ENTERPRISE NETWORKS II: WIDE AREA NETWORKS
LAPD or DL-CORE. This refers to a reduced subset of LAPD found in Annex A of ITU-
T Rec. Q.922 (Ref. 16). The basic body of Q.922 presents CCITT/ITU-T specification
for frame relay. This derivative is called LAPF rather than LAPD. The material found
in ANSI T1.618-1991 (Ref. 17) is identical for all intents and purposes with Annex A
To properly describe frame relay from our perspective, we will briefly give an overview
of the ANSI T1.618-1991 (Ref. 17) and T1.606-1990 (Ref. 18). This will be followed by
some fairly well identified variants.
Introduction to Frame Relay Operation
Frame relay may be considered a cost-effective outgrowth of ISDN, meeting high data rate
(e.g., 2 Mbps) and low delay data communications requirements. Frame relay encapsulates
data files. These may be considered "packets," although they are called frames. Thus
frame relay is compared to CCITT Rec. X.25 packet service. Frame relay was designed
for current transmission capabilities of the network with its relatively wider bandwidths
and excellent error performance (e.g., BER better than 1
The incisive reader will note the use of the term bandwidth. It is used synonymously
with bit rate. If we were to admit at first approximation 1 bit per hertz of bandwidth, such
use is acceptable. We are mapping frame relay bits into bearer channel bits probably on
a one-for-one basis. The bearer channel may be a DS0/E0 64-kbps channel, a 56-kbps
channel of a DS1 configuration, or multiple DS0/E0 channels in increments of 64 kbps
up to 1.544/2.048 Mbps. We may also map the frame relay bits into a SONET or SDH
configuration (Chapter 17). The final bearer channel may require more or less bandwidth
than that indicated by the bit rate. This is particularly true for such bearer channels riding
on radio systems and, to a lesser extent, on a fiber-optic medium or other transmission
media. The reader should be aware of certain carelessness of language used in industry
Frame relay works well in the data rate range from 56 kbps up to 1.544/2.048 Mbps.
It is being considered for the 45-Mbps DS3 rate for still additional speed.
ITU-T's use of the ISDN D-channel for frame relay favors X.25-like switched virtual
circuits (SVCs). However, ANSI recognized that the principal application of frame relay
was interconnection of LANs, and not to replace X.25. Because of the high data rate
of LANs (megabit range), dedicated connections are favored. ANSI thus focused on
permanent virtual connections (PVCs). With PVCs, routes are provisioned at the time of
frame relay contract. This notably simplified the signaling protocol. Also, ANSI frame
relay does not support voice or video.
As mentioned, the ANSI frame relay derives from ISDN LAPD core functions. The
core functions of the LAPD protocol that are used in frame relay (as defined here) are
Frame delimiting, alignment, and transparency provided by the use of HDLC flags
and zero bit insertion/extraction.
Frame multiplexing/demultiplexing using the address field.
Inspection of the frame to ensure that it consists of an integer number of octets prior
to zero bit insertion or following zero bit extraction.
We would rather use the term greater bit rate capacity.
Zero bit insertion is a technique used to assure that the unique beginning flag of a frame is not imitated inside
the frame that allows full transparency.
Where the DLCI indicates a particular channel or channel group in the multiplex aggregate for PVC operation.