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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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2.5 Integrated Services Digital Network (ISDN)<br />

This section describes how to use the PPP encapsulation over ISDN<br />

point-to-point links. PPP over ISDN is documented by elective RFC 1618.<br />

Because the ISDN B-channel is, by definition, a point-to-point circuit, PPP is well<br />

suited for use over these links.<br />

The ISDN Basic Rate Interface (BRI) usually supports two B-channels with a<br />

capacity of 64 kbps each, <strong>and</strong> a 16 kbps D-channel for control information.<br />

B-channels can be used for voice or data or just for data in a combined way.<br />

The ISDN Primary Rate Interface (PRI) can support many concurrent B-channel<br />

links (usually 30) <strong>and</strong> one 64 Kbps D-channel. The PPP LCP <strong>and</strong> NCP<br />

mechanisms are particularly useful in this situation in reducing or eliminating<br />

manual configuration <strong>and</strong> facilitating ease of communication between diverse<br />

implementations. The ISDN D-channel can also be used for sending PPP<br />

packets when suitably framed, but is limited in b<strong>and</strong>width <strong>and</strong> often restricts<br />

communication links to a local switch.<br />

PPP treats ISDN channels as bit- or octet-oriented synchronous links. These<br />

links must be full-duplex, but can be either dedicated or circuit-switched. PPP<br />

presents an octet interface to the physical layer. There is no provision for<br />

sub-octets to be supplied or accepted. PPP does not impose any restrictions<br />

regarding transmission rate other than that of the particular ISDN channel<br />

interface. PPP does not require the use of control signals. When available, using<br />

such signals can allow greater functionality <strong>and</strong> performance.<br />

The definition of various encodings <strong>and</strong> scrambling is the responsibility of the<br />

DTE/DCE equipment in use. While PPP will operate without regard to the<br />

underlying representation of the bit stream, lack of st<strong>and</strong>ards for transmission will<br />

hinder interoperability as surely as lack of data link st<strong>and</strong>ards. The D-channel<br />

interface requires Non-Return-To-Zero (NRZ) encoding. Therefore, it is<br />

recommended that NRZ be used over the B-channel interface. This will allow<br />

frames to be easily exchanged between the B- <strong>and</strong> D-channels. However, when<br />

the configuration of the encoding is allowed, NRZ Inverted (NRZI) is<br />

recommended as an alternative in order to ensure a minimum ones density<br />

where required over the clear B-channel. Implementations that want to<br />

interoperate with multiple encodings can choose to detect those encodings<br />

automatically. Automatic encoding detection is particularly important for primary<br />

rate interfaces to avoid extensive preconfiguration.<br />

Terminal adapters conforming to V.120 1 can be used as a simple interface to<br />

workstations. The terminal adapter provides asynchronous-to-synchronous<br />

conversion. Multiple B-channels can be used in parallel. V.120 is not<br />

interoperable with bit-synchronous links, because V.120 does not provide octet<br />

38 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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