CA2012361C - Transmission system with wideband virtual channel - Google Patents

Abstract

A method is described for transmitting signals having a bit-rate higher than the bit rate of the base rate channels in a digital data transmission system normally providing discrete base rate data channels. The method comprises allocating a group of n+1 predetermined bit-rate data channels as a virtual channel for the transmission of the high bit-rate signals. One of the n+1 channels is designated as an overhead channel arid the remaining n channels are designated as data channels. The high bit-rate signals are divided into n sub-signals having a bit-rate equal to or less than the predetermined bit-rate, and transmitted over the n data channels. Delay calibration signals are transmitted at intervals over the overhead and data channels, the delay calibration signals being transmitted in the data channels in slots normally allocated for data, said data slots being transmitted over said overhead channel while the calibration signals are transmitted in their place. At the far end the delay calibration signals are used to reassemble the n subsignals into the original high bit-rate data signal.

Description

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This invention relates to an apparatus for transmitting data over a digital transmission system having discrete data channels with a predetermined base bit-rate lower than the bit-rate of the data to be transmitted. More particularly, the invention provides a method and apparatus for allocating a number of base rate channels to a virtual channel capable of carrying signals having a bit-rate higher than the base rate.
One data transmission system to which the invention is especially applicable is the Integrated Services Digital Network (ISDN) for which the standards have been defined by the International Telegraph and Telephone Consultative Committee (CCITT). ISDN makes use of a 2.048 megabyte per second primary rate TDM channel onto which are time-division-multiplexed 32 sub-channels (DSO). Each sub-channel has a base bit-rate of 64 Kb per second. The primary rate TDM
channel carries 8000 frames per second, each divided into 32 time slots carrying one byte (8 bits) of data from each sub-channel. The first time slot in each frame is used to identify the start of the frame, and another time slot, which constitutes the data channel, carries routing instructions.
The remaining 30 slots are available for carrying data normally as 30 discrete channels.
The primary rate TDM channel might, for example, be used to connect a private branch exchange (PBX) to the public telephone network. A single primary rate channel will therefore give the subscriber access to thirty 64 Kbps base rate channels.. Situations often arise, such as in the transmission of image and video signals, or high volumes of computer data, where it is desirable for the subscriber to transmit the data at.a rate higher than the base rate. For example, it would be desirable to have the capability of sending a 128 Kbps bit stream over two parallel 64 Kbps base rate channels. Unfortunately, because of the switching requirements and propagation characteristics of the public 2~D~.~3~1.
network a channel which occupies a particular time slot in any given frame of the transmitted signal does not necessarily occupy the same time slot at the far end. The base rate channels are subject to different delays through the network. As a result, if the channels are merely reassembled sequentially at the far end, the transmitted data is scrambled and unusable.
International Patent Application No. WO 85/04300 describes a system wherein prior to data transmission synchronization signals are transmitted along each of the base channels to determine the delays applicable to each channel. A reframer unit then takes into account these delays to reassemble the transmitted data in the correct order. The problem with this system is that once the virtual channel has been established it cannot be changed without being completely reset. Furthermore if the delays for the various channels change during transmission, the data becomes unusable. The system cannot therefore be regarded as reliable.
U.S. Patent No. 4,805,167 describes a system for providing a variable data rate aggregate channel. In this system marker signals are sent on the base rate channels to specify the order of transmission of the sub-signals to as to ensure correct reassembly at the far end. One problem with this system is that it can only be used for packet transmission since it requires there to be idle time slots in the data channels to carry the marker signals. It cannot therefore be used with continuous signals, such as video signals because there are no slots in which to insert the marker signals. Also, since the marker signals can only be inserted when idle time slots are available, it does not permit.the delay characteristics of the network to be continually monitored.

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An object of the invention is to provide an improved transmission system which does not depend on holes being available in the data stream and which permits continuous monitoring of the channel delays.
According to a first aspect of the invention there is provided a digital data transmission system normally providing discrete data channels having a predetermined bit-rate, said channels being subject to different propagation delay characteristics through the system, a method of transmitting. signals having a bit-rate higher than said predetermined bit-rate rate, comprising allocating a group of n+1 said predetermined bit-rate data channels as a virtual channel for the transmission of said high bit-rate signals, one of said n+1 channels being designated as an overhead channel and the remaining n channels being designated as data channels, dividing said high bit-rate signals into n sub-signals having a bit-rate equal to or less than said predetermined bit-rate, transmitting said n sub-signals over said n data channels, transmitting delay calibration signals at intervals over said overhead and data channels, the delay calibration signals being transmitted in said data channels in slots normally allocated for data, said data slots being transmitted over said overhead channel while said calibration signals are transmitted in their place, and using said delay calibration signals at the far end to reassemble said n subsignals into said original high bit-rate data signal.
Tn a preferred embodiment the delay calibration signals are transmitted successively over the respective channels in a rotational pattern. For example, in the case of a virtual channel consisting of four data channels arid one overhead channel, it would take successive frames to complete one rotation. In a first frame the delay calibration signals are transmitted in the time slot corresponding to the overhead channel. In a second frame the delay calibration byte is transmitted in the time slot corresponding to the 2os~3s1 first data channel and the data that would normally be sent in that time slot is instead sent in the time slot in the overhead channel. Likewise in the third frame, data from the second channel is swapped into the overhead channel time slot and the delay calibration signal sent in its place, and so on until after the delay calibration signals have been sent in the fourth data channel the next delay calibration signals are sent in the overhead channel, whereafter the cycle is repeated.
Each data channel (DSO) may be delayed differently by the network. The delay calibration signals, hereafter referred to as delay calibration byte (DCB); form a framing pattern in each channel which can be extracted at the receiver. The contents of the DCBs permit the relative delay to be determined. In the scheme just described every end byte on the far end channels constitutes the framing pattern.
The rotating calibration may be performed continuously, or used initially and then discontinued.
To determine the relative delay between channels the overhead byte sends out a rotation count (LSB). A
rotation starts with a DCB sent out on the overhead channel and ends when the last data channel in the virtual channel has sent its DCB. At the receiver this creates the appearance of every (N + 1)th byte on the channel forming a framing pattern.
To accommodate 48 Kbps data channels, only the six most significant data bits of each byte are used, the remaining bits being set to one to ensure correct ones density on Switch 56 networks. Six bits provide 64 unique bytes for the DCBs, but since the count wraps around only half the rotation count can be resolved. One byte rotation count allows differential delays of 32 frames to be detected.
Since the DCB would occur every 2N overhead slots the maximum delay resolution would be 2N*32 frames, which in practice is insufficient.
To overcome this problem, a second slot is used to send the most significant bits (MSB) of the rotation count, which results in a 12 bit delay calibration count. This permits 2048*2N frames to be resolved. In the worst case, where the channel size is three (N=3) this results in 12228 frames, which is equal to 1.528 seconds. This is sufficient for mixed terrestrial satellite networks.
The overhead slots are used for the LSB count, the MSB count and general purpose overhead information. The LSB
appear every second overhead slot. The slots not used for LSB alternate sending the MSB and an overhead byte. The MSB
follows even LSB counts and the OHB (overhead byte) slot follows odd LSB slots.
Another aspect of the invention provides in a digital data transmission system normally providing discrete data channels having a predetermined base bit-rate, said channels being subject to different propagation delay characteristics 2o through the system, an apparatus for transmitting signals having a bit-rate higher than said predetermined bit-rate rate, comprising means for allocating a group of n+1 said predetermined bit-rate data channels as a virtual channel for the transmission of said high bit-rate signals, one of said n+1 channels being designated as an overhead channel and the remaining n channels being designated as data channels, means for dividing said high bit-rate signals into n sub-signals having a bit-rate equal to or less than said predetermined bit-rate, means transmitting said n sub-signals over said n data channels, means for transmitting delay calibration signals at intervals' over said overhead and data channels, the delay calibration signals being transmitted in said data channels in slots normally allocated for data, said data slots being transmitted over said overhead channel while said calibration signals are i~ransmitted in their place, and means at the far end for using said delay calibration signals to reassemble said n subsignals into said original high bit-rate data signal.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:-Figure 1 is a :schematic diagram of a virtual channel operating in accordance with the invention;
Figure lb show=~ the scheme. for sending DCBs in the successive time slots of a single DSO data channel;
Figure 2 is a block diagram of a data transmission system incorporation a virtual channel in accordance with the invention;
Figure 3 is a flow chart illustrating the operation of a frame state unit;
Figure 4 is a diagram illustrating the consequences of a base channel slipping within a virtual channel;
Figure 5 is a simplified block diagram of a four-node transmission network:; and Figure 6 is a block diagram of two-node network.
The invention will be described with reference to an ISDN network having a primary rate TDM carrier of 2.048 Mb per second carrying thirty data channels and two system channels. As discussed above the primary rate channel carries 8000 frames per second, each base channel (DSO) having a base rate of 64 :Kb per second.

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Figure 1 shows a transmit section T and a receiver section R. It should be noted that while for convenience the two sections are designated as the transmitter T and receiver R respectively, the system operates in the full duplex mode.
The system is symmetrical and either section can serve as transmitter or receiver.
A 256 super-rate Kbps bit stream 1 is divided into four parallel bit streams 2, 3, 4, 5 each running at 64 Kbps (256 = 4 = 64). The four bit streams 2, 3, 4, 5 are applied to a Virtual Channel Resource (VCR) unit 6 in the transmit section T which outputs four base rate bit streams 7, 8, g, 10 serving as data channels and an additional channel 11, which normally serves a data channel, but which in the virtual channel serves as an overhead channel.
The five base rate channels are transmitted over five base rate (DSO) channels on the public telephone network to a Virtual Channel Resource (VCR) unit 12 in the receiver section R, which includes delay buffers 13 respectively receiving each of the incoming channels and a a reframer unit 14 for reassembling the incoming bytes in their proper sequence so as to reconstitute the four original bit streams as 2', 3~, 4~, 5~. Figure 3 is a flow diagram showing the operation of the receiver framing unit 14.
The reconstituted bit streams are then recombined into the original 256 Kbps super-rate bit stream.
The VCR transmit unit 6 inserts delay calibration bytes (DCB) 15 among the data bytes 16 according to the scheme shown. In frame 1, the first delay calibration byte (DCB) is inserted into the time slot corresponding to the overhead channel. Ln the next frame, the DCB is inserted in the time slot corresponding to the first data channel 7 and the contents of this time slot are transmitted in the time slot corresponding to the overhead channel. In frame 3 the contents of the time slot corresponding to the second data channel 8 are transmitted in the overhead channel, and the DCB is transmitted in this time slot. The scheme is repeated in the third and fourth data channels 9 and 10, whereafter in the sixth frame the DCB is transmitted in the overhead channel time slot, and the pattern is repeated on a rotational basis.
Figure 1b shows how the bytes constituting the DCBs (LSBs' MSBs, and OHBs) are sent successively in the DCB slots of a single DSO channel. The first LSB, representing the rotation count, is sent in the first DCB slot. There then follows three regular data slots followed by the MSB in the next DCB slot. The next DCB slots contain successively the LSB = 1 byte, the OHB, the LSB = 2 byte and the next MSB.
Figure 2 shows the hardware implementation of the virtual channel. Data Transmit Unit (DTU) 20 of the transmit section T receives the incoming super-rate bit stream at 256 Kbps and outputs four parallel 64 Kbps bit streams on ports 0, 1, 2, and 3. These bit streams are connected by switch 21 to the Virtual Channel Resource (VCR) transmit unit 6 in such a way that the lower circuit number on the data transmission unit 20 is connected to the lower circuit number at the input of the VCR transmit unit 6. The VCR transmit unit 6 outputs five channels (four data channels + one overhead channel) to switch 22, which is in turn connected to the line termination transmit unit 23. This is connected via a time division multiplex line 24 forming part of the public digital transmission network to the line termination receiver unit 25 of the receiver section R. The receiver unit 25 outputs five channels (including the overhead channel) to the switch unit 26, which is connected to the VCR receiver unit 12, which extracts the overhead information and outputs four data channels to the switch unit 27. The switch 27 connects the lower incoming circuit number from the virtual channel resource receiver unit 12 to the outgoing lowest circuit _ 8 _ 21~1236~.
number on the DTU 28, which in turn outputs the original super-rate 250 Kbps bits stream.
Although the invention has been described in connection with a 64 Kbs base rate network, in order to accommodate 48 Kbps data channels, which are used in some countries such as the United States, only the six most significant data bits of each byte are used according to the scheme described above.
A rotation starts with a DCB sent out on the overhead channel 11 and ends when the last data channel 10 in the virtual channel has sent its DCB, i.e. at frame 5 in Figure 1. At the receiver R this creates the appearance of every Nth channel forming a framing pattern.
The Overhead Bytes (OHB) are described below. As discussed above, these are sent in the DCBs alternating with the LSB rotation counts. The slot following rotation count LSB = 0 is used for rotation count MSB and may not be used for overhead information.
The Overhead Bytes are six bits to allow for 48 Kbps transfer. Overhead information is sent via a one or two byte pattern. The OHBs are described below. The unused bits are set to l to comply with Switch 56 ones density requirements. The overhead bytes consist of:
~ Status Words These are used to advise the sender of the status of an incoming virtual channel.
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OOabcdll Channel Status Word a = virtual channel synch (1=in synch) b = virtual channel state bit 1 c = virtual channel state bit 1 d = data synch (1=in synch) State Virtual Channel State 00 In Service 01 Calibrate and leave l0 Out of Frame ~ Channel ID Bytes These are used to indicate the channel/virtual channel numbers of the transmitted virtual channel (VC) to the receiver.
0lnnnnll nnnnn = 0 following byte is channel #LSB
1 following byte is channel #MSB
2 following byte is VC #LSB
3 following byte is VC ~MSB
4 channel mode = start up mode 5 channel mode = continuous mode 6 channel mode = transparent mode The following overhead byte codes may only be sent in the overhead channel (channel number 0):
lOnnnnll nnnn =0 overhead channel message start 1 overhead channel message end llabcdll ~ abcd => ABCD signalling bits In a given 100 ms interval each channel must send the OH
bytes listed below one or more times.
- l0 -~

channel status word ~ channel number ~ virtual channel number ~ channel mode overhead channels in message mode are exempt from this requirement.
In order to correct delay at the receiver the sequence in which the rotation count is sent and the assignment of channel number to circuit numbers must be known by the receiver. The channel rotation count is sent first on the overhead channel and then on the data channels in order of increasing channel number. The channel numbers are assigned to the sender data circuits in the following fashion, which is given by way of example with reference to a Newbridge 3645 Mainstreet system. The channels must be numbered in some fixed manner which is the same at each end:
~ in an ST-BUS the earlier timeslots get the lowest numbers ~ in MX streams simultaneous ST-BUS timeslots are ordered by PE slot number ~ in MX links the timeslots are ordered D1 < D2 < CBI < CB2 This protocol requires that data circuits from the data source (DTU) 20 to VCR transmit unit 6 be mapped in the same fashion as the circuits from the VCR receive unit 12 to data receiver unit 28. Switches 21, 22, 26, 27 bring this about, and this arrangement ensures that data interface ports are properly connected.
The circuits between the VCRs need not be consistently mapped. . The VCR receive unit 12 will determine the channel number assignments and send the data out in increasing timeslot order on the data circuits to the data receiver. This allows independence of the network (56 Kbs networks are known to jumble group circuits).

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In the continuous calibration mode, discussed below, the overhead channel (channel number 0) may be used to send messages from end to end. The messages are sent in the overhead DCB slots following an overhead channel message start byte pattern. The overhead channel on byte pattern is repeated 10 times before the message channel is active. Once the channel is active a message of not more than 128 bytes may be sent. The message is terminated by sending overhead channel message end byte (repeated 10 times). While the overhead channel is active no other messages may be sent on channel 0.
The overhead message packets are not defined. Any suitable packet based protocol can be used to send messages.
They are used for messages for:
~ Performance summary (slips, LSB count errors etc.) ~ Dynamic channel sizing In order to accommodate a variety of network types several modes exist. All of these modes assume that the virtual channel is full duplex. The channel mode is sent in overhead bytes in all channels. Examination of any one channel is sufficient for determining the mode. The mode should be debounced for 4 super-rotations.
1. Continuous Calibration Mode Data and DCB patterns are inserted at start-up and continued for the life of the channel. For transport of N
data channels N+1 circuits between VCRs must be allocated.
The overhead channel should be used to continuously send the following information:

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~ Channel Number (LSB & MSB) ~ Virtual Channel Number (LSB & MSB) ~ Data Channel Status ~ Continuous Mode Channel 2. Start-Ub Calibration Mode Only This mode exists to allow overhead-free transmission of super-rate data following initial set-up. In this case for N data circuits only N inter-VCR circuits are required. These channels are numbered from 1 to N. This mode starts with the transmission of the standard rotating DCB pattern. The overhead DCBs send (continuously):
~ Start-Up Mode Channel ~ Channel Number LSB
~ Channel Number MSB
'15 ~ Far End Channel Data Status ~ Far End Virtual Channel Status To simplify framing at the far-end all ones are sent in place of data. Once the receiving VCR has framed and extracted the necessary information from the overhead bytes, it sends a data synch = 1 in its Channel Status Word, and virtual channel synch = 1 and state = In Service in the Virtual Channel Status Word.
The sending VGR upon receipt of virtual channel synch sends a transfer to transparent signal in the data status word of channel 1. This signal consists of 8 repetitions of~the Data Status Word in successive OH bytes with mode set to Transparent. Once the signal is complete the sender then shifts to transparent mode.

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3. Transparent Mode This mode allows for direct super-rate virtual channel connection without continuous delay calibration.
This mode can be started immediately, or entered after a delay calibrating start-up sequence. While in the transparent mode the receiver continues to monitor for a framing pattern. If framing is detected and maintained for 500 ms on all of the channels forming a virtual channel then the receiver change modes to correspond to the mode type in the data status word of the incoming data.
Ideally the maximum reframe time for a single DSO
within a virtual channel should be 500ms, so the maximum time to re-calibrate an entire virtual channel would be 2.5 secs.
The maximum time to correct a frame slip on a DSO would be 100 ms. For large values of N, these targets may not be attainable.
Channel Operation In normal operation each channel sends data status and virtual channel overhead bytes. During start-up the channel watches for far end Virtual Channel synch. If synch is not received within 10 secs, a recovery signal is broadcast. If VC synch is received within 10 secs the channel declares In-service and continues monitoring or moves to the transparent mode.
In the event of a data channel out-of-frame the receiver sends an out-of-synch signal in the channel status byte and all other channels within the virtual channel send Virtual Channel Out-of-Synch (VCOS) in their virtual channel status.words. If VCOS is detected in the incoming streams and persists for 5 secs, the sender declares Far-End-Out-Of-Frame and enters the recovery mode. If the receiver cannot 2~01:3~~.
frame on the incoming stream within 5 sees, it declares Near-End-Out-Of-Frame and continues to maintain far end alignment.
Frame slips occur as a result of asynchronism between network elements. A frame slip will result in either the duplication of deletion of a frame of information within the transmission facility. If a single channel within a virtual channel slips then it will become permanently misaligned with the other members of the virtual channel as shown in Fig. 4. In the virtual channel information the data all starts within one synchronization domain. This leads to two slip scenarios:
~ phase error slips due to an asynchronous ~ network span ~ continuous slips due to different network synch ~ sources The phase error slip case results in a maximum delay adjustment of +/- 1 frame. In Fig 5 node 2 runs slightly slower than the network. Data from 1 -> 2 arrives too quickly and the receiver must periodically throw away a frame of data. At the 2 -> 3 interface data is arriving too slowly and a frame must be duplicated occasionally to correct. The net effect on the channel is that it will periodically slip in one direction and then eventually slip back. Eaah time the slip occurs the delay of the affected channel must be changed.
In the continuous slip case, slips occur in one direction only. The slips on links A & B will be out of phase. If an attempt is made to compensate by changing delay on the channel which.slips, then as slips accumulate so will the delay. Eventually the buffer space for delaying will be exhausted and it will be necessary to disrupt service to re-calibrate. One solution, provided a knowledge of network topology is available, is to add a delay to channel 1 when 2012361.
link 1 slips and remove the delay from channel 1 when link 2 slips, in which case the need to re-calibrate can be avoided.
A frame slip will misalign data on the virtual channel. The slips should be detected and corrected quickly.
At the receiver the framing pattern will slip, and the framer must cope with slips without declaring out-of-frame.
The receiver framing operation will now be described in more detail. At the base-rate channel (DSO) level the rotating DCB pattern appears as a DCB every N
frames. This allows each DSO to be considered as an independent channel which must frame on the incoming DCB
pattern. Delay equalization is then performed by reading the current DCB count and the byte offset from it from each DSO
in a known time-sequence. This allows the relative delay between the DSO to be calculated and the variable delay buffers changed. It is permissible to allow the DSOs to reframe as a consequence of delay adjustment provided that the performance objectives are met.
The start-up time and the recovery time from 2 0 protection switches depends on the time it takes the receiver to frame on each DSO in the virtual channel. The frequency of delay calibration bytes is a function of the virtual channel size. As the frequency of DCBs decreases reframe time increases.
The VCR state framer, for which the flow chart is shown in Fig. 3, handles slips without re-framing to allow very fast slip response time. In the start state the framer selects a byte from the data stream and moves to the Load First DCB state. N data bytes later the second DCB is loaded on the transition Load Second DCB state. If the two DCBs form a valid pattern (the second is one more than the first) then the Reverse Guard state is entered (Note: if the first DCB was 0 then the following DCB is a MSB and the one after 2~1236~.
that is an overhead byte, in this case a valid pattern cannot be declared until the next LSB rotation count is examined).
From Reverse Guard the pattern is checked until GUARD-COUNT
correct DCBs have been detected at which point an in-frame is declared and the Forward Guard state entered. A 2/4 DCB
error causes loss of frame and transition to Load Second DCB.
In addition to looking in the expected timeslots, the forward guard state examines the timeslot on either side. If a valid pattern is detected between the previous timeslot DCB and one on either side of the present DCB then a Watch for Slip state is entered. If GUARD-COUNT valid DCB bytes are observed in the +/- 1 timeslot then slip is declared, the timeslot for examining framing is moved and SLIP is declared. If 2/4 DCB
error in timeslot +/- 1 are encountered in one of the Wait For Slip states then the Load Second DCB state is entered.
The parameter GUARD-COUNT is a feature of the framer. It should be adjustable on a per Virtual Channel Resource level. The ability to adjust GUARD-COUNT on a per virtual channel basis is desirable, but not required. The frequency of DCB bytes rotation time) is N x 125 ~CSec. This time is calculated for various channel sizes in the table below:
N Rotation Time Super-Rotation Time 8 1 ms 256 ms 16 2 ms 512 ms 32 4 ms 1024 ms 64 8 ms 2048 ms 128 16 ms 4096 ms 256 32 ms 8192 ms The overhead frequency is 2 x Rotation Time.
The framer slip detection depends on the size of channel and the framer parameter GUARD-COUNT. Some times for various values of parameters are indicated below:

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G C Slin Response 8 4 10 ms 8 8 18 ms 32 4 20 ms 32 8 36 ms 256 2 192 ms 256 4 320 ms 256 8 576 ms These numbers assume GUARD-COUNT + 1 rotation count l0 LSBs are required for declaration of slip. Time to implement the delay adjustment is not included.
A main advantage of the system described is that the method is adaptive and will adjust for changes in delay during traffic transmission. The system also allows a high bit rate synchronous channel to detect and recover from changes in network (i.e. frame slips, protection switches) during the course of the connection and the signalling can be passed via ABCD bits in the overhead bytes.

Claims (9)

1. A method of transmitting super-rata data signals having a bit-rate higher than a predetermined bit-rate through a digital data transmission system normally providing channels having said predetermined bit-rate, said channels being subject to different propagation delay characteristics through said system, comprising:
allocating a group of k said channels (where k is an integer) in said data transmission system as a virtual channel for the transmission of said super-rate signals;
dividing said super-rate signals into n sub-signals, where n is an integer and n ~ k, having a bit-rate equal to or less than said predetermined bit-rate;
generating delay calibration signals for transmission through said data transmission system;
defining an overhead channel in at least one of said channels;
transmitting said n sub-signals over said channels;
transmitting said delay calibration signals in a rotational pattern over said channels in slots normally containing data signals, said delay calibration signals temporarily displacing the data signals normally occupying said slots;
transmitting said displaced data signals over said overhead channel in slots normally occupied by the delay calibration signals displacing thorn; and reassembling said n sub-signals to reconstitute said original super-rate data signals with the aid of said delay calibration signals after transmission through said channels.

2. A method as claimed in claim 1, wherein the delay calibration signals include rotation count bytes.

3. A method as claimed in claim 2, wherein said rotation count bytes are interspersed with multi-purpose overhead bytes (OHB's) which carry overhead data for the virtual channel.

4. A method as claimed in claim 3, wherein a predetermined number of the most significant bits of the rotation count bytes are not used io carry rotation count information, and additional bytes, referred to as (MSBs), which are interspersed with the rotation count bytes and multi-purpose overhead bytes (OHBs), are used to carry the information missing from the unused bit positions of the rotation count bytes.

5. A method as claimed in claim 4, wherein said virtual channel is established between a sender and a receiver and said multi-purpose overhead bytes comprise status words giving status information about the virtual channel to the sender, and channel id bytes indicating the channel numbers of the transmitted virtual channel to the receiver.

6. A method as claimed in claim 1, wherein k=n+1 and one of said channels is reserved exclusively as an overhead channel dedicated to carrying, in a rotational pattern, said delay calibration signals and said data signals displaced by said delay calibration signals.

7. In a digital data transmission system normally providing discrete channels inch having a predetermined bit-rate, said channels being subject to different propagation delay characteristics through said system, an apparatus for transmiting super-rate data signals having a bit-rate higher than said predetermined bit-raft through said channels comprising:
means for allocating a group of k said channels (where k 1s an integer) in said data transmission system as a virtual channel far the transmission of said super-rate signals;
means for dividing said super-rate signals into n subsignals, whore n is an integer and n ~ k, having a bit-rate equal to or less than said predetermined bit-rate;
means for generating delay calibration signals for transmission through said data transmission system;
means for defining an overhead channel in at least one of said channels;
means for transmitting said n sub-signals over said channels;
means for transmitting said delay calibrating signals is a rotational pattern over said channels in slots normally containing data signals, said delay calibration signals temporarily displacing the data signals normally occupying said slots;
means for transmitting said displaced data signals over said overhead channel is slots normally occupied by the delay calibration signals displacing them; and means for reassembling said n sub-signals to reconstitute said original super-rate data signals with the said of said delay calibration signals after transmission through said channels.

8. An apparatus as claimed in claim 7, wherein sending and receving virtual channel resource units (VCRs) are provided respectively at each end of said data transmission system, said sending unit including means for inserting said delay calibration signals into said sub-signals in said rotational pattern and transferring the displaced data signals to the overhead channel, and said receiving VCR unit including a reframer to reassemble the received signals in the same order as said super-rate signal with the aid of said delay calibration signals.

9. An apparatus as claimed in claim 8, wherein k=n+1 and one of said channels is reserved exclusively as an overhead channel dedicated to carrying, in a rotational pattern, said delay calibration signals and said data signals displaced by said delay calibration signals.