AT&T DEFINITY 7200 series Podręcznik Użytkownika Pobierz pdf (Strona 149)

DATA CONNECTIVITY — AN OVERVIEW 7-7

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1 1 1 1 1 1 1 1SERIAL

0 0 0 0 0 0 0 0

PARALLEL

Figure 7-4. Serial vs. Parallel

Type of Communications Channel

Channels can be classiﬁed according to the the type of switch that creates them, the method used to assign

them, the type of information they pass, and the transmission mode.

Creating Switch — Packet vs. Circuit Switches

Data can be channeled through packet switches and/or circuit switches:

• Packet switches transport packets of data — consisting of header, trailer, and text information — as

integral units from their origin to destination. To transport the packets, these switches provide one or

more paths that all connections share. Along these paths, users are assigned a virtual circuit so that

individual communications streams can be distinguished. The virtual circuit consists of all packets of

data that are addressed to the same destination.

• Circuit switches provide a dedicated path between two endpoints. On circuit switches, data is sent as a

continuous stream of bits, bandwidth is guaranteed, and delay is limited to propagation time.

The DIMENSION, System 75, System 85, and Generic 1 and 2 communications systems provide only circuit

switching, although they can interface with packet switches through external packet assembler/disassembler

devices. See chapter 8 for more details on the type of connection.

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AT&T DEFINITY 7200 series Podręcznik Użytkownika Strona 149