Biblio5

nowledgments, sequence numbers and flow control. These mechanisms require certain addressing and control information to be initialized and maintained during data transfer. This collection of information is called a TCP connection. The following paragraphs describe the purpose and operation of the major TCP mechanisms.

Par mechanism. TCP uses a positive acknowledgment with retransmission (PAR) mechanism to recover from the loss of a segment by the lower layers. The strategy with PAR is for a sending TCP to retransmit a segment at timed intervals until a positive acknowledgment is returned. The choice of retransmission interval affects efficiency. An interval that is too long reduces data throughput while one that is too short floods the transmission media with superfluous segments. In TCP, the timeout is expected to be dynamically adjusted to approximate the segment round-trip time plus a factor for internal processing; otherwise performance degradation may occur. TCP uses a simple checksum to detect segments damaged in transit. Such segments are discarded without being acknowledged. Hence, damaged segments are treated identically to lost segments and are compensated for by the PAR mechanism. TCP assigns sequence numbers to identify each octet of the data stream. These enable a receiving TCP to detect duplicate and out-of-order segments. Sequence numbers are also used to extend the PAR mechanism by allowing a single acknowledgment to cover many segments worth of data. Thus, a sending TCP can still send new data, although previous data have not been acknowledged.

Flow control mechanism. TCP’s flow control mechanism enables a receiving TCP to govern the amount of data dispatched by a sending TCP. The mechanism is based on a window, which defines a contiguous interval of acceptable sequence-numbered data. As data are accepted, TCP slides the window upward in the sequence number space. This window is carried in every segment, enabling peer TCPs to maintain up-to-date window information.

Multiplexing mechanism. TCP employs a multiplexing mechanism to allow multiple ULPs within a single host and multiple processes in a ULP to use TCP simultaneously. This mechanism associates identifiers, called ports, to ULP processes accessing TCP services. A ULP connection is uniquely identified with a socket, the concatenation of a port and an Internet address. Each connection is uniquely named with a socket pair. This naming scheme allows a single ULP to support connections to multiple remote ULPs. ULPs which provide popular resources are assigned permanent sockets, called well-known sockets.

12.3.4.3 ULP Synchronization. When two ULPs (upper-layer protocols) wish to communicate (see Figure 12.11), they instruct their TCPs to initialize and synchronize the mechanism information on each to open the connection. However, the potentially unreliable network layer (i.e., the IP layer) can complicate the process of synchronization. Delayed or duplicate segments from previous connection attempts might be mistaken for new ones. A handshake procedure with clock-based sequence numbers is used in connection opening to reduce the possibility of such false connections. In the simplest handshake, the TCP pair synchronizes sequence numbers by exchanging three segments, thus the name three-way handshake.

12.3.4.4 ULP Modes. A ULP can open a connection in one of two modes: passive or active. With a passive open, a ULP instructs its TCP to be receptive to connections with other ULPs. With an active open, a ULP instructs its TCP to actually initiate a three-way handshake to connect to another ULP. Usually an active open is targeted to a passive open. This active/ passive model supports server-oriented applications where

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a permanent resource, such as a database-management process, can always be accessed by remote users. However, the three-way handshake also coordinates two simultaneous active opens to open a connection. Over an open connection, the ULP pair can exchange a continuous stream of data in both directions. Normally, TCP groups the data into TCP segments for transmission at its own convenience. However, a ULP can exercise a push service to force TCP to package and send data passed up to that point without waiting for additional data. This mechanism is intended to prevent possible deadlock situations where a ULP waits for data internally buffered by TCP. For example, an interactive editor might wait forever for a single input line from a terminal. A push will force data through the TCPs to the awaiting process. A TCP also provides the means for a sending ULP to indicate to a receiving ULP that “urgent” data appear in the upcoming data stream. This urgent mechanism can support, for example, interrupts or breaks. When a data exchange is complete, the connection can be closed by either ULP to free TCP resources for other connections. Connection closing can happen in two ways. The first, called a graceful close, is based on the three-way handshake procedure to complete data exchange and coordinate closure between the TCPs. The second, called an abort, does not allow coordination and may result in the loss of unacknowledged data. [Note: There is a certain military flavor in the TCP/ IP protocol family. There is good reason; its development (around 1975) was supported by the U.S. Department of Defense for ARPANET. Since then, this protocol family has become extremely popular in the commercial world (e.g., Internet), both for LAN and WAN operations.]

12.4 INTEGRATED SERVICES DIGITAL NETWORKS (ISDN)

12.4.1 Background and Objectives

The original concept of ISDN dates back to the early 1970s. Its design, in the context of the period, was built around the copper distribution plant (subscriber loop and local trunk plant). The designers saw and understood that by the early 1980s there would be a digital network in place controlled by CCITT Signaling System No. 7 (Chapter 13). It was revolutionary for its time by bringing 64-kbps digital channels right into the home and office. With the ISDN design, the 64-kbps digital channel handles:

•Voice telephony (digital);

•Digital data, both packet switched and circuit switched;

•Telex/ teletext;

•Facsimile (e.g., CCITT Group 4); and

•Conference television (56, 64, or 128 kbps).

The goal of ISDN is to provide an integrated facility to incorporate each of the services just listed on a common 64-kbps channel. This section provides an overview of that integration.

In North America, one gets the distinct feeling the certain technologies [e.g., ATM (Chapter 18), frame relay (Section 12.5) and gigabit enterprise networks (Chapter 11), and EHF wireless local loops)] have leap-frogged ISDN. Yet ISDN keeps seeming to have second, third, . . . renaissances in North America and has had a fairly high penetration in Europe. It seems that the ISDN market is very price/ cost driven.

ISDN, in itself, is really only an interface specification at customer premises and at

the first serving digital exchange. This digital exchange is part of a larger digital network that must have CCITT Signaling System No. 7 implemented and operational for ISDN to work end-to-end successfully (Ref. 11).

12.4.2 ISDN Structures

12.4.2.1 ISDN User Channels. Here we look from the user into the network. We consider two user classes: residential and commercial. The following are the standard bit rates for user access links:

•B-channel: 64 kbps;

•D-channel: 16 kbps or 64 kbps, and

•H-channels (discussed in the following).

The B-channel is the basic user channel. It is transparent to bit sequences. It serves all the traffic types listed in Section 12.4.1.

In one configuration, called the basic rate, the D-channel has a 16-kbps data rate; in another, called the primary rate, it is 64 kbps. Its primary use is for signaling. The 16kbps version, besides signaling, may serve as transport for low-speed data applications, particularly those using X.25 packet data (see Section 12.2).

There are a number of H-channels:

•H0 channel: 384 kbps;

•1536 kbps (H11) and 1920 kbps (H12).

The H-channel is intended to carry a variety of user information streams. A distinguishing characteristic is that an H-channel does not carry signaling information for circuit switching by the ISDN. User information streams may be carried on a dedicated, alternate (within one call or separate calls), or simultaneous basis, consistent with the H-channel bit-rates. The following are examples of user information streams:

•Fast facsimile;

•Video, such as video conferencing;

•High-speed data;

•High-quality audio or sound program channel;

•Information streams, each at rates lower than the respective H-channel bit rate (e.g., 64-kbps voice), which have been rate-adapted or multiplexed together; and

•Packet-switched information.

12.4.2.2 Basic and Primary User Interfaces. The basic rate interface structure is composed of two B-channels and a D-channel referred to as “2B + D.” The D-channel at this interface is 16 kbps. The B-channels may be used independently (i.e., two different simultaneous connections). Industry and much of the literature call the basic rate interface the BRI.

Appendix I to CCITT Rec. I.412 (Ref. 11) states that alternatively the basic access may be just one B-channel and a D-channel, or just a D-channel.

The primary rate interface (PRI) structures are composed of n B-channels and one

Figure 12.15 ISDN reference model.

•Electrical and mechanical specification;

•Channel structure and access capabilities;

•User–network protocols;

•Maintenance and operation;

•Performance; and

•Services.

ISDN specifications as set out by the ITU-T I Recommendations and relevant Bellcore/ ANSI specifications cover these six items.

Figure 12.15 shows the conventional ISDN reference model. It delineates interface points for the user. In the figure NT1, or network termination 1, provides the physical layer interface; it is essentially equivalent to OSI layer 1. Functions of the physical layer include:

•Transmission facility termination;

•Layer 1 maintenance functions and performance monitoring;

•Timing;

•Power transfer;

•Layer 1 multiplexing; and

•Interface termination, including multidrop termination employing layer 1 contention resolution.

Network termination 2 (NT2) can be broadly associated with OSI layers 1, 2, and 3. Among the examples of equipment that provide NT2 functions are user controllers, servers, LANs, and PABXs. Among the NT2 functions are:

•Layers 1, 2, and 3 protocol processing;

•Multiplexing (layers 2 and 3);

•Switching;

•Concentration;

•Interface termination and other layer 1 functions; and

•Maintenance functions.

A distinction must be drawn here between North American and European practice. Historically, telecommunication administrations in Europe have been, in general, national monopolies that are government controlled. In North America (i.e., United States and Canada) they are private enterprises, often very competitive. In Europe, NT1 is considered as part of the digital network and belongs to the telecommunica-

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tions administration. The customer ISDN equipment ISDN equipment starts at the T interface. In North America, both NT1 and NT2 belong to the ISDN user, and the U interface defines the network entry point.

It should be noted that there is an overall trend outside of North America to privatize telecommunications such as has happened in the United Kingdom and is scheduled to take place in other countries such as Germany, Mexico, and Venezuela.

TE1 in Figure 12.15 is the terminal equipment, which has an interface that complies with the ISDN terminal–network interface specifications at the S interface. We will call this equipment ISDN compatible. TE1 covers functions broadly belonging to OSI layer 1 and higher OSI layers. Among the TE1 equipment are digital telephones, computer work stations, and other devices in the user end-equipment category that is ISDN compatible.

TE2 in Figure 12.15 refers to equipment that does not meet the ISDN terminal–network interface at point S. TE2 adapts the equipment to meet that ISDN terminal–network interface. This process is assisted by the TA, the terminal adapter.

Reference points T, S, and R are used to identify the interface available at those points. T and S are identical electrically and mechanically, and from the point of view of protocol. Point R relates to the TA interface or, in essence, it is the interface of that nonstandard (i.e., non-ISDN) device. The U-interface is peculiar to the North American version of ISDN.

We will return to user–network interfaces once the stage is set for ISDN protocols looking into the network from the user.

12.4.4 ISDN Protocols and Protocol Issues

When fully implemented, ISDN will provide both circuit and packet switching. For the circuit-switching case, now fairly broadly installed in North America, the B-channel is fully transparent to the network, permitting the user to utilize any protocol or bit sequence so long as there is end-to-end agreement on the protocol utilized.5 Of course, the protocol itself should be transparent to bit sequences.

It is the D-channel that carries the circuit-switching control function for its related B-channels. Whether it is the 16-kbps D-channel associated with BRI or the 64-kbps D-channel associated with PRI, it is that channel which transports the signaling information from the user’s ISDN terminal from NT to the first serving telephone exchange of the telephone company or administration. Here the D-channel signaling information is converted over to CCITT No. 7 signaling data employing ISUP (ISDN User Part) of SS No. 7. Thus it is the D-channel’s responsibility for call establishment (setup), supervision, termination (takedown), and all other functions dealing with network access and signaling control.

The B-channel in the case of circuit switching is serviced by NT1 or NT2 using OSI layer 1 functions only. The D-channel carries out OSI layers 1, 2, and 3 functions such that the B-channel protocol established by a family of ISDN end-users will generally make layer 3 null in the B-channel where the networking function is carried out by the associated D-channel.

With packet switching two possibilities emerge. The first basically relies on the B-channel to carry out OSI layers 1, 2, and 3 functions at separate packet-switching facilities (PSFs). The D-channel is used to set up the connection to the local switching exchange at each end of the connection. This type of packet-switched offering provides 64-kbps service. The second method utilizes the D-channel exclusively for

5“Broadly installed” means that most of the public carriers can offer the service to their customers.

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Figure 12.18 Basic architectural model of ISDN. (From ITU-T Rec. I.324, Figure 1/ I.324, p. 3, [Ref. 12].)

the first three OSI layers for call setup to the local switching center at each end of the circuit. (3) The D-channel signaling data are turned over to CCITT Signaling System No. 7 (SS No. 7) at the nearand far-end local switching centers. (4) SS No. 7 also utilizes the first three OSI layers for circuit establishment, which requires the transfer of control information. In SS No. 7 terminology, this is called the message transfer part. There is a fourth layer called the user part in SS No. 7. There are three user parts: telephone user part, data user part, and ISDN user part, depending on whether the associated B-channel is in telephone, data, or ISDN service for the user.

12.4.5 ISDN Networks

In this context, ISDN networking is seen as a group of access attributes connecting an ISDN user at either end to the local serving exchange (i.e., the local digital switch). This is illustrated in Figure 12.18, the basic architectural model of ISDN. It is here that the ISDN circuits meet the national transit network (CCITT terminology). This is the public switched digital telephone network. That network, whether DS1-DS4-based or E1-E5-based, provides two necessary attributes for ISDN compatibility:6

6We should add “as well as SONET-based or SDH-based.”

Figure 12.19 A generic communication context showing ISDN’s relationship with the seven-layer OSI reference model. Note that the end-system protocol blocks may reside in the subscriber’s TE or network exchanges or other ISDN equipment.

1. 64-kbps channelization (Note: In North America this may be 56 kbps);

2. Separate channel signaling based on CCITT Signaling System No. 7 (Chapter 13).

Connections from the ISDN user at the local connecting exchange interface include:

• Basic service (BRI) 2B + D c 192 kbps (CCITT specified)

c 160 kbps (North American/ Bellcore specified). Both rates include overhead bits.

• Primary rate service (PRI) 23B + D/ 30B + D c 1.544/ 2.048 Mbps.

Note: For E-1 service, it is assumed that the user will provide the synchronization channel, Channel 0. It is not the responsibility of the ISDN service provider.

12.4.6 ISDN Protocol Structures

12.4.6.1 ISDN and OSI. Figure 12.19 shows the ISDN relationship with OSI. (OSI was discussed in Chapter 10.) As is seen in the figure, ISDN concerns itself with only the first three OSI layers. OSI layers 4 through 7 are peer-to-peer connections and are the end-user’s responsibility. Remember that the B-channel deals with OSI layer 1 exclusively. There is one exception. That is when the B-channel is used for packet data service. In this case it will deal with the first three OSI layers.

Of course, with the BRI service, the D-channel is another exception. The D-channel interfaces with CCITT Signaling System No. 7 at the first serving exchange. These (BRI) D-channels handle three types of information: (1) signaling (s), (2) interactive data (p), and (3) telemetry (t).

The layering of the D-channel has followed the intent of the OSI reference model. The handling of the p and t data can be adapted to the OSI model; the s data, by its very

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Figure 12.20 Correspondence among the ISDN D-channel, CCITT Signaling System No. 7, and the OSI mode. ( 1985 IEEE Computer Soc. Press, Washington, DC [Ref. 13].)

nature, cannot.7 Figure 12.20 shows the correspondence between D-channel signaling protocols, SS No. 7 levels, and the OSI seven-layer model.

12.4.6.2 Layer 1 Interface, Basic Rate. The S/ T interface of the reference model, Figure 12.15 (or layer 1, physical interface), requires a balanced metallic transmission medium (i.e., copper pair) in each direction of transmission (four-wire) capable of supporting 192 kbps. Again, this is the NT interface of the ISDN reference model.

Layer 1 provides the following services to layer 2 for ISDN operation:

•The transmission capability by means of appropriately encoded bit streams of the B- and D-channels and also any timing and synchronization functions that may be required.

•The signaling capability and the necessary procedure to enable customer terminals and/ or network terminating equipment to be deactivated when required and reactivated when required.

•The signaling capability and necessary procedures to allow terminals to gain access to the common resource of the D-channel in an orderly fashion while meeting the performance requirements of the D-channel signaling system.

•The signaling capability and procedures and necessary functions at layer 1 to enable maintenance functions to be performed.

•An indication to higher layers of the status of layer 1.

12.4.6.2.1 Interface Functions. The S and T functions for the BRI consist of three bit streams that are time-division multiplexed: two 64-kbps B-channels and one 16-kbps D-channel for an aggregate bit rate of 192 kbps. Of this 192 kbps, the 2B + D configura-

7For CCITT Signaling System No. 7, like any signaling system, the primary quality-of-service measure is “post dial delay.” This is principally the delay in call setup. To reduce the delay time as much as possible, it is incumbent upon system engineers to reduce processing time as much as possible. Thus SS 7 truncates OSI to four layers, because each additional layer implies more processing time.