Method and apparatus for multiplexing sub-rate channels in a digital data communication system

Abstract: 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), in a system for performing rate adaptation for synchronized sub-rate channels in a manner geared to package sub-rate data in fragment size envelopes for integer multiples thereof), the width of a fragment is matched (ideally equal) to the granularity of the system's switch. This improves the efficiency of the rate adaptation process and maximizes the use of the system's switching and bandwidth capacity. In a system having m slots of bandwidth per frame, n fragments per slot and a switch granularity of x Kbps, x is less than 64 and each fragment is x Kbps wide.

Description: BACKGROUND OF THE INVENTION This invention pertains to digital data communication systems, and more particularly relates to methods and apparatus for multiplexing sub-rate channels in a manner that takes advantage of the switching system's switching granularity, that is, the smallest channel size that can be switched through the system. 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"; and 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. 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 are 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 sub-rate 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-referenced U.S. applications, it would be desirable to inprove the efficiency of bandwidth usage over the I.463 standard for sub-rate channel bandwidth allocation. It is also desirable to be able to fully use the capabilities of a switch, more particularly those switches taught in the above-referenced applications. Since the inventions taught in these applications make it possible to switch fragments of less than 64 Kbps, we have appreciated that it would be desirable if sub-rate channel assignments were keyed to the granularity of the switch which in turn is ideally matched to the choice of fragment size. For the illustrative example set out above, where the fragment size (8 Kbps) is matched to the switching granularity, an improved I.463 allocation scheme would ideally assign sub-rate channels to fragments (or integer multiples of fragments), thereby taking maximum advantage of the system's switch capacity without wasting slot 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, methods and apparatus are set forth which inprove 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). Furthermore, methods and apparatus are set forth which are capable of performing rate adaptation for synchronized sub-rate channels in a manner geared to package sub-rate data in fragment size envelopes (or integer multiples thereof), where the width of a fragment is matched (ideally equal) to the granularity of the system's switch. The preferred embodiment of the invention is taught in the illustrative context of a switching system having the aforementioned 32 slots per frame, for each IML, where each slot is 64 Kbps wide, where 8 fragments are defined per slot, each fragment being 8 Kbps wide, and where the switch granularity is matched to the fragment width, i.e., is also 8 Kbps wide. The invention can be generalized for a switching system having m slots of bandwidth per frame, n fragments per slot and a switch granularity of x Kbps, where x is less than 64 and each fragment is x Kbps wide. Further yet, (in the illustrative context), the efficiency of bandwidth usage over the I.463 standard can be achieved by implementing a new rate adaptation scheme in which sub-rate data channels are assigned to and 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. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described by way of example with reference to the 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-referenced 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 simply 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 preferred rate adaption scheme embodying the present invention. 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 synchronized, 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. At initialization, a program sequencer can be easily set up to map sub-rate 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 immedilately hereinafter with reference to Figure 6. Figure 6 depicts a typical user interface, such as UI 112 of Figure 2. The I.463 support facility for a bank of CPE (e.g. CPE 1-N) is shown in Figure 6 as block 601. Multiple I.463 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 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, an 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 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 clock (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 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 multiplexing a sub-rate channel, 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 sub-rate channel to means capable of performing single step rate adaptation, wherein said means for performing rate adaptation is operative to insert data bits from said channel into a "t" bit frame in accordance with said protocol; (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 information 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 storing in said sequencer a map indicating the proper place to insert sub-rate channel data bits in a frame, in accordance with said protocol. A method as set forth in claim 1 or 2 wherein said predefined protocol is the CCITT I.463 protocol. A method as set forth in claim 1, 2 or 3 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 taking maximum advantage of switching granularity. A method as set forth in any preceding claim further comprising the steps of: (a) receiving a synchronous 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 information bits include framing bits. Apparatus for multiplexing 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 envelopes that are integer multiples of x bits per second. Apparatus according to claim 8, 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; and Apparatus as set forth in claim 9 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 interleaving 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 10 wherein said means for interleaving is a program sequencer. Apparatus as set forth in claim 10 or 11 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 10, 11 or 12 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 predetermined demultiplexing protocol; and (b) means, coupled to said frame detector means, for de-interleaving sub-rate channel data from predetermined bit positions in an input data stream, for insertion into a sub-rate channel.

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