Methods and apparatus for performing time interleaved multiplexed rate adaptation for sub-rate channels in a digital data communication system

Abstract: The sub-rate channel carrying capacity and the efficiency of bandwidth usage are improved over the CCITT I.463 standard, for switching systems having a granularity, that is, the smallest channel size that can be switched through the system, of better than 64 Kbps (e.g. 8 Kbps). The system is capable of performing rate adaptation for multiple synchronized sub-rate channels in accordance with a predefined multiplexing protocol. The protocol calls for "time interleaved" multiplexed rate adaptation to be performed, i.e., multiple sub-rate channel data is packaged into fragment size envelopes (or integer multiples thereof). The width of a fragment is also matched (ideally equal) to the granularity of the system's switch. As a result of using the time interleaved multiplexed rate adaptation scheme, the rate adaptation process, the channel carrying capacity of the system at large, and the use of the system's switching and bandwidth capacity are all improved.

Description: BACKGROUND OF THE INVENTION This invention pertains to digital data communication systems and more particularly relates to method and apparatus for performing time interleaved multiplexed rate adaptation for sub-rate channels in order to improve the system's individual channel carrying capacity and thereby take advantage of both the system's switching granularity, that is, the smallest channel size that can be switched through the system, and available bandwidth. State of the art digital data communication switching systems are typified by the systems described in our United States patent application Serial Number 07/103,611, filed October 1, 1987, entitled "High Speed Communication Processing System"; in our United States patent application Serial Number 07/103,612, also filed October 1, 1987, entitled "A Digital Data Communications System", and published July 25, 1989, as United States Patent 4,852,089; and in WO-A-8 606 231 (Bell Communications Research). The system taught in US Patent 4,852,089 is defined as having m slots of data per frame and n fragments per slot to yield a total of m x n fragments per frame. For example, the invention taught in US Patent 4,852,089 permits a T1 line to be divided up into 24 slots, each 64 Kbps wide, with 8 fragments being defined per slot (each representing 8 Kbps bandwidth). This yields 192 fragments with an 8 Kbps signalling channel left on the T1 line. Also, bandwidth allocation is contemplated for 2.048 Mbps Inter Module Links ("IMLs") within a given node, where 32 slots each 64 Kbps wide can be defined (each again with eight 8 Kbps fragments). Also taught in the above-referenced applications are means for allocating and de-allocating bandwidth on the communication lines of the system using bit maps, how to perform allocation in a manner which minimizes call blocking, contention, etc., and how to interconnect Customer Premises Equipment (CPE) to the node oriented network via User Interfaces (UI), Network Processors (NPs), Switch Matrices (SMs), Network Interfaces (NIs), etc., via the IMLs in a given node. The systems taught in the referenced applications support the CCITT I.463 standard for multiplexing of subrate channels. Also, these systems have the capability of switching fragment vs. slot packets, i.e. have an improved switching granularity as compared with the 64 Kbps switch granularity to which the I.463 standard was designed. For the illustrative 32 slot, 8 fragments per slot IML bandwidth scheme set forth hereinbefore, I.463 mandates that subrate channels (defined herein as a 19.2 Kbps channel or any submultiple thereof) be assigned to 64 Kbps of bandwidth (a whole slot) before being transmitted to the switch. According to the I.463 standard, two layers of rate adaptation are performed to condition sub-rate channel data to be transmitted in the 64 Kbps packets. Given the improved switching granularity and bandwidth allocation capabilities of systems such as those set forth in the above-reference U.S. applications, our co-pending European patent publication number EP-A-0 382 362 filed herewith, claiming priority from U.S. patent application number 07/308,705 filed February 9, 1989 and entitled "Methods and Apparatus for Multiplexing Sub-Rate Channels in a Digital Data Communication System", is concerned to improve the efficiency of bandwidth usage over the I.463 standard for sub-rate channel bandwidth allocation. This copending application is hereinafter referred to as the "Improved I.463 Patent Application". The Improved I.463 Patent Application is also concerned with being able to fully use the switching capabilities (granularity) of the communication system. Since the inventions taught in the above-referenced U.S. patent applications made it possible to switch fragments of less than 64 Kbps, it was recognized that it would be desirable if sub-rate channel assignments were keyed to the granularity of the switch which in turn could be ideally matched to the choice of fragment size. In short, the Improved I.463 Patent Application, describes how to improve the efficiency of bandwidth usage over the CCITT I.463 standard, for switching systems having a granularity of better than 64 Kbps (e.g. 8 Kbps). The efficiency of bandwidth usage over the I.463 standard is achieved (in the context of the illustrative example set out above) by implementing a new rate adaptation scheme in which sub-rate data channels are assigned to an 8 Kbps fragment, or multiples thereof. For example, a 2.4 Kbps channel is assigned to an 8 Kbps envelope, 9.6 Kbps is assigned to two 8 Kbps envelopes, etc. Multiple envelopes (each 8 Kbps wide) are then multiplexed effecting a potential 8-fold improvement over existing methods and apparatus for implementing the I.463 standard. In the illustrative context, the key is to use the 8 Kbps granularity of the switch rather than the I.463 standard of assigning sub-rate data to a whole 64 Kbps slot. Although definite improvements in the utilization of bandwidth and switching system granularity are made in the invention disclosed in the Improved I.463 Patent Application, it would be desirable if the individual channel carrying capacity of the system described therein could be improved as well. Such an improvement would result in the ability to take even greater advantage of the communication system's switching granularity and available bandwidth. SUMMARY OF THE INVENTION The present invention is defined in the appended claims, to which reference should now be made. In a preferred embodiment of the invention, described in more detail below, the individual channel carrying capacity in a digital communication system is improved, and more particularly sub-rate channel rate and adaptation is performed in a way that improves available bandwidth usage when compared with the methods and apparatus taught in the Improved I.463 Patent Application. The use of fragment bandwidth per se is improved by allowing a plurality of sub-rate channels to be carried within a given fragment. This degree of freedom provides the potential for improving bandwidth usage substantially in the communication system as a whole. Also the switching system granularity is effectively improved by matching granularity and fragment bandwidth, while at the same time permitting multiple sub-rate channels to be embedded in a given fragment. The ability to embed a plurality of sub-rate channels in an envelope previously reserved for one sub-rate channel is referred to hereinafter as "time interleaved" rate adaptation. In accordance with the invention, methods and apparatus are set forth which improve upon prior rate adaptation schemes by supporting time interleaved rate adaptation. In the preferred embodiment of the invention a program sequencer is used to perform rate adaptation as specified by a predefined time interleaved multiplexing scheme. The protocol for the predefined scheme is used by the sequencer to insert data bits from at least one active sub-rate channel into predetermined bit positions in a "t" bit frame. In this manner a single level of rate adaptation can be used to support, for example, the insertion of three 2.4 Kbps channels into a single 8 Kbps channel. For this 2.4 Kbps example, a 24 fold improvement over the standard CCITT I.463 rate adaptation scheme, and a 3 fold improvement of the rate adaptation scheme taught in the Improved I.463 Patent Application, can be achieved. The opposite of the time interleaved multiplexed rate adaptation function may be performed, referred to hereinafter as "time de-interleaving", in order to detect and extract any and all sub-rate channel data that may be embedded in fragments received from the network. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of the invention will now be described in more detail, reference being made to the accompanying drawings, in which:- Figure 1 (prior art) is a high-level functional representation of a typical digital data communication network comprising nodes interconnected by communication lines; Figure 2 is a block diagram of one such node showing a variety of communication paths both within and outside of a given node; Figure 3 depicts the allocation of a communication line into "slots" and "fragments"; Figure 4 depicts a bit map organization that is suitable for use with the present invention; Figure 5 depicts, in block diagram form, (a) the prior art CCITT I.463 two level rate adaption scheme for inserting sub-rate channels into 64 Kbps slots compared with (b) the preferred rate adaptation scheme embodying the present invention; and Figure 6 depicts a functional block diagram for a user interface that supports the preferred rate adaption scheme embodying the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 depicts a typical network consisting of nodes 101, 102, 103 and 104 interconnected by communication lines. Some of the nodes are shown as having customer premises equipment (CPE) connected to them. Node 101 is shown connected to CPE 105, and node 104 is shown connected to CPE 106. In practice, any of the nodes may have CPE connected to them. Figure 2 shows internal detail of a typical node, comprising a wideband time-slot switch matrix (110, 120, 130) and a number of dual-ported network processors (140, 141) connected to each other via 2.048 Mbps serial links. These internal node paths are the inter-module links (IML) referred to hereinbefore. The switch matrix permits connectivity between the network processors, as well as connecting them to the network, via network interface (NI) means such as NI 111, and to local terminals, personal computers (PCs) and private branch exchanges (PBXs), labeled as CPE 115, via other similar serial links and devices such as user interface (UI) means 12. Rate adaptation as contemplated by the CCITT I.463 standard, takes place at the UI level of the network. The prior art process is described hereinafter with reference to Figure 5. Figure 2 also shows that each network processor is connected to a typical parallel computer bus 160. The network processors (NPs) of the system can either be connected together with a single such bus, or in groups to more than one such bus. This way the necessary computing resources, such as memory 170, can be connected to the switch processors in a manner traditional to the computer industry. The first network processor installed in the system is also programmed to control the switch matrix connections, such that when particular users in the network require access to a given network processor, that connection is carried out by the first network processor upon receiving a command from that specific user. On the other hand, asynchronous network processor to network processor interconnectivity is achieved over the computer bus, such as is required when two such network processor units are sharing a single communications processing function such as packet switching. For example, a packet received by network processor 140 which was destined to network processor 141, if sharing the same bus, simply hands off such packet to network processor 141 using either direct memory access (DMA) or a "mail-slot" technique. If a node that has been installed with N network processors is showing inadequate performance, or must then perform additional functionality, network processor N+1 is added. Furthermore, if the bus bandwidth is inadequate for all the functions being performed in a single-bus system, a second computer bus can be added to the system to separate into separate computing groups the functions that are not interrelated. Figure 3 depicts the allocation of bandwidth of a 2.048 Mbps IML communication line in a switching system assumed, for the sake of illustration only, to have an 8 Kbps switching granularity. The available bandwidth is shown divided into thirty-two 64 Kbps slots. (32 x 64 Kbps = 2.048 Mbps). Each 64 Kbps slot is further divided into eight "fragments" of 8 Kbps each. Not shown in Figure 3, is a bandwidth allocation scheme for other data path frequencies, for example, T1 lines which could have twenty-four 64 Kbps slots, each with eight 8 Kbps fragments and a framing/signalling channel of 8Kbps left over. In the illustrated embodiment of the invention, the ideal fragment bandwidth is the same as the maximum switching system granularity. Thus, for a switching system having a switching granularity of "x" bps, the ideal fragment width is also x bps. Dividing the fragment width into the frame bandwidth will indicate the number of fragments possible per frame. These can be grouped into slots such that each frame has an integral number of slots, m, where each slot has an integral number of fragments, n. As data begins flowing within a node (for the IML example) for transmission between devices within the node, a portion of the available bandwidth on a given link on which that data appears is inherently taken up by the data. Bandwidth allocation may be accomplished, as taught in the above-reference applications, using bit maps for each link connected within the node. The bit maps may be maintained by the network processor (NP) within a memory, such as memory 170. The bit map for each end of a link contains one bit position corresponding to each fragment of that link. Each bit position will contain a ONE to indicate that the corresponding fragment is in use and unavailable, or a ZERO to indicate that the corresponding fragment is available. The network processor can thus easily identify available fragments imply by searching through the bit map looking for ZERO bits. If more than one fragment is required, there is no necessity to assign contiguous fragments, as available fragments can be located anywhere in the link. By adopting the convention that fragments will be allocated for transactions originating from one end of a link, from one end of the bit map, and for transactions originating at the other end of the same link, from the other end of the bit map, there is no need to pre-allocate to either, enabling the system to be more efficient when a preponderance of transactions is originating from one side or the other. The same type of allocation scheme will enhance performance of the internode T1 lines as well. An example of a suitable bit map for a 32 slot per frame IML is delineated in Figure 4. This can easily be modified to support 24 slot T1 lines, etc. Turning to Figure 5, a comparison of the prior art rate adaptation scheme used to support I.463, is made against the rate adaption scheme for inserting at least one sub-rate channel into an 8 Kbps slot. Although well known to those skilled in the art, a brief review of the standard I.463 rate adaptation scheme will be explained for the sake of completeness. CCITT I.463 implies two layers of synchronous rate adaptation. As illustrated by Figure 5, sub-rate data, e.g. an asynchronous 2.4 Kbps signal, can be input via link 501 from CPE. The input data is shown synchronized at RAO in Figure 5. Methods for synchronizing sub-rate data are well known by those skilled in the art by, for example, decoding framing bits in synchronized framing bit patterns. Once sychronized, CCITT I.463 calls for the performance of two layer rate adaptation at RA1 and RA2 of Figure 5(a) (prior art). At the first layer, RA1, sub-rate data is mapped into envelopes that are integer multiples of 8 Kbps wide. For example, the 2.4 Kbps signal would be "stuffed" into an 8 Kbps envelope, a 19.2 Kbps signal would be stuffed into a 32 Kbps envelope, etc., per this well known protocol. The standard protocol goes on to call for a second level of rate adaptation where each set of envelopes corresponding to a given sub-rate channel signal, are in turn stuffed into a 64 Kbps slot for transmission to the switch matrix. This second level of rate adaptation is illustrated in Figure 5(a) (prior art) at RA2. The output to the switch at 505 has individual sub-rate channels embedded in, and taking up, entire 64 Kbps slots. Since the switch granularity of systems taught in the above-referenced patent applications is better than 64 Kbps (8Kbps for the illustrated example), we now contemplate only 1 layer of rate adaptation as shown in the lower portion of Figure 5, Figure 5(b). Thus sub-rate user inputs on link 511 can be output to the switch, via links 512, 513 and 514, via RA1 and MUX 520, without the second layer of rate adaptation called for in the I.463 standard. RA1 performs the same functions in both the prior art and new approach. RA2 can also be performed as an option for compatibility with other systems. Also shown in Figure 5(b), at 535, is the option of performing yet another layer of rate adaptation depending on choice of fragment size and switch granularity. The illustrative embodiment of the invention with 8 Kbps fragments (envelopes) and an 8 Kbps granularity switch, does not require a second level of rate adaptation. Before going on to the details of implementing the new rate adaptation scheme, described hereinafter with reference to Figure 6, it will be useful to visualize the I.463 protocol as mandating that sub-rate data in the range of 600 baud to 19.2 Kbps, be inserted into 8 Kbps fragments by means of an 80-bit frame (or envelope). Thus, for example, for a 2.4 Kbps signal, 24 data bits of sub-rate information need to be inserted into an 80 bit frame for the 8 Kbps operating rate. The other 56 bits of information in an 80 bit frame are filler bits. The illustrated embodiment contemplates the use of what will be referred to hereinafter as an "I.463 Plus" protocol. This protocol specifies that multiple sub-rate channels be inserted into a given fragment. Thus, for the sake of illustration only, three 2.4 Kbps channels can be inserted into one 8 Kbps fragment, i.e. 72 data bits of sub-rate data (from 3 separate channels) are inserted into the above defined 80 bit frame. An initialization, a program sequencer can be easily set up to map multiple sub-rate channel data bits into any desired location in the aforesaid frame. Also, such a sequencer can be used to reverse the process, i.e., extract sub-rate information. The ability to perform these functions using well known program sequencers and mapping techniques will add to the understanding of the illustrative embodiment of the invention to be described immediately hereinafter with reference to Figure 6. Figure 6 depicts a typical user interface, such as UI 112 of Figure 2. The I.463 Plus support facility for a bank of CPE (e.g. CPE 1-N) is shown in Figure 6 as block 601. Multiple I.463 Plus support facility can be located at a UI, as illustrated by block 602 which is intended to perform the same function as block 601. A frame generator, such as device 610 of Figure 6, is a device well known by those skilled in the art for generating framing information. To support the I.463 Plus protocol, frame generator 610 generates a frame clock (for outputting data) for the 80 bit frames referred to hereinbefore. Frame generator 610 is driven off of clock 605, and outputs not only the frame clock for data output (on link 680), but also control information and frame overhead data, for use by a program sequencer such as program sequencer 620. Program sequencer 620 is depicted in Figure 6 as having two portions, as interleave portion and a de-interleave portion. The program sequencer can be realized by a microprocessor that is initialized to direct sub-rate channel data from one or more such channels into prespecified bit positions in each 80-bit frame (for interleaving). The same sequencer can be used to extract sub-rate data in each frame for data destined to the CPE side of the network (via the de-interleaver). The de-interleave portion of program sequencer 620 is shown operating in conjunction with frame detector 640, a device that is also well known to those skilled in the art. Frame detector 640 determines frame boundaries and enables program sequencer 620 to appropriately remove sub-rate information from a given frame. Control and frame block (for input data) signals are depicted in Figure 6 as input to program sequencer 620, from frame detector 640. The combination of devices 610, 620 and 640 are all that is necessary to perform both the desired interleaving and de-interleaving in support of any desired protocol and particularly the I.463 protocol sought to be more efficiently supported. Figure 6 also indicates how the output of the sequencer, destined for the switch, can be multiplexed onto an internal UI bus, e.g., bus 650, via mux/demux device 675, in accordance with a time slot plan stored at device 680 which could be realized by RAM. UI bus 650 is shown taking output from (and also distributing data to) a plurality of banks of CPE, and interfacing with IMLs to the switch via concentrator 699. What has been described is a rate adaptation scheme that meets all of the objectives set forth hereinbefore, and in particular features increased individual channel carrying capacity, improved bandwidth usage, efficiency in bandwidth allocation, and is designed to take advantage of the speed and flexibility of the systems' switching capacity. Those skilled in the art will recognize that the foregoing description of a preferred embodiment of the novel methods and apparatus has been presented for the purposes of illustration and description only. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment and examples set forth herein were presented in order to best explain the principles of the instant invention and its practical application to thereby enable others skilled in the art to best utilize the instant invention in various embodiments and with various modifications as are suited to the particular use contemplated.

Claims: A method for performing time interleaved multiplexing of a plurality of active sub-rate channels, in accordance with a predefined multiplexing protocol, on a data path in a digital communication switching system that has a switching granularity of "x" bits per second, wherein said data path has a bandwidth divided into "m" slots, the slots each being further subdivided into "n" fragments, characterised by the steps of: (a) inputting said plurality of active sub-rate channels into means capable of performing single step rate adaptation, wherein said means for performing rate adaptation is operative to insert data bits from each of said channels into a "t" bit frame in accordance with said protocol, effectively multiplexing said channels in a given fragment up to the fragment's bandwidth; (b) creating said t bit frame utilizing frame generator means; (c) interleaving the sub-rate data into the proper position in said t bit frame as defined by said protocol; (d) inserting filler bits into the remaining bit positions of said t bit frame; and (e) outputting the t bit frame for transmission through the switching system. A method as set forth in claim 1 further comprising the step of initializing a program sequencer, coupled to said frame generator means, by loading into said sequencer a map indicating the proper place to insert multiple sub-rate channel data bits in a frame, in accordance with said protocol. A method as set forth in claim 1 or 2, wherein the value of t is directly related to the bandwidth of a fragment which in turn is directly related to the granularity, x bits per second, of the switching system to thereby permit sub-rate channel data to be switched to take maximum advantage of switching granularity. A method as set forth in any preceding claim further comprising the steps of: (a) receiving a asynchronous sub-rate channel data from a system user; and (b) synchronizing the data received prior to performing said step of inputting. A method as set forth in any preceding claim further comprising the step of optionally performing at least one further level of rate adaptation. A method as set forth in any preceding claim wherein said filler bits include framing bits. Apparatus for multiplexing a plurality of sub-rate channels, in accordance with a predefined multiplexing protocol, in a digital switching system having a plurality of system users operating at sub-rate frequencies and having a switching granularity of "x" bits per second, characterised by means for performing a single level of rate adaptation to insert synchronized sub-rate channel data into individual envelopes that each have x bits per second of bandwidth. Apparatus according to claim 7, further characterised by: (a) means for generating a clock signal; (b) means for receiving asynchronous sub-rate channel data from said system users; (c) means for synchronizing received asynchronous data in relation to said clock signal. Apparatus as set forth in claim 8 wherein said means for performing rate adaptation further comprises: (a) frame generator means, operative in response to said clock signal, for generating "t" bit frames in accordance with a predefined multiplexing protocol; and (b) means, coupled to said frame generator means, for time interleaving multiple sub-rate channel data into predetermined bit positions in said t bit time frame in accordance with said protocol. Apparatus as set forth in claim 9 wherein said means for interleaving is a program sequencer. Apparatus as set forth in claim 9 or 10 wherein said means for interleaving further comprises means for optionally performing at least one additional level of rate adaptation. Apparatus as set forth in claim 9, 10 or 11 wherein said means for performing rate adaptation further comprises: (a) frame detector means, operative in response to said clock signal, for identifying the boundary of said t bit frames in accordance with a predefined demultiplexing protocol; and (b) means, coupled to said frame detector means, for de-interleaving multiple sub-rate channel data from predetermined bit positions in said t bit frames.

Inventors: ALMOND GARY ROBERT, STOREY DAVID W, ALMOND, GARY ROBERT, STOREY, DAVID W.
Applicants: DATA GENERAL CORP, DATA GENERAL CORPORATION
Application year: 1990
Main classification: H04J3
IPC codes: H04Q11, H04J3