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Configuring SMDS

1-4

117361-A Rev. A

Data Exchange Interface Protocol

For SMDS high-speed access classes, that is, speeds from 1.2 Mb/s to 34 Mb/s,

the SMDS Interest Group (SIG) wanted to simplify the integration of SMDS into

existing network equipment, and to hasten SMDS into the market. To do this, they

divided the functions deﬁned by the SIP levels between devices that handle local

network packets (for example, a router) and devices that interface with the digital

services provided by common carriers (a DSU/CSU).

The Data Exchange Interface (DXI) protocol deﬁnes this division of tasks and

describes the router and DSU/CSU relationship (F

igure 1-4). Refer to the next

section for more information.

Figure 1-4. DXI Protocol

For low-speed SMDS (56 Kb/s to 64 Kb/s), the DXI is extended out to the SMDS

network. In this case, you do not need a special SMDS DSU/CSU.

PDU Assembly

SMDS PDU assembly begins when the router receives a network-generated

packet. The router takes the entire packet and encapsulates it within a 36-byte

header (containing addressing, length, and control information) and a 4-byte

trailer (containing a CRC value), thereby creating an SMDS Level-3 PDU

igure 1-5).

Router

DSU/CSU

Router

DSU/CSU

DXI protocol defines the router

and DSU/CSU interface

SMDS

network

SMDS

switch

SMDS

switch

SMS0005A

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Avaya Configuring SMDS Bedienungsanleitung Seite 18