Intel ER8100STUS - Express 8100 Router, Справочное руководство

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Reference.bok : Chapter05 33 Wed Apr 22 10:58:08 1998
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X.25 Characteristics
X.25 Services
X.25 device
characterization
At the physical layer (layer 1 of the OSI reference model), X.25 defines LAN
communications devices (for example a router) as Data Terminal Equipment
(DTE) and PSDN nodes as Data Communication-terminating Equipment (DCE).
The X.25 protocol defines the communication interactions between the DTE and
DCE devices, and is independent of the structure and protocols used between the
two DCE devices and over the PSDN.
Physical layer
X.25 specifies the use of full duplex, point-to-point synchronous circuits for the
physical layer.
Link layer
The primary responsibility of the link layer protocol is to ensure the error-free ex-
change of data over a link which has variable delays. The X.25 standard defines
the use of either Link Access Procedure (LAP) or Link Access Procedure, Bal-
anced (LAPB) where LAPB is the preferred link layer protocol and is the proto-
col implemented in the Intel Express 8100 Router.
Network layer
X.25 network layer (also known as packet layer) services give X.25 its virtual cir-
cuit characteristics. In addition to sending and receiving data packets, the net-
work layer is responsible for:
■
establishing and clearing virtual circuits across the PSDN
■
recovering from error conditions (reset and restart procedures)
1226
DCE
X.25 X.25
Packet Switched
Data Network
(PSDN)
DTE DCE DTE