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Protocol Conversion

As an option, packet-switched networks will accomplish protocol conversion (Figure 8.8). Protocol conversion can include any protocol that is well-established, well-understood, widely-deployed and, therefore, supported by the carrier. As this process of protocol conversion adds value, packet networks (X.25) are widely recognized as the first Value-Added Networks (VANs). Protocols supported typically include asynchronous, IBM Bisync (BSC), and IBM SDLC.


Figure 8.8  Disparate packet networks interconnected via X.75, with protocol conversion.

Latency

Latency or delay, is a troublesome and limiting characteristic of packet networks. As each packet may take a different route though the network, each may travel a route of a different length; therefore, propagation delay may vary from packet to packet. Additionally, each packet may travel through a different number of packet nodes, each of which must act on the packet to read its address, check for errors, request retransmissions of errored packets, and so on—this fact compounds the issue of packet delay. Further, each packet may encounter different levels of congestion in the network, which may add to packet delays. Finally, additional delay is imposed on each packet if protocol conversion is required. While this process adds value, it does add to the latency factor. The end result is that some level of latency not only is assured, but also is variable and uncertain in magnitude [8-6].

While this characteristic of packet switching does not affect some applications, it renders others ineffective. For instance, e-mail communications over the Internet is not seriously impacted, although the delays may be aggravating at times. On the other hand, isochronous (stream-oriented) communications such as realtime audio, voice, or video are not effectively supported over a packet network.

Access

X.25 actually is the ITU-T standard describing the physical, link and packet level protocols between the user DTE/DCE and the network [8-17] and [8-18]. Devices capable of packetizing the data are connected over a X.25 link. In a true X.25 environment, the user accomplishes the packetizing process through DCE in the form of a PAD (Packet Assembler/Disassembler), the standard for which is X.3. The PAD also may contain intelligence for password protection and performance reporting [8-8].

Occasional or casual users typically access a packet network on a dialup basis, from asynchronous PCs through modems. In such a scenario, the actual packetizing of the data is performed at the originating network node. For example, individuals accessing the Internet through an online information service often use this approach.

Large user organizations often access the network via a dedicated, leased-line link to the closest network node. Such access often is in the form of an unchannelized T1 facility, perhaps supporting frame relay. Large users connecting substantial hosts to a X.25 network present data to the node under the terms of a protocol known as subscriber Link Access Procedure Balanced (LAPB), which ensures error-free local access and egress on a full duplex (FDX) basis. The user data is segmented into a packet by the PAD, and encapsulated in a LAPB frame before presentation to the network [8-18] and [8-20]. The frame level procedure described in X.25 can be either the ISO High Level Data Link Control Procedure (HDLC) or IBM Bisync (BSC) [8-20]. Internet Service Providers (ISPs), for instance, make heavy use of such dedicated facilities.

Network Interconnection

X.25 networks are widely available as a Public Data Network (PDN) service offering, generally using packets of 128B or 256B. However, certain applications are supported more effectively by transmission of larger packets. For example, the airline reservation systems (e.g., American Airlines’ SABRE and United Airlines’ APOLLO) have deployed custom packet networks that use packet payloads of 1,028B. As this application involves the frequent transmission of relatively large sets of data (e.g., flight schedules, fares, and seating availability), a larger packet size is more appropriate. The larger packet size improves efficiency, because the payload is very large, while the overhead information is roughly the same as in the case of a smaller packet. While a larger packet is more likely to contain an errored bit, require retransmission and, therefore, reduce throughput; the custom reservation networks employ digital facilities in order to minimize this exposure.

The interconnection of such disparate networks is accomplished through an ITU-T standard known as X.75. Through an X.75 network-to-network interface, as depicted in Figure 8.8, issues of packet size are resolved in order that information flow is not affected.

Packet-Switching Hardware

The user of a X.25 packet network may require no hardware other than a PC and modem. Occasional and casual users of the Internet through an online information service (e.g., America Online, Australia Online, CompuServe and Prodigy) fall into this category. The packetizing of the asynchronous data is performed at the local X.25 node. Larger users will install DCE in the form of a Packet Assembler/Disassembler (PAD), which is specified by the ITU-T as X.3. The PAD performs the packet assembly (segmentation) of the data for the transmitting device, disassembling the packet for the receiving device in order to reconstitute the data in its native format.

Packet carriers, of course, must invest in packet nodes, rather than circuit switches. Such packet nodes are intelligent devices capable of supporting complex routing tables, buffering packets in temporary memory, resolving packet errors, and accomplishing protocol conversions.

Packet Switching Standards

The ITU-T sets standards recommendations for packet switching. Those standards include the following [8-6], [8-8] and [8-20]:

  X.3: Packet Assembly/Disassembly functions
  X.25: Interface between DCE and DTE for public packet networks
  X.28: Terminal-to-PAD communications formats
  X.29: Host-to-PAD communications formats
  X.31: Packet-mode services over ISDN
  X.32: Defines X.25 synchronous dialup mode
  X.75: X.25 internetwork call control procedures


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