Juniper JUNOSE 11.1.X - LINK LAYER CONFIGURATION 4-7-2010, Руководство по настройке

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Depending on the type of connection you choose, you can specify one or more PVCs
on each interface. For a standard point-to-point ATM interface, you configure only
one PVC. For NBMA ATM connections, you configure multiple circuits.
ATM Virtual Connections
A virtual connection (VC) defines a logical networking path between two endpoints
in an ATM network. ATM cells travel from one point to the other over a virtual
connection. An ATM cell is a package of information that is always 53 bytes in length,
unlike a frame or packet, which has a variable length. An ATM cell has a cell header
and a
payload . The payload contains the user data.
The cell header includes an 8-bit virtual path identifier (VPI) and a 16-bit virtual
channel identifier (VCI).
An ATM network can have two types of VCs, depending on the addressing used to
switch the traffic:
■
Virtual channel connection (VCC)
■
Virtual path connection (VPC)
Virtual Channel Connection
A VCC uses all the addressing bits of the cell header to move traffic from one link to
another. The VCC is formed by joining a series of virtual channels (VCs), which are
logical circuits uniquely identified for each link of the network. On a VCC, switching
is done based on the combined VPI and VCI values.
Virtual Path Connection
A VPC uses the higher-order addressing bits of the cell header to move traffic from
one link to another. A VPC carries many VCCs within it. A VPC can be set up
permanently between two points, and then switched.
VCCs can be assigned within the VPC easily and quickly. The VPC is formed by joining
a series of virtual paths, which are the logical groups of circuits uniquely defined for
each link of the network. On a VPC, switching is done based on the VPI value only.
ATM SVCs
JUNOSe software does not support configuration and monitoring of ATM switched
virtual circuits (SVCs) on the router.
ATM Adaptation Layer
The ATM Adaptation Layer (AAL) defines the conversion of user information into
cells by segmenting upper-layer information into cells at the transmitter and
reassembling them at the receiver. AAL1 and AAL2 handle intermittent traffic, such
as voice and video, and are not relevant to the router. AAL3/4 and AAL5 support data
communications by segmenting and reassembling packets.
Overview
■ 5
Chapter 1: Configuring ATM