ADTRAN 1200212L1, Руководство пользователя

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Chapter 1. Introduction
61200212L1-1 DSU IQ User Manual 1-3
UNDERSTANDING FRAME RELAY
Frame relay is a wide area network (WAN) service designed to
minimize physical connections. This is accomplished by using
virtual connections within the frame relay cloud and accessing
these virtual circuits with normally one physical connection at each
location to the frame relay service. Virtual circuits are addressed
using header information at the beginning of each frame. These
frames are formatted by the user's customer premise equipment
(CPE) such as the ADTRAN DSU IQ.
ANSI (American National Standards Institute) standards describe
how each frame must be constructed to provide inter-operability
between CPE equipment and frame relay switching equipment.
Each frame must contain a header, at least one byte of information
data, two bytes of CRC16, and a trailing flag 0x7E.
This header information contains a virtual circuit address known as
a DLCI (data link connection identifier). The header information
also contains bits used for network congestion control.
Frame relay virtual circuits may be defined as permanent (PVC) or
switched (SVC). PVCs have the same DLCI for a given path each
time a user protocol session is established. The network service
provider assigns these DLCIs at subscription time. SVCs, on the
other hand, have DLCIs dynamically assigned each time a user
protocol session is established. The CPE equipment must request a
call and the DLCI is assigned by the network switching equipment.
This DLCI is valid until the call is disconnected and may be
assigned a different value each time a call is requested.
DDS OPERATION
DDS (digital data service) is a nationwide service that allows
interconnection and transport of data at speeds up to 64 kbps. The
local exchange carriers provide the local loop service to DDS
customers and may provide data for routing Inter-LATA to an
inter-exchange carrier. The DSU IQ supports 56 or 64 kbps DDS
service rates yielding DTE rates of 56 kbps and 64 kbps.