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Signalling system number 7 network services part and X.25: A comparative study
COM ISDN SYSTEMS ELSEVIER
Computer Networks and ISDN Systems26 (1994) 1495-1502
Signalling system number 7 network services part and X.25: A comparative study Roch H. Glitho Ericsson Communications Inc., Systems Engineering Department, 8400 Decarie Bh,d., Town of Mount Royal, Quebec H4P 2N2, Canada
Abstract
Despite the clear-cut differences in the stated aims, Signalling System Number 7 Network Services Part and X.25 are sometimes used interchangeably. We present in this paper a comparative study that shows that although many aspects of the two standards are quite similar, significant dissimilarities do exist. Caution is therefore required whenever the use of any of the two standards, in place of the other, is contemplated. Key words: Data link layer; Link layer; Message Transfer Part (MTP); Network layer; Network Services Part (NSP);
OSI Reference Model (RM); Packet layer; Signalling Connection Control Part (SCCP); Signalling System Number 7 (SS7); X.25
1. Introduction Signalling System Number 7 (SS7) and X.25 standards have been specified for very different purposes: - In the words of the CCITI" the overall objective of Signalling System No 7 is to provide an internationally standardized general purpose C o m m o n Channel Signalling (CCS) system: • Optimized for operations in digital telecommunications network in conjunction with stored program controlled exchanges; • that can meet present and future requirements of information transfer for inter-processors transactions within telecommunications networks for call control, remote control, and m a n a g e m e n t and maintenance signalling; • that provides a reliable means for transfer of information in correct sequence, and without loss or duplication [1].
SS7 is divided in two parts: the User Part (UP), and the Network Services Part (NSP). The NSP consists of the Message Transfer Part (MTP) and the Signalling Connection Control Part (SCCP), and is somewhat functionally equivalent to the first three layers of the O p e n System Interconnection Reference Model (OSI RM) [2]. For an overview see Ref. [3]. - C C I T T defines X.25 as an interface between Data Terminal Equipment ( D T E ) and Data Circuit-terminating Equipment ( D C E ) f o r terminals operating in the packet mode and connected to public data networks by dedicated circuits [4]. X.25 comprises three layers of communications physical, link, packet - which can somewhat be m a p p e d one to one into the first three OSI layers. For an overview see Refs. [5,6]. However notwithstanding the clear-cut differences in the stated goals, SS7 NSP and X.25 are sometimes used interchangeably in practice:
0169-7552/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-7552(93)E0092-S
R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502
1496
sccP
PACKEr LAYER
NETWORK LAY~
MTP3 DATA l I N K
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LINK LAYER
.................. L A Y E R
......................
PHYSICAL LAYEi ~I"P1
PHYSICAL LAYEI~ .
$S7 NSP
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X.25
Fig. 1. SS7 NSP and X.25 communication layers mapping into the equivalent OSI R M layers.
- SS7 NSP is used to support classical X.25 applications. For instance, file transfer capability is a requirement on the protocols for the operation and maintenance procedures of SS7 Operation, Maintenance, and Administration Part (OMAP) 117]. - X.25 is used for CCS. Some Mobile Switching Centres (MSCs) use it for mobile telephony signalling, - Telecommunication Management Network (TMN) (Recommendation M3010, [8]) will use indiscriminately SS7 NSP and X.25 as lower layer protocol profiles at the Q3 interface [9] 2 A comparative study is therefore worthwhile. For this purpose, the OSI RM framework is used in this article. Fig. 1 shows the mapping of the communications layers of the two standards into the equivalent OSI layers, - The first section presents the similarities and dissimilarities in the ways the two standards accomplish the OSI Data link layer functions, - The second presents the similarities and dissimilarities in the OSI Network layer services they offer to the user, and in the protocols they use to realize the services,
I O M A P is a SS7 NSP user [7]. 2 The future tense is used because the function of SCCP at the boundary of the network layer and the transport layer in the Q3 interface is still u n d e r study.
The physical layer is not part of the study for obvious reasons, the maximum standardized bit rate being the same for the two standards: 64 kbits/s. A glossary of terms and acronyms is given in the Appendix.
2. SS7 and X.25 data link layers: Similarities and dissimilarities MTP2 of SS7 corresponds to the OSI Data link layer (Fig. 1). It ensures the transfer of three different types of Signal Unit (SU) over MTP1 (the physical layer): The Message Signal Unit (MSU) which carries the actual user data, the Link Status Signal Unit (LSSU) and the Fill-InSignal Unit (FISU) which carry control and status information. Fig. 2 shows the SUs layout. X.25 link layer corresponds to the OSI Data link layer (Fig. 1). It ensures the transfer of three different types of frames over the physical layer: The Information frame (I-frame)which conveys the actual user data, the Supervisory frame (Sframe) and the Unnumbered frame (U-frame) which convey control and status information. Fig. 3 shows the frames layout. Both standards use bit-oriented protocols which show strong similarities to the well known bit-oriented protocols HDLC, ADCCP, and SDLC. For the relationships the three last protoCOIS have to each other, see Ref. [10]. X.25 link
R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502 16
8
MSU
F
CRC
8
LSSU
F
F
8
SIF
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2
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6
2
1
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8
1
7
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7
8
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II cI lil lil lil l
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2
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Variable
LI
6
1
LI
7
1
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F= Flag e R e = Cycle Redundancy Check LI = Length Indicstor BSN = Backward Sequence Indicator number FSN = Forvvmd Sequence Indicator numbs" BIB = Backward Indicator Bit
FSN
7
BSN
1497
7
8
BSN
F
8
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FIB = Forward Indicator Bit SF = Status Field SIF = Signaling lnfonmlioa Field SIO = Service lnfonnafiee Octet
Fig. 2. Signal units.
8
8
F
8
A
C
I
2
2
Variable
I
2
F = Flag A = Add~
C = Conuol I = Infonmdon
1
8
CRC
F
3
1
2
16
3
3
N(R) = Tmmmitter n~oeive I~laence number SC = Supervilm.'y C.ode UN = Unnumbered Code
c . Cyc~ P.ed.nd~ Ch~k N(S) = Trmmniner tend sequence number P/F = Poll/Final bit Fig. 3. X.25 link layer frames.
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R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502
layer protocols (LAP and LAP-B) are in fact considered as H D L C subsets [6]. Nevertheless, they accomplish some of the OSI data link functions in a quite dissimilar manner, The similarities and dissimilarities are illustrated in this section by giving first typical examples of functions accomplished in a quite similar manner, and after that typical examples of functions accomplished in a quite dissimilar manner. For an overview of the OSI Data link layer functions, see Ref. [11].
2.1. Functions accomplished in a quite similar manner Typical examples of functions accomplished in a quite similar manner are: segmenting, transparency, flow control, and sequence control. Segmenting. The role of the segmenting function is to divide the user data into segments suitable for transmission through the physical medium, The two standards accomplish it the same way, by using the CCITT standard flag 01111110 to open and close SU (or frame), Transparency. The transparency function permits the data link layer to be totally transparent to the format or structure of the user information. The bit stuffing technique is used in both cases: - The sender stuffs a 0 bit into the outgoing bit stream whenever it encounters five consecutive ones in the user data. - The receiver automatically destuffs the 0 bit whenever it encounters five consecutive 1 followed by a 0 bit in the incoming bit stream, Flow control. The flow control function regulates the flow of information to prevent the receiver from being overwhelmed by incoming data in case the input rate exceeds its capacity to accept and process data. Both standards use a window-based mechanism. For MTP2 the maximum size of the window is 127, and for X.25 link layer, 7 if the basic mode is used, and 127 if the extended mode is used. The mechanism is enhanced in both cases by an additional feature which allows the receiver to
inform the sender of its temporary inability to receive additional SU (or frame). In the first case the receiver sends an LSSU indicating busy to the sender, and in the second case it sends a Receive Not Ready (RNR) which is an S-frame. Sequence control. The sequence control function guarantees the detection of missing segmerits. Both standards use sequence numbers: - The first sequence number (Backward Sequence Number (BSN) in SS7 terminology and N(R) in X.25 terminology) acknowledges segments. - The second (Forward Sequence Number (FSN) in SS7 terminology and N(S) in X.25 terminology) identifies the segment in which it resides.
2.2. Functions accomplished in a quite dissimilar manner Typical functions accomplished in a quite dissimilar manner are synchronization and error control. Synchronization. The role of the synchronization function is to bring the receiver's decoding mechanisms into alignment with the transmitter's encoding mechanisms. In the first case FISUs are exchanged, and in the second flags are. The reason is that MTP2 (unlike X.25 link layer)provides a consistent error monitoring method. Error control. The error control function detects and monitors errors induced by the transmission medium. It also requests the retransmission of segments containing errors. Both standards use the standard CCITT 16 bits Cyclic Redundancy Check (CRC) to detect errors. But in addition: - MTP2 monitors the errors. When the physical link is in service, a SU error rate is used to provide the criteria for taking it out of service because of an excessive error rate. When it is in the proving state, an alignment error rate (based on the errors detected in the FISUs) is used to provide the criteria for rejecting it for service due to an excessive error rate. - MTP2 provides two forms of error correction, the Basic Method and the Preventive Cyclic
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R.H. Glitho / Computer Networksand ISDN Systems 26 (1994) 1495-1502
Retransmission (PCR) Method, while X.25 link layer provides only the first one. The Basic Method is a non compelled positive/negative acknowledgement retransmission error correction system, which uses the well known Go Back N technique. The P C R Method is also a non compelled positive/negative acknowledgement retransmission error correction system, but preventive cyclic retransmission takes place whenever there is no message available for transmission. The examples above show that SS7 NSP and X.25 accomplish most of the OSI Data link functions in a quite similar manner. The dissimilarities in the accomplishment of few functions such as synchronization and error control can be explained by the stringent dependency and availability objectives an SS7 network has. Those objectives are listed in Ref. [3].
[ GFI . Chamaelnuml~r
Byte1 Byte2
I
Control info
[ C/D I
Byte
3
Additional ControlLifo
an~or u~-rdata
Gn = GeneralFormatIdentifier
LCG = Logical ChannelGroupnumber
C/D = Packet
Fig. 5. X.25 packets generallayout. different interface [5]. It allows the multiplexing
3. SS7 and X.25 network layers: Similarities and dissimilarities
SS7 Network Layer (NL) corresponds to the OSI Network layer. It is split into SCCP and
of up to 4095 logical channels on the same user interface. It offers two types of facilities: standard or basic (available on all X.25 implementations), and optional. The facilities referred to in this section are standard, if not stated otherwise. Fig. 5 shows the general layout of X.25 PL packets.
MTP3. It can support up to 255 simultaneous users called subsystems. Each subsystem is identi-
3.1. Comparison o f the services offered to the user
fied by a Subsystem N u m b e r (SSN). Fig. 4 shows the general layout of SS7 N L messages (SCCP messages), X.25 Packet Layer (PL) corresponds to the
The OSI R M defines two categories of Network Service (NS): the Connectionless NS and the Connection-Oriented NS [12].
OSI Network layer. It is unique to the X.25 standard and its replacement will result in a
3.1.1. Connectionless N S SS7 N L offers two classes of Connectionless
Routinglabel
Byte
1
Messagetype code Byte2 l~ndat~ fix~l part Mandatoryvariablepart
NS: • Class 0 - Basic Connectionless class • Class 1 - Sequenced Connectionless class 3 Class 1 enhances class 0 by an additional feature which allows a subsystem to indicate to SS7 N L that a particular stream of messages should be delivered in sequence. X.25 PL offers none. A connectionless NS feature did exist in an earlier version of the standard (1980), but was removed in 1984 [6]. However it offers an optional facility, the Fast
Optionalpart Fig. 4. SS7 NL messages general layout.
3 This class has no equivalent in the OSI RM.
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R.H. Glitho/Computer Networksand ISDN Systems 26 (1994) 1495-1502
Select which is based on the same premises, i.e the elimination of the overhead due to session establishment and disestablishment. Thanks to that facility the user can send a limited amount of data (up to 128 bytes) during the connection establishment phase,
3.1.2. Connection-oriented NS SS7 NL offers two classes of Connection-Oriented NS: • Class 2 - Basic Connection-Oriented class 4 • Class 3 - Flow control Connection-Oriented class Class 3 enhances class 2 by two features: flow control and detection of lost and mis-sequenced messages, X.25 offers only the equivalent of class 3. SS7 NL and X.25 PL offer basically the same OSI NS to the user. X.25 offers no Connectionless NS, but it provides the optional Fast Select facility which is somewhat equivalent, 3.2. Comparison of the protocols An SS7 N L data message can convey up to 254 octets of user data and an X.25 PL up to 128 octets. In the last case, optional facilities can increase the figure. In both cases segmenting and reassembling are allowed. The general rules which govern the exchange of messages between two SS7 NLs are compared below to the ones which govern the exchange of packets between two X.25 PLs. The comparison is limited to the two topics of NL protocols that are currently most significant: routing and congestion control,
3.2.1. Routing SS7 NL distinguishes two basic categories of addresses: (1) Global Title (GT). A G T is a logical address such as a dialled number which does not
explicitly contain information that would allow routing in the SS7 network, and which requires translation by SCCP. (2) Destination Point Code + SSN. A Destination Point Code plus an SSN allow direct routing by SS7 NL, without prior translation by SCCP. The actual routing is done by MTP3. In view of Bell and Jabbour's [13] point to point network routing algorithms classification, the algorithm used by MTP3 is adaptive and distributed: - It is adaptive because alternate routes are found when nodes a n d / o r lines fail or come back to service. - It is distributed because there is no Routing Control Center which generates centrally routing tables. X.25 PL addressing scheme is specified in recommendation X.121 [14]. It is similar to the public switched telephone network scheme with each user identified by a decimal number consisting of a country code, a network code, and an address within the specific network. The actual routing mechanism within the public data network is outside the scope of X.25 PL specifications, since X.25 standard scope is restricted to the interface between the D T E and the DCE.
3.2.2. Congestion control Both SS7 N L and X.25 PL use a window-based flow control protocol in attempt to quench congestion. The window size cannot exceed 127 in the first case, and 7 in the second if the optional maximum size of 127 is not used. As enhancement to the window based flow control mechanism, X.25 provides an additional feature which is not supported by SS7 NL. It is the Receive Not Ready - Receive Ready feature which allows an X.25 PL to indicate to its peer a temporary inability to accept packets and the end of the inability. The protocols used by SS7 N L and X.25 PL to realize the services offered to the user are basically different: SS7 N L has a complex routing mechanism while routing is outside the scope of X.25 specifications. -
4 This class has no equivalent in the OSI RM.
R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502 - Although the two standards use a windowbased flow control protocol for congestion control, only X.25 PL provides the additional feature of "Receive Not Ready - Receive Ready".
4. S u m m a r y a n d c o n c l u s i o n s While SS7 NSP has been designed with telephone signalling in mind, X.25 has been designed with general data communications in mind. This explains the salient dissimilarities we have pointed out in this paper and that can be summarized as follow: - SS7 NSP offers more reliable mechanisms for error detection and recovery such as the specifications for removing faulty links from service before they actually go down. - SS7 NSP supports features that are specially designed for telephony. An example is the G T translation feature supported by the routing mechanism and which allows the SS7 NSP user to route a message in the network using normal dialled telephone numbers instead of the actual address of the nodes. Two practical conclusions can be drawn from the similarities and dissimilarities between SS7 NSP and X.25: - Whenever SS7 NSP is to be used in place of X.25 two cases should be considered. The first, the simplest is when only general data communications traffic (no signalling traffic) is to be carried: no problem should arise. The second is when both general data communications traffic and signalling traffic are to be carried. In that case the potential effect on the signalling traffic should be studied carefully. The reason is that signalling applications have stringent delay objectives that can hardly be met when they share the NSP with general data communication applications. Ref. [3] gives a flavour of the delay objectives, - X.25 can be used in place of SS7 NSP to carry signalling traffic only when X.25 and the subjacent data network meet the stringent availability and dependency objectives of signalling applications. As illustration, the network should not fail more than 10 minutes a y e a r . . .
1501
Appendix - G l o s s a r y o f terms a n d a c r o n y m s ADCCP BSN CCS CCITT CRC FISU FSN GT HDLC LAP LSSU MTP NL NS NSP OMAP OSI RM PL PCR SCCP SDLC SS7 SU TMN UP
Advanced Data Communication Control Procedure Backward Sequence Number Common Channel Signalling International Consultative Committee for Telephone and Telegraph. Cyclic Redundancy Check Fill-In-Signalling-Unit Forward Sequence Number Global Title High level Data Link Control Link Access Protocol Link Status Signalling Unit Message Transfer Part Network Layer Network Services Network Service Part Operation, Maintenance, and Administration Part Open System Interconnection Reference Model Packet Layer Preventive Cyclic Retransmission Signalling Connection Control Part Synchronous Data Link Control Signalling System Number 7 Signalling Unit Telecommunication Management Network User Part
References
[1] CCITT Study Group XI, Specifications of signalling system No. 7, CCITT Blue Book, Fascicle VI.7, Geneva, Switzerland, 1989.
[2] CCITF Recommendation X.200, Reference model of open systems interconnection for CCITT applications,
Blue Book, Fascicle VillA, Geneva, Switzerland, 1989. [3] A. Modarressi and R. Skoog, Signalling system No 7, IEEE Comm. Mag. (July 1990). [4] CCITT Recommendation X.25, Interface between DTE and DCE for terminals operating in the packet mode on public data networks, CCITT Blue Book, Fascicle VIII.2,
Geneva, Switzerland, 1989.
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R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502
[5] C.R. Dhas and V.K. Konangi, X.25: an interface to public packet networks, IEEE Comm. Mag. (September 1986). [6] tice,U'D'Prentice-HallBlaData ck' Networks.'international,Concepts,1989. Theory, and Prac[7] CCITT Recommendation Q.795, Operations, Maintenance, and Administration Part (OMAP), Blue Book, Fascicle VI.9, Geneva, Switzerland, 1989. [8] CCITI" Recommendation M.3010, Principles for telecommunication management network, Geneva, Switzerland, 1993. [9] CCITT Recommendation Q.811, Lower layer protocol profiles for the Q3 interface, Geneva, Switzerland, 1993. [10] W.D. Brodd, HDLC, ADCCP, and SDLC: What's the difference?, Data Comm. (August 1983). [11] J.W. Conard, Services and protocols of the data link layer, Proc. IEEE, December 1983. [12] C. Ware, The OSI network layer: Standards to cope with the real world, Proc. IEEE, December 1983. [13] P.R. Bell and K. Jabbour, Review of point-to-point network routing algorithms, IEEE Comm. Mag. (January 1986). [14] CCITT Recommendation X.121, Fascicle VIII.3, Geneva, Switzerland, 1989.
|! :
~ ~1
Roch H. Glitho, a national of Benin, West Africa, received M.Sc. degrees in Computer Science and Mathemat-
ics from the University of Geneva (Switzerland), and an M.Sc. in Business Economics from the University of Grenoble (France), in 1984, 1985, and 1990, respectively. ~ From 1985 to 1989, he worked as R ~ and D Engineer with Norsk Data A.S. in Oslo (Norway) and his responsibilities included LANs and X.25 protocols implementation in firmware/software. During the first year of his career with Norsk Data A.S., he had an international assignment on LANs interconnection with the Data Division (DD) of the European Center for Nuclear Research (CERN) in Geneva. He has been working at Ericsson Telecom AB in Stockholm (Sweden) since 1990. He holds the position of Senior Specialist, Network Management, in the Transit Exchange Development Department. He is currently on international assignment at Ericsson Communications in Montreal (Canada) where he is involved in standardization activities and in the design of Operation Systems (OS) architectures for cellular and signalling networks. He is a member of the New York Academy of Sciences, the IEEE Communications Society and the IEEE Computer Society. He participates in the editorial activities of the IEEE Communications Magazine as Associate Technical Editor.
Computer Networks and ISDN Systems26 (1994) 1495-1502
Signalling system number 7 network services part and X.25: A comparative study Roch H. Glitho Ericsson Communications Inc., Systems Engineering Department, 8400 Decarie Bh,d., Town of Mount Royal, Quebec H4P 2N2, Canada
Abstract
Despite the clear-cut differences in the stated aims, Signalling System Number 7 Network Services Part and X.25 are sometimes used interchangeably. We present in this paper a comparative study that shows that although many aspects of the two standards are quite similar, significant dissimilarities do exist. Caution is therefore required whenever the use of any of the two standards, in place of the other, is contemplated. Key words: Data link layer; Link layer; Message Transfer Part (MTP); Network layer; Network Services Part (NSP);
OSI Reference Model (RM); Packet layer; Signalling Connection Control Part (SCCP); Signalling System Number 7 (SS7); X.25
1. Introduction Signalling System Number 7 (SS7) and X.25 standards have been specified for very different purposes: - In the words of the CCITI" the overall objective of Signalling System No 7 is to provide an internationally standardized general purpose C o m m o n Channel Signalling (CCS) system: • Optimized for operations in digital telecommunications network in conjunction with stored program controlled exchanges; • that can meet present and future requirements of information transfer for inter-processors transactions within telecommunications networks for call control, remote control, and m a n a g e m e n t and maintenance signalling; • that provides a reliable means for transfer of information in correct sequence, and without loss or duplication [1].
SS7 is divided in two parts: the User Part (UP), and the Network Services Part (NSP). The NSP consists of the Message Transfer Part (MTP) and the Signalling Connection Control Part (SCCP), and is somewhat functionally equivalent to the first three layers of the O p e n System Interconnection Reference Model (OSI RM) [2]. For an overview see Ref. [3]. - C C I T T defines X.25 as an interface between Data Terminal Equipment ( D T E ) and Data Circuit-terminating Equipment ( D C E ) f o r terminals operating in the packet mode and connected to public data networks by dedicated circuits [4]. X.25 comprises three layers of communications physical, link, packet - which can somewhat be m a p p e d one to one into the first three OSI layers. For an overview see Refs. [5,6]. However notwithstanding the clear-cut differences in the stated goals, SS7 NSP and X.25 are sometimes used interchangeably in practice:
0169-7552/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-7552(93)E0092-S
R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502
1496
sccP
PACKEr LAYER
NETWORK LAY~
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.................. L A Y E R
......................
PHYSICAL LAYEi ~I"P1
PHYSICAL LAYEI~ .
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Fig. 1. SS7 NSP and X.25 communication layers mapping into the equivalent OSI R M layers.
- SS7 NSP is used to support classical X.25 applications. For instance, file transfer capability is a requirement on the protocols for the operation and maintenance procedures of SS7 Operation, Maintenance, and Administration Part (OMAP) 117]. - X.25 is used for CCS. Some Mobile Switching Centres (MSCs) use it for mobile telephony signalling, - Telecommunication Management Network (TMN) (Recommendation M3010, [8]) will use indiscriminately SS7 NSP and X.25 as lower layer protocol profiles at the Q3 interface [9] 2 A comparative study is therefore worthwhile. For this purpose, the OSI RM framework is used in this article. Fig. 1 shows the mapping of the communications layers of the two standards into the equivalent OSI layers, - The first section presents the similarities and dissimilarities in the ways the two standards accomplish the OSI Data link layer functions, - The second presents the similarities and dissimilarities in the OSI Network layer services they offer to the user, and in the protocols they use to realize the services,
I O M A P is a SS7 NSP user [7]. 2 The future tense is used because the function of SCCP at the boundary of the network layer and the transport layer in the Q3 interface is still u n d e r study.
The physical layer is not part of the study for obvious reasons, the maximum standardized bit rate being the same for the two standards: 64 kbits/s. A glossary of terms and acronyms is given in the Appendix.
2. SS7 and X.25 data link layers: Similarities and dissimilarities MTP2 of SS7 corresponds to the OSI Data link layer (Fig. 1). It ensures the transfer of three different types of Signal Unit (SU) over MTP1 (the physical layer): The Message Signal Unit (MSU) which carries the actual user data, the Link Status Signal Unit (LSSU) and the Fill-InSignal Unit (FISU) which carry control and status information. Fig. 2 shows the SUs layout. X.25 link layer corresponds to the OSI Data link layer (Fig. 1). It ensures the transfer of three different types of frames over the physical layer: The Information frame (I-frame)which conveys the actual user data, the Supervisory frame (Sframe) and the Unnumbered frame (U-frame) which convey control and status information. Fig. 3 shows the frames layout. Both standards use bit-oriented protocols which show strong similarities to the well known bit-oriented protocols HDLC, ADCCP, and SDLC. For the relationships the three last protoCOIS have to each other, see Ref. [10]. X.25 link
R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502 16
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CRC
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F
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F= Flag e R e = Cycle Redundancy Check LI = Length Indicstor BSN = Backward Sequence Indicator number FSN = Forvvmd Sequence Indicator numbs" BIB = Backward Indicator Bit
FSN
7
BSN
1497
7
8
BSN
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8
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FIB = Forward Indicator Bit SF = Status Field SIF = Signaling lnfonmlioa Field SIO = Service lnfonnafiee Octet
Fig. 2. Signal units.
8
8
F
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A
C
I
2
2
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I
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N(R) = Tmmmitter n~oeive I~laence number SC = Supervilm.'y C.ode UN = Unnumbered Code
c . Cyc~ P.ed.nd~ Ch~k N(S) = Trmmniner tend sequence number P/F = Poll/Final bit Fig. 3. X.25 link layer frames.
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R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502
layer protocols (LAP and LAP-B) are in fact considered as H D L C subsets [6]. Nevertheless, they accomplish some of the OSI data link functions in a quite dissimilar manner, The similarities and dissimilarities are illustrated in this section by giving first typical examples of functions accomplished in a quite similar manner, and after that typical examples of functions accomplished in a quite dissimilar manner. For an overview of the OSI Data link layer functions, see Ref. [11].
2.1. Functions accomplished in a quite similar manner Typical examples of functions accomplished in a quite similar manner are: segmenting, transparency, flow control, and sequence control. Segmenting. The role of the segmenting function is to divide the user data into segments suitable for transmission through the physical medium, The two standards accomplish it the same way, by using the CCITT standard flag 01111110 to open and close SU (or frame), Transparency. The transparency function permits the data link layer to be totally transparent to the format or structure of the user information. The bit stuffing technique is used in both cases: - The sender stuffs a 0 bit into the outgoing bit stream whenever it encounters five consecutive ones in the user data. - The receiver automatically destuffs the 0 bit whenever it encounters five consecutive 1 followed by a 0 bit in the incoming bit stream, Flow control. The flow control function regulates the flow of information to prevent the receiver from being overwhelmed by incoming data in case the input rate exceeds its capacity to accept and process data. Both standards use a window-based mechanism. For MTP2 the maximum size of the window is 127, and for X.25 link layer, 7 if the basic mode is used, and 127 if the extended mode is used. The mechanism is enhanced in both cases by an additional feature which allows the receiver to
inform the sender of its temporary inability to receive additional SU (or frame). In the first case the receiver sends an LSSU indicating busy to the sender, and in the second case it sends a Receive Not Ready (RNR) which is an S-frame. Sequence control. The sequence control function guarantees the detection of missing segmerits. Both standards use sequence numbers: - The first sequence number (Backward Sequence Number (BSN) in SS7 terminology and N(R) in X.25 terminology) acknowledges segments. - The second (Forward Sequence Number (FSN) in SS7 terminology and N(S) in X.25 terminology) identifies the segment in which it resides.
2.2. Functions accomplished in a quite dissimilar manner Typical functions accomplished in a quite dissimilar manner are synchronization and error control. Synchronization. The role of the synchronization function is to bring the receiver's decoding mechanisms into alignment with the transmitter's encoding mechanisms. In the first case FISUs are exchanged, and in the second flags are. The reason is that MTP2 (unlike X.25 link layer)provides a consistent error monitoring method. Error control. The error control function detects and monitors errors induced by the transmission medium. It also requests the retransmission of segments containing errors. Both standards use the standard CCITT 16 bits Cyclic Redundancy Check (CRC) to detect errors. But in addition: - MTP2 monitors the errors. When the physical link is in service, a SU error rate is used to provide the criteria for taking it out of service because of an excessive error rate. When it is in the proving state, an alignment error rate (based on the errors detected in the FISUs) is used to provide the criteria for rejecting it for service due to an excessive error rate. - MTP2 provides two forms of error correction, the Basic Method and the Preventive Cyclic
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Retransmission (PCR) Method, while X.25 link layer provides only the first one. The Basic Method is a non compelled positive/negative acknowledgement retransmission error correction system, which uses the well known Go Back N technique. The P C R Method is also a non compelled positive/negative acknowledgement retransmission error correction system, but preventive cyclic retransmission takes place whenever there is no message available for transmission. The examples above show that SS7 NSP and X.25 accomplish most of the OSI Data link functions in a quite similar manner. The dissimilarities in the accomplishment of few functions such as synchronization and error control can be explained by the stringent dependency and availability objectives an SS7 network has. Those objectives are listed in Ref. [3].
[ GFI . Chamaelnuml~r
Byte1 Byte2
I
Control info
[ C/D I
Byte
3
Additional ControlLifo
an~or u~-rdata
Gn = GeneralFormatIdentifier
LCG = Logical ChannelGroupnumber
C/D = Packet
Fig. 5. X.25 packets generallayout. different interface [5]. It allows the multiplexing
3. SS7 and X.25 network layers: Similarities and dissimilarities
SS7 Network Layer (NL) corresponds to the OSI Network layer. It is split into SCCP and
of up to 4095 logical channels on the same user interface. It offers two types of facilities: standard or basic (available on all X.25 implementations), and optional. The facilities referred to in this section are standard, if not stated otherwise. Fig. 5 shows the general layout of X.25 PL packets.
MTP3. It can support up to 255 simultaneous users called subsystems. Each subsystem is identi-
3.1. Comparison o f the services offered to the user
fied by a Subsystem N u m b e r (SSN). Fig. 4 shows the general layout of SS7 N L messages (SCCP messages), X.25 Packet Layer (PL) corresponds to the
The OSI R M defines two categories of Network Service (NS): the Connectionless NS and the Connection-Oriented NS [12].
OSI Network layer. It is unique to the X.25 standard and its replacement will result in a
3.1.1. Connectionless N S SS7 N L offers two classes of Connectionless
Routinglabel
Byte
1
Messagetype code Byte2 l~ndat~ fix~l part Mandatoryvariablepart
NS: • Class 0 - Basic Connectionless class • Class 1 - Sequenced Connectionless class 3 Class 1 enhances class 0 by an additional feature which allows a subsystem to indicate to SS7 N L that a particular stream of messages should be delivered in sequence. X.25 PL offers none. A connectionless NS feature did exist in an earlier version of the standard (1980), but was removed in 1984 [6]. However it offers an optional facility, the Fast
Optionalpart Fig. 4. SS7 NL messages general layout.
3 This class has no equivalent in the OSI RM.
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Select which is based on the same premises, i.e the elimination of the overhead due to session establishment and disestablishment. Thanks to that facility the user can send a limited amount of data (up to 128 bytes) during the connection establishment phase,
3.1.2. Connection-oriented NS SS7 NL offers two classes of Connection-Oriented NS: • Class 2 - Basic Connection-Oriented class 4 • Class 3 - Flow control Connection-Oriented class Class 3 enhances class 2 by two features: flow control and detection of lost and mis-sequenced messages, X.25 offers only the equivalent of class 3. SS7 NL and X.25 PL offer basically the same OSI NS to the user. X.25 offers no Connectionless NS, but it provides the optional Fast Select facility which is somewhat equivalent, 3.2. Comparison of the protocols An SS7 N L data message can convey up to 254 octets of user data and an X.25 PL up to 128 octets. In the last case, optional facilities can increase the figure. In both cases segmenting and reassembling are allowed. The general rules which govern the exchange of messages between two SS7 NLs are compared below to the ones which govern the exchange of packets between two X.25 PLs. The comparison is limited to the two topics of NL protocols that are currently most significant: routing and congestion control,
3.2.1. Routing SS7 NL distinguishes two basic categories of addresses: (1) Global Title (GT). A G T is a logical address such as a dialled number which does not
explicitly contain information that would allow routing in the SS7 network, and which requires translation by SCCP. (2) Destination Point Code + SSN. A Destination Point Code plus an SSN allow direct routing by SS7 NL, without prior translation by SCCP. The actual routing is done by MTP3. In view of Bell and Jabbour's [13] point to point network routing algorithms classification, the algorithm used by MTP3 is adaptive and distributed: - It is adaptive because alternate routes are found when nodes a n d / o r lines fail or come back to service. - It is distributed because there is no Routing Control Center which generates centrally routing tables. X.25 PL addressing scheme is specified in recommendation X.121 [14]. It is similar to the public switched telephone network scheme with each user identified by a decimal number consisting of a country code, a network code, and an address within the specific network. The actual routing mechanism within the public data network is outside the scope of X.25 PL specifications, since X.25 standard scope is restricted to the interface between the D T E and the DCE.
3.2.2. Congestion control Both SS7 N L and X.25 PL use a window-based flow control protocol in attempt to quench congestion. The window size cannot exceed 127 in the first case, and 7 in the second if the optional maximum size of 127 is not used. As enhancement to the window based flow control mechanism, X.25 provides an additional feature which is not supported by SS7 NL. It is the Receive Not Ready - Receive Ready feature which allows an X.25 PL to indicate to its peer a temporary inability to accept packets and the end of the inability. The protocols used by SS7 N L and X.25 PL to realize the services offered to the user are basically different: SS7 N L has a complex routing mechanism while routing is outside the scope of X.25 specifications. -
4 This class has no equivalent in the OSI RM.
R.H. Glitho / Computer Networks and ISDN Systems 26 (1994) 1495-1502 - Although the two standards use a windowbased flow control protocol for congestion control, only X.25 PL provides the additional feature of "Receive Not Ready - Receive Ready".
4. S u m m a r y a n d c o n c l u s i o n s While SS7 NSP has been designed with telephone signalling in mind, X.25 has been designed with general data communications in mind. This explains the salient dissimilarities we have pointed out in this paper and that can be summarized as follow: - SS7 NSP offers more reliable mechanisms for error detection and recovery such as the specifications for removing faulty links from service before they actually go down. - SS7 NSP supports features that are specially designed for telephony. An example is the G T translation feature supported by the routing mechanism and which allows the SS7 NSP user to route a message in the network using normal dialled telephone numbers instead of the actual address of the nodes. Two practical conclusions can be drawn from the similarities and dissimilarities between SS7 NSP and X.25: - Whenever SS7 NSP is to be used in place of X.25 two cases should be considered. The first, the simplest is when only general data communications traffic (no signalling traffic) is to be carried: no problem should arise. The second is when both general data communications traffic and signalling traffic are to be carried. In that case the potential effect on the signalling traffic should be studied carefully. The reason is that signalling applications have stringent delay objectives that can hardly be met when they share the NSP with general data communication applications. Ref. [3] gives a flavour of the delay objectives, - X.25 can be used in place of SS7 NSP to carry signalling traffic only when X.25 and the subjacent data network meet the stringent availability and dependency objectives of signalling applications. As illustration, the network should not fail more than 10 minutes a y e a r . . .
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Appendix - G l o s s a r y o f terms a n d a c r o n y m s ADCCP BSN CCS CCITT CRC FISU FSN GT HDLC LAP LSSU MTP NL NS NSP OMAP OSI RM PL PCR SCCP SDLC SS7 SU TMN UP
Advanced Data Communication Control Procedure Backward Sequence Number Common Channel Signalling International Consultative Committee for Telephone and Telegraph. Cyclic Redundancy Check Fill-In-Signalling-Unit Forward Sequence Number Global Title High level Data Link Control Link Access Protocol Link Status Signalling Unit Message Transfer Part Network Layer Network Services Network Service Part Operation, Maintenance, and Administration Part Open System Interconnection Reference Model Packet Layer Preventive Cyclic Retransmission Signalling Connection Control Part Synchronous Data Link Control Signalling System Number 7 Signalling Unit Telecommunication Management Network User Part
References
[1] CCITT Study Group XI, Specifications of signalling system No. 7, CCITT Blue Book, Fascicle VI.7, Geneva, Switzerland, 1989.
[2] CCITF Recommendation X.200, Reference model of open systems interconnection for CCITT applications,
Blue Book, Fascicle VillA, Geneva, Switzerland, 1989. [3] A. Modarressi and R. Skoog, Signalling system No 7, IEEE Comm. Mag. (July 1990). [4] CCITT Recommendation X.25, Interface between DTE and DCE for terminals operating in the packet mode on public data networks, CCITT Blue Book, Fascicle VIII.2,
Geneva, Switzerland, 1989.
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[5] C.R. Dhas and V.K. Konangi, X.25: an interface to public packet networks, IEEE Comm. Mag. (September 1986). [6] tice,U'D'Prentice-HallBlaData ck' Networks.'international,Concepts,1989. Theory, and Prac[7] CCITT Recommendation Q.795, Operations, Maintenance, and Administration Part (OMAP), Blue Book, Fascicle VI.9, Geneva, Switzerland, 1989. [8] CCITI" Recommendation M.3010, Principles for telecommunication management network, Geneva, Switzerland, 1993. [9] CCITT Recommendation Q.811, Lower layer protocol profiles for the Q3 interface, Geneva, Switzerland, 1993. [10] W.D. Brodd, HDLC, ADCCP, and SDLC: What's the difference?, Data Comm. (August 1983). [11] J.W. Conard, Services and protocols of the data link layer, Proc. IEEE, December 1983. [12] C. Ware, The OSI network layer: Standards to cope with the real world, Proc. IEEE, December 1983. [13] P.R. Bell and K. Jabbour, Review of point-to-point network routing algorithms, IEEE Comm. Mag. (January 1986). [14] CCITT Recommendation X.121, Fascicle VIII.3, Geneva, Switzerland, 1989.
|! :
~ ~1
Roch H. Glitho, a national of Benin, West Africa, received M.Sc. degrees in Computer Science and Mathemat-
ics from the University of Geneva (Switzerland), and an M.Sc. in Business Economics from the University of Grenoble (France), in 1984, 1985, and 1990, respectively. ~ From 1985 to 1989, he worked as R ~ and D Engineer with Norsk Data A.S. in Oslo (Norway) and his responsibilities included LANs and X.25 protocols implementation in firmware/software. During the first year of his career with Norsk Data A.S., he had an international assignment on LANs interconnection with the Data Division (DD) of the European Center for Nuclear Research (CERN) in Geneva. He has been working at Ericsson Telecom AB in Stockholm (Sweden) since 1990. He holds the position of Senior Specialist, Network Management, in the Transit Exchange Development Department. He is currently on international assignment at Ericsson Communications in Montreal (Canada) where he is involved in standardization activities and in the design of Operation Systems (OS) architectures for cellular and signalling networks. He is a member of the New York Academy of Sciences, the IEEE Communications Society and the IEEE Computer Society. He participates in the editorial activities of the IEEE Communications Magazine as Associate Technical Editor.