3Com LANPLEX 2500, Руководство по эксплуатации

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11-8 C
HAPTER
11: ATM N
ETWORKS
The primary layers of the BISDN reference model are:
■
The Physical Layer
■
The ATM Layer
■
The ATM adaptation Layer
The Physical Layer The Physical Layer defines how cells are transported over the network. This
are of influence includes physical interfaces, media, and information rates.
The Physical Layer also defines how cells are converted to a line signal
depending on the media type. ATM is media independent in that it is not
tied to any particular physical layer. The ATM Forum UNI Specification
Version 3.0 provides specifications for physical layer interfaces for both the
public and private user-network interfaces.
SONET Physical Layer Interface
Synchronous Optional Network (SONET) is the physical layer most often
associated with ATM. SONET provides, through a framing structure, the
payload envelope necessary for the transport of ATM cells.
The LANplex system uses SONET OC-3 as the Physical Layer interface for the
ATM network.
The ATM Layer The ATM Layer provides a single mode of transport for many types of
telecommunications services. Except for the quality of service (QOS)
requested for the virtual circuit, the ATM Layer is unaware of the type of
information (voice, video, or data) that it is carrying.
Quality of Service
Traffic management is concerned with ensuring that users get their desired
quality of service. The ATM layer Quality of Service (QOS) is defined by a set
of parameters such as
delay , delay variation , and cell loss ratio . While setting
up a connection on an ATM network, specific rate parameters related to the
desired quality of service can be requested.
The applications that are transported by the ATM Layer are divided into four
service classes: constant bit rate (CBR), variable bit rate (VBR), available bit
rate (ABR), and unspecified bit rate (UBR).
The LANplex supports unspecified bit rate (UBR) only.