a profile for wide area x.25 operating at 2 mbps

Computer Networks and ISDN Systems 21 (1991) 307-314 307 North-Holland

A profile for wide area X.25 operating at 2 Mbps

Bernard Sales Brussels Universities, Helios-B group, Bld. du Triomphe CP 230, B-1050 Brussels, Belgium

Abstract

Sales, B., A profile for wide area X.25 operating at 2 Mbps, Computer Networks and ISDN Systems 21 (1991) 307-314.

We present some elementary performance results indicating that LAPB and X.25/packet layer protocol should be to be adapted in order to provide a 2 Mbps wide area data transmission service. This implies changes in the LAPB retransmission procedures and to window size of both link level and packet level to avoid the "window starvation" problem. For the link level retransmission, we propose to use standardised solutions (i.e. Selective retransrnission and Multi link); we do not investigate "another one" new data link layer protocol. We present possible ways to ensure that the X.25 protocol family will participate in the provision of 2 Mbps service.

Keywords. High speed networks, X.25 packet layer protocol, LAPB, protocol performance, OSI, satellite transmission, digital transmission.

1. Introduction

The X.25 Recommendation from the CCITT describes the interfacing of Data Terminal Equipment (DTE) and Data Circuit terminating Equipment (DCE) to access a Packet Switched Public Data Network (PSPDN). This recommendation specifies the procedures to be used at the physical interface (X.21/X.21 bis interface), at the link level interface (LAPB protocol) and at the packet layer interface (X.25 level 3). The X.25 Recommendation is mainly concerned with aspects of the DCE operation while aspects concerning DTEs are detailed in ISO standards. ISO 7776 describes the LAPB operation for a DTE and ISO 8208 describes the operation of the Packet Layer Protocol (X.25/PLP). However, the scope of these ISO standards is expanded beyond that of just interfacing a DTE and a PSPDN in order to cover also their operation over private networks.

X.25/PLP includes the operation of both a D T E / D C E and a D T E / D T E mode. For instance, a D T E / D C E mode is used to access a PSPDN but also a packet handler integrated within ISDN. The D T E / D T E mode of X.25/PLP can be used to provide a packet operation over LANs [1], the ISDN providing a bearer mode, or over any other

environments (e.g., leased line, satellite channel . . . . ). X.25 supports a large variety of applica- tions (telematic services, OSI services, PAD traffic) and it is therefore widely used over various environments. For the time being, X.25 Recom- mendations and its derivatives are limited to throughput up to 64 kbps. There is an important market demand (especially from the Research and Development community) for 2 Mbps packet services. Additionally, a number of telecommunication operators provides 2 Mbps point to point digital circuits based on G.700 series recom- mendations from the CCITT. Some other organi- sations envisage providing in the near future a 2 Mbps service over PSPDN (the switching technology is based on the frame relay). Data communication over satellite channels offers inter-continen- tal services at 2 Mbps.

In this context, we propose in this paper to investigate the impacts of high speed data transmission services in the order of 2 Mbps on packet level and link level protocol operations and design. This paper covers only the aspects related to the protocols. The design of systems and the aspects concerning resource allocation are beyond the scope of this paper. In the remainder of this paper, we assume that the reader is familiar with

0169-7552/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

308 B. Sales / Wide area X.25 operating at 2 Mbps

the terminology and the technical background de- scribed in [2-4] and additional standards covering the H D L C operation.

2 . P r o t o c o l p e r f o r m a n c e i s s u e s

LAPB is a subset of the H D L C protocol known as the Balanced Asynchronous Class ( H D L C BAC) (the BAC identifies the use of I, RR, R N R frames and some U-frames) with option 2 (use of REJ) and option 8 (I frames are command frames only). The frame numbering is performed modulo 8. Optionally, option 10 may be supported allowing the extension of the modulo to 128. Basically, H D L C applies a Go Back N retransmission: when an 1-frame is negatively acknowledged by a RE- JECT or when a time out condition is detected, all the unacknowledged I-frames are re-transmitted. In addition, H D L C may provide a Selective Re- transmission mechanism (known as option 3.2): when a SREJ is received, only the I frames listed in this SRFA frame need to be retransmitted. However, as LAPB does not include option 3.2, it operates as a pure Go Back N protocol.

The problem is to determine if LAPB can operate as it is or if it has to be improved in order to ensure data transmission at 2 Mbps.

All the aspects concerning the data transfer phase of both LAPB and X .25 /PLP are to be evaluated, including:

the study of the LAPB retransmission procedures in order to determine their ability to support 2 Mbps service and, possibly, study of substituted mechanisms,

- the determination of the length of an I-frame and of the timer T1 value to be used at the link level, and

- the determination if the frame numbering modulo 8 or modulo 128 as used by LAPB and by X .25 /PLP is enough or if it must be extended above 128. As we wish to estimate the maximum efficiency

of the retransmission procedures, we assume that the transmitter operates following a saturated scheme (i.e. the transmitter is always sending an I-frame, the time between two successive transmission is neglected). We assume that station A is connected to a station B by a physical connection offering the following characteristics:

/bit is the propagation delay for a bit information from station A to station B; the propagation delay from B to A is assumed to be the same as from A to B; the round trip delay of a bit information /rd a c ro s s a physical connection is defined as 2/bit;

- E r is the error rate of the physical connection (this error rate is considered to be constant);

- d and c are, respectively, the length of the information field and of the control field of an I-frame; all the I-frames are assumed to have the same length; and

- D is the throughput in b i t s / s supported by this physical connection. As a result,

- the time tf to transmit an I-frame is given by (d -t- c)/D. As If is small in regard to trd, we can consider that trd is a multiple of tf; and

- the probabili ty p that an I-f lame has been transmitted in error is given by 1 - (1 - E r ) (d+c)

Bernard Sales is Licenci6 en Informatique from the Universit~ Libre de Bruxelles in 1985. He works as Assistant/Researcher at the Laboratoire d'Informatique Thtorique and in the research group Helios-B at the same university. He is a member of RARE WG 4. He participates in the work of EWOS EG LL. In this group, he is acting as Editor of the EWOS Technical Guide of Lower Layer Relays. He is a member of ISO/SC6 WG1, WG2 and WG3 as a Belgian expert. His current research activities include lower layer protocol design and performance, interconnection of heterogeneous environments and formal description techniques.

B. Sales / Wide area )(.25 operating at 2 Mbps 309

2.1. Go Back N efficiency

First, we consider an idealised Go Back N protocol where if an I-frame has been transmitted in error, the transmitter re-sends this frame trd

time later. The window size and the numbering scheme are considered to be infinite. Let t be the time to transmit correctly an I-frame, P[t = tf +

k(trd + tf)] is given by pk(1 - p ) (k is the number of retransmissions of an I-frame). Thus, the average transmission time of an I-frame is

/trans = (1 - - p ) t f q- ~ p i (] _ _ p ) ( / f _]_ i ( t r d + t f ) ) i = l

As p < 1, ttran s may be written:

p ( t r d + t f ) ft . . . . = t f + (1 - -p ) (2)

The efficiency of the protocol is given by

/fd d 1 t t . . . . = d + c l + ( t r d + t f ) p / t f ( 1 - - p ) (3)

checkpoint cycle" to know which is the N(S) of the next I flame to be sent.

Studies have shown that, for a Go Back N protocol, the optimal value for d is in the range of 128 to 500 octets [5]. Moreover, as concatenation of TPDU DT in one X.25 DATA packet and the concatenation of X.25 DATA packets in one I- frame are not possible, the frame size is actually fixed both by the Transport and by the Network layer. As 128 octets is the default value of the TPDU size when the TPDU size is not negotiated during the Transport layer connection establish- ment phase and as 128 octets is also the default value for X.25 data packets, d = 128 (we neglect the octets for the DATA packet control field) for the link level seems the more realistic size for the I-flames.

Table 1 gives the efficiency of the idealised Go Back N scheme as a function of the bit error rate for some typical values of Ld in s with a data rate of 2 Mbps (d = 128 octets). This table shows that two cases exist: the first one for which the Go Back N scheme can be used without decreasing the global efficiency and the second one for which this strategy is inefficient.

w h e r e tfd is the d i D . Expression (3) is the efficiency maximum for

any protocol using a Go Back N strategy when the physical layer provides an error rate of Er. The main difference between this ideal Go Back N and LAPB is that LAPB uses a finite sequence numbering scheme and that the maximum outstanding I-frames is bounded by the value of the k parameter. Another difference is that a transmitter implementing LAPB detects that an I-frame has been transmitted in error when it receives a REJ frame or when the timer T1 is expired. In this last case, the transmitter is obliged to initiate a " P / F

2.2. Selective retransmission efficiency

Here, we make the same assumptions as for the Go Back N case: an I-frame transmitted with an error across the Physical connection is re-transmitted after a delay of t rd. What is important here is the number i of transmissions of the same I-frame, P[i = k] -- p k - 1(1 _ p). Thus, the average number of transmissions of an I-frame is given by:

N = ( J + 1)pJ(1 - P ) = (1 - p ) (4) j = 0

Table 1

10 -8 10 -7 10-6 10-5 10-4 10 -3

0.035 0.9564 0.9497 0.8929 0.5777 0.1122 0.0073 0.060 0.9553 0.9450 0.8531 0.4313 0.0692 0.0043 0.120 0.9542 0.9340 0.7706 0.2793 0.0361 0.0021 0.200 0.9527 0.9196 0.6825 0.1900 0.0220 0.0013 0.300 0.9508 0.9022 0.5972 0.1357 0.0148 0.0008 0.700 0.9433 0.8389 0.39112 0.0633 0.0064 0.0003


Table 2

10 -8 10 -7 10 -6 10 -5 10 -4 10 -3

0.9565 0.9564 0.9554 0.9460 0.8566 0.3172

case) is 0.70 rather than 0.006 in the simple Go Back N scheme.

2.4. Throughput analysis

As N" tf is the average time to transmit correctly an I-frame, the maximum efficiency of the selective retransmission scheme is

tro _ d ( 1 - E r ) d+" (5) N . t f d + c

This scheme is not dependent on the number of outstanding frames (i.e., tro/tf, eqn. (5) corre- sponds to eqn. (3) where trd/t f = 0, [5]). Table 2 shows the efficiency of the selective retransmission scheme (d = 128 octets). Table 2 indicates that this error recovery mechanism is con- venient for an error rate value up to 10 - 4 bits (the efficiency is not dependent on tr0 ) • d = 128 octets is one of the values offering better efficiency for the selective retransmission scheme [5].

The throughput of a link level protocol is the ratio between d and the time during which the transmission of an 1-frame monopolises the physical connection. Thus, the average throughput of the Go Back N protocol and of the Selective Retransmission are, respectively, given by (8) and (9)

d d D t t . . . . - - d + c 1 + (trd + t f ) p / t f ( 1 --p) (8)

d = ~ c D ( 1 _ Er)a+c (9) Ntf

The throughput is closely related to the efficiency of the protocol (see (3) and (5)).

2.5. Time delay analysis

2.3. Multilink procedure

This technique is based on the following ob- servation: as far as the design of the Data Link layer protocols is concerned, the only parameter in eqn. (3) we can control is (trd + t f ) / t f (i.e., the number of 1-frames needed to be retransmitted when an I-frame has been transmitted in error). (Note that it is exactly what we have done with the selective retransmission case for which this number is reduced to one.)

We can consider that the transmitter and the receiver multiplex n links over a single 2 Mbps channel (this solution has been proposed in the satellite case in [6]). On each of these links, the data retransmission mechanism could be achieved by a Go Back N procedure where the number of outstanding I frames is O i such as

n tr d Y~ Oi = ~ + 1 (6)

i=1

The efficiency of this procedure is in the order of:

1 d ~-~ 1 n d + c "~ l + O i p / ( 1 - p ) (7)

i=1

For instance, if we structure a 2 Mbps channel in 31 × 64 kbps channels, the efficiency of the multi link scheme for an error rate of 10 -5 (satellite

A time delay analysis is important for at least two reasons: firstly, the transit delay of an 1-frame from a station A to a station B is a crucial parameter for the application. Secondly, we have to take into account the transit delay provided by the frame level for the determination of the window size at the the packet level.

For the idealised Go Back N scheme, the average time between the submission by the packet layer of the first bit of an I-frame at a station A and the reception of the last bit of this I-frame to the packet layer at a station B depends only on the time required to transmit all the unacknowledged I-frames, on trd and on tf. If we consider that (frO + t f ) / t f is the number of the unacknowledged I-frames, we may write:

/rd /9 ( t2 d "[- 3 / rd q- 2tf ) tg°bN-~ tin + tf + " ~ q- ( 1 -p)

+ too ~ (10)

tin and tou t , respectively, are the time between the submission of the data by the packet entity to the link level and its first transmission on the physical connection and the time between the reception of this frame and its delivery to the receiving packet level entity.

For the selective retransmission mechanisms, the time between the first transmission of the

B. Sales / Wide area X.25 operating at 2 Mbps 311

Table 3

10-8 10-7 10 -6 10-5 10 -4

0.035 0.018 0.018 0.020 0.043 0.281 0.060 0.030 0.031 0.037 0.103 0.798 0.120 0.060 0.0063 0.089 0.351 3.119 0.200 0.101 0.0108 0.189 0.907 8.582 0.300 0.152 0.168 0.330 1.963 19.21 0.700 0.360 0.448 1.31 10.21 104.0

I-frame and its reception by the remote link entity is given by ( N - 1 ) ' t r d + Lo/2 and is therefore minimal. However, tou t includes a re-ordering procedure of 1-frames delaying the submission of the I-frames to the packet layer.

Table 3 gives the values for the average transit delay of an I-frame for the ideal Go Back N. The selective retransmission scheme gives acceptable values up to an error rate of 10 -5. At 10 -4, the transit delay increases considerably for trd > 0.150 [71.

For the multi link procedure, a minimal protocol is required to re-sequence the I-frames received from the parallel channels. The global performance of the multi link is between that of the Go Back N and that of the selective retransmission scheme.

2.6. Window size starvation problem

Both the LAPB and X.25 packet level protocol use a sliding window mechanism to allow the receiver to allocate resources for the support of this link. However, to ensure maximum use of the 2 Mbps channel, the window size used by the (N)-protocol should be sufficient to guarantee a continued pipelining of the ( N - 1) connection, i.e.

W( N ) > (time tO process the ( N ) - PDUs

+2 × ( N - 1) Transit delay}

/ ( [ ( N ) - PD U length]

/ ( U - 1) Throughput} (11)

Table 4 indicates for various values of t rd, the minimum window size for D = 9.6 kbps and for D = 2 Mbps.

If we used the default for W and d as proposed by the X.25 documentation (i.e. W = 2, d = 128), for a tra of 0.050 s, a 9.6 kbps physical channel is used at its maximum, while for a trd of 0.100 S, a 2 Mbps channel could be idle during 99%o of the time. For physical layer round trip time values up to 0.050 s (d = 128), a frame level window of 8 or of 128 is not enough. The problem is more pro- nounced because the frame size is not fixed by the link level but by the packet layer. It is, for instance, possible that the packet layer acknowl- edges the DATA packets with an RR packet in such a way that the number of I-frames carrying an RR packet increases and therefore accelerates the closing of the LAPB window. As the remote packet can send a maximum of (trd + t f ) / t f , in the worst case, WLAPB should be greater than:

trd ( trd ) tsp 2-~-r + 1 - ~-f + 1 ~ (12)

where tsp is the time required to transmit an I-frame containing an RR packet [7]. (Note that we can avoid this problem if concatenation of "supervisory" packets and a single DATA packet is done but it cannot be, according to X.25 recom- mendations.)

Another way consists in increasing the d value. However, for performance reasons, the frame length can only be increased up to 1000 octets. This can reduce the window starvation problem for 2 Mbps operation. However, this just post- pones the problem because if speed services higher than 2 Mbps are required (e.g 10 Mpbs), we could be faced again with the "window starvation problem".

For the packet layer, the problem is also critical because the window size should be sufficient to take into account the transit delay provided by the link level.

Table 4

0.035 0.060 0.120 0.200 0.300 0.700

9.6 kbps 2 2 3 3 4 8 2 Mbps 65 109 219 364 545 1270


3. Impacts on LAPB and X.25 /PLP

We have shown that for various trip delay values, the actual data transfer procedures do not operate satisfactorily. We could object that X.25 is an access protocol, and therefore, we could add that t~d is small and that X.25 uses a local window between a DTE and its local DCE.

However, in Section 2, we have explicitly ex- cluded the processing time from our analysis. This should be taken into account for the window calculation and for the efficiency of retransmission mechanisms when we design real systems. Moreover, as X.25 is the only connection-mode network protocol for OSI, it can be used not only to access a packet network but also for point to point operation over a bearer network ( D T E / D T E mode). In addition, for the D T E / D C E mode, the D bit facility has end to end significance, if it is used, the end to end transit delay becomes signifi- cant. Finally, in any case, the packet has to suffer from the transit delay of the link level. A packet level window size works only in some limited cases (for a Go Back N case, see Table 3. Selective retransmission also gives values for which a window of 128 is not sufficient [7]).

3.1. Link level

3.1.1. Single LAPB procedure over 2 Mbps channel (a) The discussion on T1 and T2 values as in

[8] is applicable. (b) A modulo 8 is insufficient in all the cases.

Modulo 128 works up to t rd = 0.70 (without taking into account the RR packet mentioned in (12); beyond this value, the numbering is to be extended. This can be done by defining a third modulo class for LAPB. A simple way consists in adding an octet on each N(S) and N(R) field as used for modulo 128; this extends the modulo to 32768.

Table 1 indicates that LAPB does not work satisfactorily for a physical layer error rate greater than 1 0 - 6 . However, conventional terrestrial digital transmission links exhibiting an error rate greater than 1 0 - 6 tend to provide a bursty error rate. As a result, the transmission line has only two error states, one where Er is greater or equal to 10 -7 and another one where when Er is 10 .3 [9].

The probabili ty that an I-frame has been transmitted in error is given by:

p , [ 1 - ( 1 - 1 0 - 7 ) a+']

+ ( 1 - P,)[1 - ( 1 - 10-3) d+C] (13)

where P, is the fraction of the time the channel provides a "good" error rate. Therefore, if P, < 0.999 for digital terrestrial transmission or in the satellite channel when Er > 6 x 10 -6, LAPB does not work satisfactorily to achieve a high throughput data transfer. A possibility consists in introducing the option 3.2 of H D L C in the LAPB specification.

3.1.2. Use of the Multi Link procedure The LAPB specification includes a Multi Link

option called MLP. This procedure encapsulates X.25 packets into Multi Link frames and distrib- utes them to single link entities for further transmission. Two corresponding single link entities operate the LAPB over a single physical channel. Multi Link frames always include a multi link control field containing flags and multi link sequence numbers with a modulo of 4096.

On the one hand, the disadvantage of the current MLP is that each single link uses a separate physical channel. This could be a source of con- cern when, for instance, the physical layer provides a single access to a 2 Mbps channel through a V.11 interface. A solution could be to enhance the MLP by adding a "Virtual Channel Number" (VCN) as the first octet of an MLP frame. With this option, it is possible to reduce as we want the O i values of (7) in such a way that the performance the MLP can tend to the one of the selective retransmission scheme.

On the other hand, the most important ad- vantages are the following:

(1) The LAPB specification for a single link case can remain unchanged. The MLP works up to t~d = 2.20 S. If necessary (e.g., if higher throughputs than 2 Mbps are required), the MLP numbering scheme can be increased to a larger value. For this purpose, we can add an octet to the MNH(S) field increasing the modulo to 1 Mega (~).

(2) The MLP is the only way to provide a 2 Mbps service to the packet layer when the physical configuration offers " low throughput" parallel connections instead of a single 2 Mbps access. As

B. Sales / Wide area )625 operating at 2 Mbps 313

a result, the same procedure is used in any cir- cumstance when a 2 Mbps packet service is required.

3.2. Impact on the packet layer

Again window size must extend beyond 128 because the packet level has to suffer from the transit delay implied by the link level.

We propose to define a modulo 32768 in addition to modulo 8 and 128. The following X.25 parameters are affected: - General Format Identifier: definition of a new

GFI value (e.g. 0011) identifying modulo 32768 - Extended Sequence numbering facilities: add the

codification of this new facility in the Extended Packet Sequence Numbering parameter (REG- ISTRATION packet)

- P(S) and P(R): addition of an octet to each P(S) and P(R) number carried in DATA, RR, RNR, REJECT packets

- Flow control parameters: modify the coding of each parameter conveying window value (Non Standard Default Window Size (REGISTRA- TION), Flow Control Negotiation Parameter - Window size (Call packets)). X.25 allows also to transport and to negotiate

Throughput values. In the present X.25 recommendation, the throughput is limited to 64 Kbps. We propose to modify the coding of the throughput class (Throughput Class Negotiation and Non Standard Default Throughput Class). A possible way is to use the pattern of bits 1111 to identify an "extended format" for the Throughput related parameters as indicated in Fig. 1.

Finally, the X.25 recommendation should also be revised to allow an appropriate default value for W when the packet layer supports high speed services.

xxxx and yyyy are values xxxx yyyy as defined in Rec X.25

7 3 0

1 1 1 1 N1 1111 N2

23 19 11 7 0

N1 and N2 are binary values corresponding to N x 64 Kbps

Fig. 1.

4 . C o n c l u s i o n

We have analysed the impact of 2 Mbps Wide Area service on the X.25 protocol stack operation presently defined in CCITT Blue Book recom- mendations and in ISO standards. This study demonstrates that the X.25 procedures must be adapted to achieve high throughput data transmission in the order of 2 Mbps. Both link level and packet level are affected. The link level has to take into account the round trip delay offered on the physical layer connections. For a physical layer round trip delay greater than 50 ms, the window size must be extended beyond 128. Another way to reduce the window starvation problem is to increase the I frame length. However, this is only a medium term solution because for services in the order of 10 Mbps, we will be faced again with the window starvation problem. For a terrestrial digital transmission which does not guarantee an error rate greater than 10 -7 during 99.9% of the time (round trip time greater than 20 ms), the retransmission procedures of LAPB based on the Go Back N scheme do not work satisfactorily. The same problem occurs for the satellite case when the error rate is greater than 6 x 10 -7. Two solutions have been analysed: use of option 3.2 of HDLC (SREJ) or use of the multi link procedure. It should be noted that the Selective Retransmis- sion mechanism offers in any case better performance in terms of throughput and transit delay. However, if the LAPB Multi Link Procedure is extended as proposed in this paper, the link level is modified in a minimal way. Use of the MLP serves two purposes: to provide a 2 Mbps service when only physical connections offering low throughput are available (e.g. N x 64 k configuration) and to increase performance by reducing the number of retransmitted 1-frames of the Go Back N scheme.

The "window starvation" problem also occurs at the packet layer. Some other parameters are also to be studied (e.g., throughput class codification). The study we have conducted concerns "ex- treme cases" for 2 Mbps operation (saturated model . . . . ). However, the solution we proposed is also valid for throughput values higher than 2 Mbps (e.g. 8 Mbps . . . . ). Our proposal corre- sponds to a long term approach and is valid for a wide range of underlying physical media (e.g. digital link, trans-oceanic cables, satellite chan-


nels . . . . ). Today, networks built on the f lame relay technology can offer switching rates of 2 Mbps or beyond. A Packet Switched Data Net- work offering 2 Mbps virtual calls today is not a dream and could be a reality in the near future.

References

[1] B. Sales, A study of protocols supporting the connection- oriented network service in a LAN/~/AN environment, in: Proc. ICCC'90, New Delhi, November 90.

[2] Data Communication Networks: Services and Facilities, Interfaces. Recommendation X.25- Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) for terminals operating in the packet mode and connected to public data network by dedicated circuit, Blue Book, Fasc. VIII.2, CCITT, Geneva 1989.

[3] ISO/IEC 8208 Information Technology--Data communi- cations--X.25 Packet Layer Protocol for Data Terminal Equipment, ISO/IEC, 1990.

[4] ISO 7776 Information Processing Systems--Data Com- munications-High-level data link control procedures: De- scription of the X.25 LAPB compatible DTE data link procedure, ISO, 1986.

[5] M. Schwartz, Telecommunication Networks, Protocols, Mod- elling and Analysis, (1987).

[6] G. Pujolle, D. Seret, D. Dromard and E. Horlait, R~seaux et T~lematique, Editions Eyrolles (in French), 1985.

[7] B. Sales, X.25/2 Mbps Wide Area services, Helios-B Inter- nal Report, IIHE, to be published.

[8] D. Wells, 10 Mbps X.25! Comput. Networks ISDN Systems 19 (3-5) (1990) 224-227.

[9] Load Increase Due to End-to-End Error Recovery for Large Window Size, contribution to ECMA TC 32/TG6, ECMA/TC32-TG6/87/122 (ECMA, Geneva, 1987).

a profile for wide area x.25 operating at 2 mbps

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