Avaya Configuring SMDS Bedienungsanleitung PDF herunterladen (Seite 16)

Configuring SMDS

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117361-A Rev. A

The point at which the DSU/CSU meets the network is the subscriber network

interface (SNI). This is the access point to the SMDS network. Within the

switching system of the SMDS network, T3 communication lines connect the

network switches together.

Because data trafﬁc and network equipment are different for each SMDS

customer, SMDS deﬁnes several

access classes

. Access classes are speeds at

which data travels from the customer site to the network switch.

SMDS currently offers six network access classes: 1.2 Mb/s, 4 Mb/s, 10 Mb/s,

16 Mb/s, 25 Mb/s, and 34 Mb/s. The 1.2 Mb/s class is for T1 lines. Classes

4 Mb/s through 34 Mb/s are for T3 lines. SMDS also offers low-speed SMDS

classes, which operate at fractional T1/E1 speeds (56 Kb/s up to 64 Kb/s).

How SMDS Sends LAN Data over WANs

LAN data is typically connectionless data. This means that it has addressing

information in each frame, so there is no need for a prior connection between the

origination and destination devices. WAN data is typically connection-oriented

data. This means that it needs a virtual circuit -- that is, a predeﬁned path across

physical lines -- between the two connection points prior to sending data. SMDS

provides connectionless data transfer across a wide area network without

establishing a logical end-to-end connection.

SMDS Interface Protocol

The SMDS Interface Protocol (SIP) deﬁnes SMDS addressing, formatting,

framing, and error-detection requirements.

The SIP has three levels that are similar to, but do not match, the protocol layers

that make up the OSI protocol model.

• Level 3 speciﬁes the addressing, formatting, and encapsulation of packet data,

referred to as Level-3 PDUs.

• Level 2 speciﬁes the segmentation of Level-3 Protocol Data Units (PDUs)

into short, ﬁxed-length SMDS cells, referred to as Level-2 PDUs.

• Level 1 speciﬁes the physical connectivity that enables transmission.

igure 1-2 illustrates the SIP protocol stack.

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Avaya Configuring SMDS Bedienungsanleitung Seite 16