Biblio5
.pdfREVIEW EXERCISES |
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parameters, the network may accept or reject the call. In the case of the permanent virtual circuit, the logical identifier and other associated parameters are defined by means of administrative procedures (e.g., at the time of subscription).
The user–network interface structure allows for the establishment of multiple virtual calls or permanent virtual circuits, or both, to many destinations over a single access channel. Specifically, for each connection, the bearer service:
1. Provides bidirectional transfer of frames;
2. Preserves their order as given at one user–network interface if and when they are delivered at the other end. (Note: No sequence numbers are kept by the network. Networks are implemented in such a way that frame order is preserved);
3. Detects transmission, format, and operational errors such as frames with an unknown label;
4. Transports the user data contents of a frame transparently; only the frame’s address and FCS fields may be modified by network nodes; and
5. Does not acknowledge frames.
At the user–network interface, the FRAD (frame relay access device), as a minimum, has the following responsibilities:
1. Frame delimiting, alignment, and transparency provided by the use of HDLC flags and zero bit insertion;
2. Virtual circuit multiplexing/ demultiplexing using the address field of the frame; 3. Inspection of the frame to ensure that it consists of an integer number of octets
prior to zero bit insertion or following zero bit extraction;
4. Inspection of the frame to ensure it is not too short or too long; and 5. Detection of transmission, format, and operational errors.
A frame received by a frame handler may be discarded if the frame:
1. Does not consist of an integer number of octets prior to zero bit insertion or following zero bit extraction;
2. Is too long or too short; and
3. Has an FCS that is in error.
The network will discard a frame if it:
1. Has a DLCI value that is unknown; or
2. Cannot be routed further due to internal network conditions. A frame can be discarded for other reasons, such as exceeding negotiated throughput.
Section 12.5 is based on ANSI standards T1.618-1991, T1.606-1990, and T1.606a-1992 (Refs. 23, 24, 26).
REVIEW EXERCISES
1. X.25 deals primarily with which OSI layer? and LAPB?
2. How does a processor know where a frame’s field boundaries are?
3. Where is an X.25 DTE located? its DCE located?
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ENTERPRISE NETWORKS II: WIDE AREA NETWORKS |
4. |
What are the three approaches used with X.25 to manage the transfer and routing |
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of data packets? |
5. |
What is the purpose of a call request and incoming call packet? |
6. |
Name at least two types of flow control packets used with X.25. |
7. |
Even though TCP/ IP predates OSI, in what OSI layers would we expect to find |
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TCP and IP? |
8. |
What is the purpose of the ARP (address resolution protocol)? |
9. |
What is the primary function of IP? |
10. |
What is the function of a router in an IP network? |
11. |
What are the three types of routing carried out by an IP routing table? |
12. |
How is ICMP used as an adjunct to IP? |
13. |
What is the term used in TCP/ IP parlance for segmentation? |
14. |
What important mechanisms does TCP offer IP with its potentially unreliable ser- |
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vices? |
15. |
What is the purpose of the three-way handshake? |
16. |
Name at least four communication services that ISDN will support. |
17. |
What is the purpose of the D-channel with ISDN? |
18. |
What are the three basic variants of the H-channel? |
19. |
Using your imagination and what you have previously learned, relate the two |
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higher bit rate H-channels to the digital network. |
20. |
How does an ISDN user derive its timing? |
21. |
How many B-channels can carry traffic in PRI service in North America? |
22. |
What is the function of the balancing bit in the BRI configuration? |
23. |
Is North American ISDN BRI 2-wire or 4-wire? |
24. |
What are the three types of LAPD control field formats? |
25. |
Name and describe at least three functions carried out by ISDN layer 3. |
26. |
What BRI line code is used with CCITT ISDN? with North American ISDN? |
27. |
What is the first field in an LAPD frame? |
28. |
Compare frame relay and X.25 for “speed” of operation. |
29. |
What does a frame relay network do with errored frames? |
30. |
Frame relay operation derives from which predecessor? |
31. |
There are six possible causes for declaring a frame relay frame invalid. Name four |
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of them. |
32. |
Discuss the use of CLLM as an alternative for congestion control. |
33. |
Name some actions a congested node can take to alleviate the problems. |
REFERENCES 383
34. If no sequence numbers are used with frame relay, how are frames kept in order?
35. How does an end-user know that a frame or frames have been lost or discarded?
REFERENCES
1. Interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Connected to the Public Data Networks by Dedicated Circuit, ITU-T Rec. X.25, Helsinki, 1993.
2. Internet Protocol, RFC 791, DDN Network Information Center, SRI International, Menlo Park, CA, 1981.
3. Transmission Control Protocol, RFC 793, DDN Network Information Center, SRI International, Menlo Park, CA, 1981.
4. Packet-Switched Signaling System between Public Networks Providing Data Transmission Services, ITU-T Rec. X.75, Helsinki, 1993.
5. Internet Protocol Transition Workbook, SRI International, Menlo Park, CA, 1982.
6. Assigned Numbers, RFC 1060, DDN Network Information Center, SRI International, Menlo Park, CA, 1990.
7. An Ethernet Address Resolution Protocol, RFC 826, DDN Network Information Center, SRI International, Menlo Park, CA, 1984.
8. A Reverse Address Resolution Protocol, RFC 903, DDN Network Information Center, SRI International, Menlo Park, CA, 1984.
9. Military Standard, Transmission Control Protocol, MIL-STD-1778, U.S. Dept. of Defense, Washington, DC, 1983.
10. IEEE Standard Dictionary of Electrical and Electronic Terms, 6th ed., IEEE Std. 100-1996, IEEE, New York, 1996.
11. ISDN User-Network Interfaces—Interface Structure and Access Capabilities, CCITT Rec. I.412, Fascicle III.8, IXth Plenary Assembly, Melbourne, 1988.
12. ISDN Network Architecture, CCITT Rec. I.324, ITU Geneva, 1991.
13. W. Stallings, ed., Integrated Services Digital Networks (ISDN), IEEE Computer Society Press, Washington, DC, 1985.
14. Digital Subscriber Signaling System No. 1 (DSS1): ISDN User-Network Interface, Data Link Layer—General Aspects, ITU-T Rec. Q.920, ITU, Geneva, 1993.
15. ISDN User-Network Interface—Data Link Layer Specification, ITU-T Rec. Q.921, ITU, Geneva, 1993.
16. ISDN User-Network Interface: Layer 3—for Basic Call Control, ITU-T Rec. Q.931, ITU, Geneva, 1993.
17. Interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Accessing a Public-Switched Telephone Network or a Circuit-Switched Public Data Network, CCITT Rec. X.32, Fascicle VIII.2, IXth Plenary Assembly, Melbourne, 1988.
18. Support of Packet Mode Terminal Equipment by an ISDN, ITU-T Rec. X.31, ITU, Geneva, 1993.
19. Framework for Frame Mode Bearer Services, ITU-T Rec. I.122, ITU, Geneva, 1993. 20. Frame Mode Bearer Services, CCITT Rec. I.233, Geneva, 1992.
21. ISDN Signaling Specification for Frame Relay Bearer Service for Digital Subscriber Signaling System No. 1 (DDS1), ANSI T1.617-1991, ANSI, New York, 1991.
22. ISDN Data Link Layer Service for Frame Mode Bearer Services, CCITT Rec. Q.922, ITU, Geneva, 1992.
384 ENTERPRISE NETWORKS II: WIDE AREA NETWORKS
23. Integrated Services Digital Network (ISDN)—Core Aspects of Frame Protocol for Use with Frame Relay Bearer Service, ANSI T1.618-1991, ANSI, New York, 1991.
24. ISDN—Architectural Framework and Service Description for Frame Relay Bearer Service, ANSI T1.606-1990, ANSI, New York, 1990.
25. Frame Relay and SMDS seminar, Hewlett-Packard Co., Burlington, MA, Oct. 1993.
26. Integrated Services Digital Network (ISDN)—Architectural Framework and Service Description for Frame-Relay Bearer Service (Congestion Management and Frame Size), ANSI T1.606a-1992, ANSI, New York, 1992.
Fundamentals of Telecommunications. Roger L. Freeman
Copyright 1999 Roger L. Freeman
Published by John Wiley & Sons, Inc.
ISBNs: 0-471-29699-6 (Hardback); 0-471-22416-2 (Electronic)
13
CCITT SIGNALING SYSTEM NO. 7
13.1 INTRODUCTION
CCITT Signaling System No. 7 (SS No. 7) was developed to meet the stringent signaling requirements of the all-digital network based on the 64-kbps channel. It operates in quite a different manner from the signaling discussed in Chapter 7. Nevertheless, it must provide for supervision of circuits and address signaling, and carry call progress signals and alerting notification to be eventually passed to the called subscriber. These requirements certainly look familiar and are no different than the ones discussed in Chapter 7. The difference is in how these requirements are met. CCITT No. 7 is a data network entirely dedicated to interswitch signaling.1
Simply put, CCITT SS No. 7 is described as an international standardized commonchannel signaling system that:
•Is optimized for operation with digital networks where switches use stored-program control (SPC), such as the DMS-100 series switches and the 5ESS, among others, which were discussed in Section 6.11;
•Can meet present and future requirements of information transfer for interprocessor transactions with digital communications networks for call control, remote control, network database access and management, and maintenance signaling;
•Provides a reliable means of information transfer in correct sequence without loss or duplication (Ref. 1).
CCITT SS No. 7, in the years since 1980, has become known as the signaling system for ISDN. This it is. Without the infrastructure of SS No. 7 embedded in the digital network, there will be no ISDN with ubiquitous access. One important point is to be made. CCITT SS No. 7, in itself, is the choice for signaling in the digital PSTN without ISDN. It can and does stand on its own in this capacity.
As mentioned, SS No. 7 is a data communication system designed for only one purpose: signaling. It is not a general-purpose system. We then must look at CCITT SS No. 7 as (1) a specialized data network and (2) a signaling system (Ref. 2).
1This would be called interoffice signaling in North America.
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386 CCITT SIGNALING SYSTEM NO. 7
13.2 OVERVIEW OF SS NO. 7 ARCHITECTURE
The SS No. 7 network model consists of network nodes, termed signaling points (SPs), interconnected by point-to-point signaling links, with all the links between two SPs called a link set. When the model is applied to a physical network, most commonly there is a one-to-one correspondence between physical nodes and logical entities. But when there is a need (e.g., a physical gateway node needs to be a member of more than one network), a physical network node may be logically divided into more than one SP, or a logical SP may be distributed over more than one physical node. These artifices require careful administration to ensure that management procedures within the protocol work correctly.
Messages between two SPs may be routed over a link set directly connecting the two points. This is referred to as the associated mode of signaling. Messages may also be routed via one or more intermediate SPs that relay messages at the network layer. This is called nonassociated mode of signaling. SS No. 7 supports only a special case of this routing, called quasiassociated mode, in which routing is static except for relatively infrequent changes in response to events such as link failures or addition of new SPs. SS No. 7 does not include sufficient procedures to maintain in-sequence delivery of information if routing were to change completely on a packet-by-packet basis.
The function of relaying messages at the network layer is called the signaling transfer point (STP) function.2 Although this practice results in some confusion, the logical and physical network nodes at which this function is performed are frequently called STPs, even though they may provide other functions as well. An important part in designing an SS No. 7 network is including sufficient equipment redundancy and physical-route diversity so that the stringent availability objectives of the system are met. The design is largely a matter of locating signaling links and SPs with the STP function, so that performance objectives can be met for the projected traffic loads at minimum cost.
Figure 13.1 is an SS No. 7 network structure model. The STP function is concentrated in a relatively small number of nodes that are essentially dedicated to that function. The STPs are paired or mated, and pairs of STPs are interconnected with a quad configuration, as shown in the figure. We could also say that the four STPs are connected in mesh. This has proved to be an extremely reliable and survivable backbone network. Other nodes, such as switching centers and service control points (SCPs), are typically homed on one of the mated pairs of STPs, with one or more links to each of the mates, depending on traffic volumes (Ref. 3).
13.3 SS NO. 7: RELATIONSHIP TO OSI
SS No. 7 relates to OSI (Section 10.10.2) up to a certain point. During the development of SS No. 7, one group believed that there should be complete compatibility with all seven OSI layers. However, the majority of the CCITT working group responsible for the concept and design of SS No. 7 was concerned with delay, whether for the data, telephone, or ISDN user of the digital PSTN. Recall from Chapter 7 that post-dial delay is probably the most important measure of performance of a signaling system. To minimize delay, the seven layers of OSI were truncated at layer 4. In fact, CCITT Rec. Q.709 specifies no more than 2.2 seconds of postdial delay for 95% of calls. To accomplish
2Bellcore (Ref. 6) reports that “purists restrict this further to MTP relaying.” (MTP stands for message transfer part.)
13.3 SS NO. 7: RELATIONSHIP TO OSI |
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Figure 13.1 Signaling System No. 7 network structure model.
this, a limit is placed on the number of relay points, called STPs, that can be traversed by a signaling message and by the inherent design of SS No. 7 as a four-layer system. Figure 13.2 relates SS No. 7 protocol layers to the OSI reference model. Remember that reducing the number of OSI layers reduces processing, and thus processing time. As a result postdial delay is also reduced.
We should note that SS No. 7 layer 3 signaling network functions include signaling message-handling functions and network management functions. Figure 13.3 shows the general structure of the SS No. 7 signaling system.
There seem to have been various efforts to force-fit SS No. 7 into OSI layer 4 upwards. These efforts have resulted in the sublayering of layer 4 into user parts and the SCCP (signaling connection control part).
Figure 13.2 How SS No. 7 relates to OSI.
388 CCITT SIGNALING SYSTEM NO. 7
Figure 13.3 General structure of signaling functions. (From ITU-T Rec. Q.701, Figure 6/ Q.701, p. 8 [Ref. 4].)
In Section 13.4 we briefly describe the basic functions of the four SS No. 7 layers, which are covered in more detail in Sections 13.5 through 13.7.
13.4 SIGNALING SYSTEM STRUCTURE
Figure 13.3, which illustrates the basic structure of SS No. 7, shows two parts to the system: the message transfer part (MTP) and the user parts. There are three user parts: (1) telephone user part (TUP), (2) data user part (DUP), and (3) the ISDN user part (ISUP). Figures 13.2 and 13.3 show OSI layers 1, 2, and 3, which make up the MTP. The following paragraphs describe the functions of each of these layers from a system viewpoint.
Layer 1 defines the physical, electrical, and functional characteristics of the signaling data link and the means to access it. In the digital network environment the 64-kbps digital path is the normal basic connectivity. The signaling link may be accessed by means of a switching function that provides the capability of automatic reconfiguration of signaling links.
Layer 2 carries out the signaling link function. It defines the functions and procedures for the transfer of signaling messages over one individual signaling data link. A signaling message is transferred over the signaling link in variable-length signal units. A signal unit consists of transfer control information in addition to the information content of the signaling message. The signaling link functions include:
•Delimitation of a signal unit by means of flags;
•Flag imitation prevention by bit stuffing;
13.4 SIGNALING SYSTEM STRUCTURE |
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•Error detection by means of check bits included in each signal unit;
•Error control by retransmission and signal unit sequence control by means of explicit sequence numbers in each signal unit and explicit continuous acknowledgments; and
•Signaling link failure detection by means of signal unit error monitoring, and signaling link recovery by means of special procedures.
Layer 3, signaling network functions, in principle, defines such transport functions and procedures that are common to and independent of individual signaling links. There are two categories of functions in layer 3:
1. Signaling Message-Handling Functions. During message transfer, these functions direct the message to the proper signaling link or user part.
2. Signaling Network Management Functions. These control real-time routing, control, and network reconfiguration, if required.
Layer 4 is the user part. Each user part defines the functions and procedures peculiar to the particular user, whether telephone, data, or ISDN user part.
The signal message is defined by CCITT Rec. Q.701 as an assembly of information, defined at layer 3 or 4, pertaining to a call, management transaction, and so on, which is then transferred as an entity by the message transfer function. Each message contains “service information,” including a service indicator identifying the source user part and possibly whether the message relates to international or national application of the user part.
The signaling information portion of the message contains user information, such as data or call control signals, management and maintenance information, and type and format of message. It also includes a “label.” The label enables the message to be routed by layer 3 through the signaling network to its destination and directs the message to the desired user part or circuit.
On the signaling link such signaling information is contained in the message signal units (MSUs), which also include transfer control functions related to layer 2 functions on the link.
There are a number of terms used in SS No. 7 literature that should be understood before we proceed further:
Signaling Points. Nodes in the network that utilize common-channel signaling;
Signaling Relation (similar to traffic relation). Any two signaling points for which the possibility of communication between their corresponding user parts exist are said to have a signaling relation;
Signaling Links. Signaling links convey signaling messages between two signaling points;
Originating and Destination Points. The originating and destination points are the locations of the source user part function and location of the receiving user part function, respectively;
Signaling Transfer Point (STP). An STP is a point where a message received on one signaling link is transferred to another link;
Message Label. Each message contains a label. In the standard label, the portion that is used for routing is called the routing label. The routing label includes:
390CCITT SIGNALING SYSTEM NO. 7
•Destination and originating points of the message;
•A code used for load sharing, which may be the least significant part of a label component that identifies a user transaction at layer 4.
The standard label assumes that each signaling point in a signaling network is assigned an identification code, according to a code plan established for the purpose of labeling.
Message Routing. Message routing is the process of selecting the signaling link to be used for each signaling message. Message routing is based on analysis of the routing label of the message in combination with predetermined routing data at a particular signaling point.
Message Distribution. Message distribution is the process that determines to which user part a message is to be delivered. The choice is made by analysis of the service indicator.
Message Discrimination. Message discrimination is the process that determines, on receipt of a message at a signaling point, whether or not the point is the destination point of that message. This decision is based on analysis of the destination code of the routing label in the message. If the signaling point is the destination, the message is delivered to the message destination function. If not, the message is delivered to the routing function for further transfer on a signaling link.
13.4.1 Signaling Network Management
Three signaling network-management functional blocks are shown in Figure 13.3. These are signaling traffic management, signaling link management, and signaling route management.
13.4.1.1 Signaling Traffic Management. The signaling traffic management functions are:
1. To control message routing. This includes modification of message routing to preserve, when required, accessibility of all destination points concerned or to restore normal routing;
2. In conjunction with modifications of message routing, to control the resulting transfer of signaling traffic in a manner that avoids irregularities in message flow; and
3. Flow control.
Control of message routing is based on analysis of predetermined information about all allowed potential routing possibilities in combination with information, supplied by the signaling link management and signaling route management functions, about the status of the signaling network (i.e., current availability of signaling links and routes).
Changes in the status of the signaling network typically result in modification of current message routing and thus in the transfer of certain portions of the signaling traffic from one link to another. The transfer of signaling traffic is performed in accordance with specific procedures. These procedures are changeover, changeback, forced rerouting, and controlled rerouting. The procedures are designed to avoid, as far as circumstances permit, such irregularities in message transfer as loss, missequencing, or multiple delivery of messages.