, AERM  414  monitors the number of signaling unit errors detected by DAEDR  402 . If too many errors (determined based on a stored parameter which may be changed by an operator of VoIP switch  20 —FIG. 3) are detected, an ABORT message is relayed to IAC  430 . 
     The transition of AERM  414  from IDLE to MONITORING is instigated by a START-AM message received from IAC  430 . The reverse transition, from MONITORING to IDLE, results from a STOP-AM message received by AERM  414  from IAC  430 . IAC  430  may also transmit an EMERGENCY message to AERM  430 , indicating that the alignment between a link  14  and SG  24  is in an emergency mode. If IAC  430  sends an EMERGENCY message, IAC  430  will set also an internal counter (T i ) equal to an emergency counter (T ie ) which defines the parameters in the emergency testing or proving period. If the number of errors in signaling units received by MG  22  and detected by AERM  430  exceed the value of counter T i  (which has been previously set equal to T ie ) during the emergency proving or testing period (the emergency proving period being defined by the timer P e —described in greater detail below), AERM  430  will issue an ABORT message to IAC  430 . AERM  430  may also receive from LSC  428  a NORMAL message indicating that SG  24  is operating in normal alignment mode. In response to the reception of a NORMAL message, AERM  414  will set the T i  parameter to T in . If AERM  414  identifies, during the NORMAL proving or testing period (defined by the time P n  also described in greater detail below), a quantity of signaling unit errors greater than the T i  parameter, AERM  414  will issue the ABORT message to IAC  430 . The values for T i  in an EMERGENCY proving period may be, for example, set to single error detected over a short period, such as, for example, 0.7 s. For NORMAL operation, the number of detected errors allowed before triggering an ABORT message may be much greater, such as for example, for signaling unit errors, over a longer period, such as, for example, 2.4 s. A summary of the messages, the signal source or signal destination of the message (i.e., an entry in the Signal Source column indicates that the entered block transmits the associated signal message to AERM block  414  while an entry in the Signal Destination column indicates that AERM block  414  transmits the associated signal message to the entered block) and information about the purpose of the message is found in Table 6. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 AERM Block 414 Message Table 
               
             
          
           
               
                   
                   
                 Signal 
                 Signal 
                   
               
               
                 Message 
                 MSG ID 
                 Source 
                 Destination 
                 Comments 
               
               
                   
               
             
          
           
               
                 Start AM 
                 START-AM 
                 IAC 
                 — 
                 IAC starts the error monitor 
               
               
                 (Alignment 
                   
                   
                   
                 when entering into alignment. 
               
               
                 Monitor) 
               
               
                 Stop AM 
                 STOP-AM 
                 IAC 
                 — 
                 IAC stops the error monitoring. 
               
               
                 (Alignment 
               
               
                 Monitor) 
               
               
                 Emergency 
                 EMERGENCY 
                 IAC 
                 — 
                 Indicates Emergency alignment 
               
               
                   
                   
                   
                   
                 mode. Set T i  to T ie  which is equal 
               
               
                   
                   
                   
                   
                 to one signal unit in error in the 
               
               
                   
                   
                   
                   
                 proving period. 
               
               
                 Normal 
                 NORMAL 
                 LSC 
                 — 
                 Indicates Normal alignment 
               
               
                   
                   
                   
                   
                 mode. Set T i  to T in  which is 
               
               
                   
                   
                   
                   
                 equal to four signal units in error 
               
               
                   
                   
                   
                   
                 in the proving period. 
               
               
                 Abort 
                 ABORT 
                 — 
                 IAC 
                 Abort proving if sent when 
               
               
                 Proving 
                   
                   
                   
                 number of errors = T i . 
               
               
                 Correct SU 
                 CORRECT-SU 
                 — 
                 IAC 
                 In the data links 14, this signal is 
               
               
                   
                   
                   
                   
                 generated by DAEDR, however 
               
               
                   
                   
                   
                   
                 AERM should really be sending 
               
               
                   
                   
                   
                   
                 it and should only send one when 
               
               
                   
                   
                   
                   
                 it has been started since IAC is 
               
               
                   
                   
                   
                   
                 only waiting for one. 
               
               
                   
               
             
          
         
       
     
     SUERM (Signalling Unit Error Rate Monitor) block  412  also has two modes of operation: IDLE, wherein no functions are performed; and IN-SERVICE, wherein SUERM block  412  monitors the number of errors detected by DAEDR block  402 . A signal flow diagram for SUERM block  412  is illustrated in FIG.  5 . SUERM  412  transitions from IDLE to IN-SERVICE on receipt of a START-EM message from LSC  428 . The reverse transition, from IN-SERVICE to IDLE, results from the receipt of a STOP-EM message also transmitted by LSC  428 . If SUERM  412  detects too many errors on link  14  (an error having been determined by DAEDR  402 ), SUERM transmits a LINK-FAIL message to LSC  428  indicating this condition. Moreover, the connection between MG  22  and link  14  is also brought down. A summary of the messages, the signal source or signal destination of the message (i.e., an entry in the Signal Source column indicates that the entered block transmits the associated signal message to SUERM block  412  while an entry in the Signal Destination column indicates that SUERM block  412  transmits the associated signal message to the entered block) and an explanatory comment for each message is found in Table 7. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 SUERM Block 412 Message Table 
               
             
          
           
               
                   
                   
                 Signal 
                 Signal 
                   
               
               
                 Signal 
                 MSGID 
                 Source 
                 Destination 
                 Comments 
               
               
                   
               
               
                 Start EM 
                 START-EM 
                 LSC 
                 — 
                 Start Error monitoring 
               
               
                 (Error 
                   
                   
                   
                 by SUERM. 
               
               
                 Monitor) 
               
               
                 Stop EM 
                 STOP-EM 
                 LSC 
                 — 
                 Stop Error monitoring 
               
               
                 (Error 
                   
                   
                   
                 by SUERM. 
               
               
                 Monitor) 
               
               
                 Link 
                 LINK-FAIL 
                 — 
                 LSC 
                 Generated by SUERM 
               
               
                 Failure 
                   
                   
                   
                 when too many errors 
               
               
                   
                   
                   
                   
                 are detected on the 
               
               
                   
                   
                   
                   
                 signaling link. 
               
               
                   
               
             
          
         
       
     
     BERT (Bit Error Rate Testing) block  404  generally performs the procedures for testing and maintaining SS7 links as described in T1.111.17. Similar to other blocks, BERT  404  has two states: IDLE, where no functions performed; and IN-SERVICE. A transition from IDLE to IN-SERVICE results from the reception of a START-BERT message received by BERT  404  from LSC  428 . A reverse transition, from IN-SERVICE to IDLE, results from the reception by BERT  404  of a STOP-BERT transmitted by LSC  428 . Upon receipt of a STOP-BERT message, BERT  404  will return a ACK-STOP-BERT, which will include the results of error testing. Error testing of messages sent between SG  24  and MG  22 , or between MG  22  and an SSP  12  (FIG.  3 ), may be performed by establishing a loopback system. A “local” loopback will be established between SG  24  and MG  22  upon receipt of a LOCAL-LOOP message at BERT  404  which has been issued by LSC  428 . The signalling units used in the local loopback will be issued by SG  24 , received by MG  22  and returned. “Remote” loopbacks, conducted between MG  22  and link  14 , can also be established by the reception of a REM-LOOP message issued by LSC  428 . LSC  428  will periodically request, by the issuance of a STATUS-BERT message, the status of the BERT testing. Generally, during testing, MG  22 , through operation of BERT  404 , will transmit BERT testing results to SG  24  on a periodic basis, such as, every 5 seconds. A summary of the messages, the signal source or signal destination of the message (i.e., an entry in the Signal Source column indicates that the entered block transmits the associated signal message to BERT block  404  while an entry in the Signal Destination column indicates that BERT block  404  transmits the associated signal message to the entered block) and comments about the message is found in Table 8. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 BERT Block 404 Message Table 
               
             
          
           
               
                   
                   
                 Signal 
                 Signal 
                   
               
               
                 Signal 
                 MSG ID 
                 Source 
                 Destination 
                 Comments 
               
               
                   
               
               
                 Start 
                 START-BERT 
                 LSC 
                 — 
                   
               
               
                 BERT 
               
               
                 Stop 
                 STOP-BERT 
                 LSC 
                 — 
                 The acknowledge 
               
               
                 BERT 
                   
                   
                   
                 for this message 
               
               
                   
                   
                   
                   
                 will also return 
               
               
                   
                   
                   
                   
                 the result of the test. 
               
               
                 Status 
                 STATUS- 
                 — 
                 LSC 
                 Periodically out- 
               
               
                 BERT 
                 BERT 
                   
                   
                 put the status of 
               
               
                   
                   
                   
                   
                 the BERT test. 
               
               
                   
                   
                   
                   
                 The MG may return 
               
               
                   
                   
                   
                   
                 the test status to the 
               
               
                   
                   
                   
                   
                 SG every 5 seconds 
               
               
                   
                   
                   
                   
                 while the test is 
               
               
                   
                   
                   
                   
                 running. 
               
               
                 Local 
                 LOCAL- 
                 LSC 
                 — 
                 Establish local 
               
               
                 Loopback 
                 LOOP 
                   
                   
                 loopback between 
               
               
                   
                   
                   
                   
                 SG 24 and MG 24 
               
               
                   
                   
                   
                   
                 (SG 24 sends 
               
               
                   
                   
                   
                   
                 the SUs) 
               
               
                 Remote 
                 REM-LOOP 
                 LSC 
                 IN 
                 Start Remote 
               
               
                 Loopback 
                   
                   
                   
                 Loopback which 
               
               
                   
                   
                   
                   
                 loops data at 
               
               
                   
                   
                   
                   
                 conventional 
               
               
                   
                   
                   
                   
                 MTPL2 between 
               
               
                   
                   
                   
                   
                 MG 22 and link 
               
               
                   
                   
                   
                   
                 14. 
               
               
                   
               
             
          
         
       
     
     OM-MG block  416  (of MG  22 ) and OM-SG  424  (of SG  24 ) provide general operational management support for VoIP switch  20  (FIG.  4 ). OM-MG  416  records OMs about the signals received by MG  22  and uploads these messages, when requested by OM-SG  424  via a OM-START message, to SG  24 . Once the OM signal units are uploaded to SG  24 , via an OM-DATA message generated by OM-MG  416 , the OM signal units are cleared. A summary of the messages, the signal source or signal destination of the message (i.e., an entry in the Signal Source column indicates that the entered block transmits the associated signal message to OM-MG block  416  while an entry in the Signal Destination column indicates that OM-MG block  416  transmits the associated signal message to the entered block) and general comments regarding the messages is found in Table 9. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 OM-MG Block 416 Message Table 
               
             
          
           
               
                   
                   
                 Signal 
                 Signal 
                   
               
               
                 Signal 
                 MSG ID 
                 Source 
                 Destination 
                 Comments 
               
               
                   
               
               
                 OM 
                 OM-START 
                 OM-SG 
                 — 
                 Start upload of OM 
               
               
                 Upload 
                   
                   
                   
                 data from OM-MG 
               
               
                 Start 
                   
                   
                   
                 that is to be 
               
               
                   
                   
                   
                   
                 transmitted to OM-SG 
               
               
                 OM Data 
                 OM-DATA 
                 — 
                 OM-SG 
                 Will return all OMs 
               
               
                   
                   
                   
                   
                 kept on the MG. 
               
               
                   
               
             
          
         
       
     
     As mentioned previously, a number of timers are used during the operation of VoIP switch  20  (FIG.  3 ). These timers are mostly resident on SG  24  with a few timers also existing on MG  22 . A list of the timers and their purpose is include in Table 10. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Timers on SG 24 
               
             
          
           
               
                   
                 Duration 
                   
               
               
                 Timer 
                 (Exemplary 
               
               
                 Name 
                 Only) 
                 Purpose 
               
               
                   
               
               
                 T 1   
                 13.1 sec.  
                 Started when LSC 428 enters the Aligned/Ready 
               
               
                   
                   
                 state waiting for a valid FISU/MSU from SSP 12. 
               
               
                 T 2   
                 11.6 sec.  
                 Started when IAC 430 enters the Not Aligned 
               
               
                   
                   
                 state and it is waiting for SIO, SIN or SIE 
               
               
                   
                   
                 from SSP 12. 
               
               
                 T 3   
                 11.6 sec.  
                 Started when IAC 430 enters the Aligned state 
               
               
                   
                   
                 waiting for SIE or SIN. 
               
               
                 P n   
                 2.4 sec. 
                 Started when IAC 430 enters proving period 
               
               
                   
                   
                 for normal alignment. 
               
               
                 P e   
                 0.7 sec. 
                 Started when IAC 430 enters proving 
               
               
                   
                   
                 period for emergency alignment. 
               
               
                 T 5   
                 0.2 sec. 
                 Started when CC 422 enters the Level 2 
               
               
                   
                   
                 Congestion state and specifies the interval 
               
               
                   
                   
                 SIBs are sent while in congestion. 
               
               
                 T 6   
                 3.1 sec. 
                 Started when SIB are received by TXC-SG. 
               
               
                   
                   
                 Link is taken down if remote congestion 
               
               
                   
                   
                 last too long. 
               
               
                 T 7   
                 0.6 sec. 
                 Used by TXC-SG to time acknowledgements 
               
               
                   
                   
                 from far-end. If no ACK is received in 
               
               
                   
                   
                 a period of T 7 , link is taken down (excessive 
               
               
                   
                   
                 delay of acknowledgement). 
               
               
                 T SUERM   
                 1.0 sec. 
                 This timer is not used presently. 
               
               
                 T RTV   
                 0.2 sec. 
                 Implementation dependent timer started when 
               
               
                   
                   
                 messages are still awaiting confirmation 
               
               
                   
                   
                 from driver. 
               
               
                 T ACK   
                 1.0 sec. 
                 Timer started indicating how long to wait for 
               
               
                   
                   
                 an ACK (used by both the SG 24 and MG 22). 
               
               
                   
                   
                 If an ACK is not received in T ACK , the 
               
               
                   
                   
                 message is retransmitted. Two timeouts in a 
               
               
                   
                   
                 row will cause the SG 24 or MG 22 to take 
               
               
                   
                   
                 the link down. 
               
               
                 T SANITY   
                   1 sec. 
                 Timer used at RC-MG 406 to periodically send 
               
               
                   
                   
                 the LSSU or FISU received by RC-SG 420 
               
               
                   
                   
                 when no new messages are being received 
               
               
                   
                   
                 from the link. This timer is also used at RC-SG 
               
               
                   
                   
                 420 to monitor that RC-MG 406 is periodically 
               
               
                   
                   
                 sending SUs. Failure to receive SUs from 
               
               
                   
                   
                 the MG 22 during two T SANITY  periods will 
               
               
                   
                   
                 cause the SG 24 to take down the link. This 
               
               
                   
                   
                 timer is also used at TXC-SG 426 to 
               
               
                   
                   
                 periodically send the proper LSSU or FISU to 
               
               
                   
                   
                 the TXC-MG 408 when no new SUs need to 
               
               
                   
                   
                 be transmitted. This timer is also used by 
               
               
                   
                   
                 TXC-MG 408 to verify that it is receiving SUs 
               
               
                   
                   
                 from TXC-SG 426. Failure to receive an SU 
               
               
                   
                   
                 from the SG during two consecutive T SANITY   
               
               
                   
                   
                 periods will cause MG 22 to take down the 
               
               
                   
                   
                 link. This implies that when the link is idle, 
               
               
                   
                   
                 the SG 24 and MG 22 will still exchange a 
               
               
                   
                   
                 message every T SANITY  seconds. 
               
               
                   
               
             
          
         
       
     
     In an alternative embodiment communications links  28  (FIG. 3) and network  30  (FIG. 4) over which communication between MGs  22 , SG  24  and MGC  26  takes place could, instead of an IP network, be another type of routed or switched packet network. 
     While the MGs  22  and SG  24  of exemplary embodiment disclosed separate the functions of UL-MTPL2 block  34  and LL-MTPL2 block  32  as existing on separately on SG  24  and MG  22 , respectively, an alternative embodiment falling within the scope of the present invention combines the functions of UL-MTPL2 block  34  and LL-MTPL2  32  on a single component of VoIP switch  20 , such as MGs  22 . Accordingly, the same functionality, including a fully associated network, of the exemplary embodiment would be maintained but without requiring any SGs  24 . 
     While one (or more) embodiment(s) of this invention has been illustrated in the accompanying drawings and described above, it will be evident to those skilled in the art that changes and modifications may be made therein without departing from the essence of this invention. All such modifications or variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.