20
Frame Relay
For data transfer, X.25-based packet switching has established itself worldwide as
a
standard and
very reliable means. However, X.25 is not a technique suited to the higher quality and speeds of
modern data communications networks, and
so
it is beginning
to
be supplanted by new techniques,
among them ‘frame relay’. In this chapter we start by discussing the shortcomings of X.25-based
packet switching in carrying highspeed bitrates and explain how frame relay was designed to
overcome these problems.
We
conclude with a more detailed review of the frame relay protocols
themselves.
20.1
THE THROUGHPUT LIMITATIONS
OF
X.25
PACKET SWITCHING
The reliability of X.25 packet switching has resulted from worldwide accepted
standards and the huge availability of compatible hardware and software products
enabling computer devices made by different manufacturers and strewn around the
world to intercommunicate without difficulty. X.25 was the first universal data
communication protocol and it stimulated rapid growth in data communication traffic
volumes, because of its reliability and its robustness. Paradoxically, its robustness is
now leading to the demise of X.25, because one of the main limitations of packet
switching based on X.25 is its unsuitability for carriage of high speed information
channels and its relative inefficiency when used in conjunction with high quality

large number of bits in transit on the line at any point in time (because of its length), in
our example around 20
000
bits or 2500 bytes (line length
X
bitrate/speed of light). (That
should blow any preconception you might have had that electricity travels
so
fast that
we can consider sender and transmitter to be in synchronism with one another!) These
bits in transit on the line must be considered when designing high speed data networks,
if
the network is to operate efficiently.
X.25 lays a very high priority on the safe arrival of bits, in the correct order and
without errors. One of the methods used to ensure safe arrival is the use of an
acknowledgement
window.
Only
so
many packets (as defined by the
window size,
typically 7) may be transmitted by the sending device before an acknowledgement is
received confirming safe arrival.
As
the typical maximum packet size is defined as
256 bytes, this means that a maximum of 1792 bytes (7
X
256) may be transmitted by
the sender before an acknowledgement
is

n
~
pulse travels at around
10'
m/s
4
1-
bit length
=
speed in m/S
=
50 m
bitrate
2
X
10'
sender network
(2048
kbitls) receiver
number
of
bits in transit
on
the line
=
line length
I
bit length
=
1000

wide area
computer and LAN-to-LAN connection needs of 64 kbit/s or
greater,
frame relay
is today’s preferred method. However, for bitrates above
2
Mbit/s,
native ATM (see Chapter 26) should be considered.
20.2 THE NEED FOR FASTER RESPONSE DATA NETWORKS
Although the basic need to carry high bandwidth signals drove the need for the
development of the
frame relay
protocols,
so
did the need for faster responding net-
works. It may not be immediately obvious, but the time required to propagate even low
bandwidth information across a digital network is dependent on the bitspeed employed:
the higher the bitspeed, the lower the propagation delay. Even data applications with
limited information transport needs appear to run faster when carried by a high speed
network, even though sufficient bandwidth may have previously already been available.
The following discussion gives two reasons why.
Let us imagine two rainwater conduits, one of small bore and one of large bore. Let us
assume that the first has throughput capacity of 5litres/s and the second of 10litres/s.
Now let us assume that the rainfall rate is 4 litre+. Why should
I
bother with the large
bore conduit? The answer is that the rainfall rate is not constant. Over the course of time
the rate may vary between, say
2
and 6 litres per second, so that during moments in time

bits) is sensitive to the transmission bitspeed. The propagation
time in this case is equal to the sum of the raw propagation time and the signal duration
(Figure 20.2(b)).
Taking the example of a 9600 bit/s dataline of
100 km length (as might be employed
in a corporate packet switching network today), we can calculate propagation times for
both cases
(a)
and (b) of our example:
382
FRAME RELAY
(a)
DL-,
pulse travels at around
10’
mh
sender
network
receiver
(b) signal pattern travels at
108
mlS
l+ +
signal duration
=
number
of
bitslbitspeed
Figure
20.2

In other words, before enough bits
(8)
have been received to interpret the frame (in our
case a data or
ASCII
character),
1.8
ms have elapsed. This compares with the
1
ms
needed for conveyance of a pulse across the line. Despite the fact that the average
throughput required from the line may be far less than the 9600 bit/s available (say
2400 bit/s or
300
characters/s), the effective propagation time of characters across the
line is much longer than the
1
ms that you might expect.
No
human being will notice the extra 0.8ms, you might say? Indeed they will not
where a simple one-way transmission is involved with a human end user. However,
where an interactive dialogue is taking place between two computers (question-answer-
question-answer), then this will take around
80%
longer to conduct.
A
human waiting
for the computer’s response may see a response in around 4 seconds, where previously it
was around 2 seconds. Such intercomputer dialogues are the main cause of delays for
modern computer software. (Typical dialogues run ‘please send first character’

transmission line bitspeed (e.g.
9600
bit/s,
64
kbit/s,
2
Mbit/s, etc.)
0
message, packet or frame length in number of bits
0
the number of inter-computer interactions (request and response dialogue)
necessary to complete an action before responding to the human user.
Though our example is of a lowspeed data application, similar principles apply for all
types of data applications. Thus the higher the bitspeeds employed in the network, the
faster the application response time.
THE EMERGENCE AND
USE
OF
FRAME RELAY
383
router
G-
frame relay
switch
wide-area
frame relay
Figure
20.3
Typical use of frame relay
to

PVC
(permanent virtual channel)
ser-
vice between pairs of fixed end-points. The service to the user was therefore somewhat
akin to a
64
kbit/s leaseline, but without the full costs of a leaseline, because
statistical
multiplexing
in the wide area part of the network allowed resources to be shared across
a number of users and therefore costs to be saved by each of them (Figure
20.3).
The
pricing strategy of public network operators has also encouraged the use of frame relay
service as a leaseline replacement service where high speed but relatively low volume
usage is required, because flat rate charging based on the
committed information rate
(CIR)
has become the industry standard.
20.4
FRAME RELAY UN1
In the arrangement of Figure
20.3,
which is typical for a frame relay network, each
of
the routers is connected to the frame relay network using a single connection, typically
of
64
kbit/s, employing the Frame relay
VNI

supported at
OS1
layer 3. These are the functions which provide within the network for
reliable end-to-end transfer. Saving these functions simplifies the frame relay protocol
(in comparison with
X.25),
making it more efficient and faster running.
As is also shown in Figure 20.3, it is common in frame relay networks to build fully
meshed networks of logical connections (PVCs) between individual routers (a triangle
in our case). This circumvents the need for the routers themselves to act as transit nodes
for inter-router traffic, and
so
improves the overall performance perceived by the
LAN
users without having much effect on the overall cost of either the network hardware or
the transmission lines needed in the
wide area network
(WAN).
20.5
FRAME RELAY SVC SERVICE
The main drawback of the arrangement shown in Figure 20.3 is the management effort
needed to establish and maintain the large number of PVC connections within the
network. Potentially, each time a link in the network fails or a new link is added,
administrative work may be necessary to reconfigure some or all
of
the PVCs to new,
more efficient paths. To get around this problem, the Europeans in particular have
driven the development of an enhancement of the frame relay
UN1
to include the

and frames will be lost, so affecting all the
data links
sharing the congested link. Worse
still, once the end user devices detect the loss of information, retransmission will
commence, and the load on the network only further increases.
CONGESTION CONTROL IN FRAME RELAY NETWORKS
385
end user
end user
device
device
(sending)
(receiving)
excess frames
overflowing the
buffer and being
discarded
Figure
20.4
Congestion
in
a frame relay
network
As the connection from the sending device of Figure 20.4
to
the network is not con-
gested, the sending of frames would continue unabated, were it not for the
congestion
notzjication
procedures within the frame relay protocols.

is the agreed minimum
bitrate to be provided by the network between the two ends
of
the frame relay
data
link.
The CIR is agreed at the time of setting up the connection. Provided the frame
transmission rate of the sending device is at or below the CIR, then the network is
not permitted to force a reduction in the frame sending rate of the sending device,
and is not permitted wilfully to discard frames. However, where the sending device
is exceeding the CIR at a time when the BECN message is received, then the
network may first request reduction in the rate of frame transmission to the CIR.
Should the reduction not be undertaken (for example, because the sending device
cannot respond to the request), then the network is permitted to discard the
excess
frames.
At times
of
no congestion, sending devices are permitted for defined short periods of
time (called the
excess burst,
or
excess burst duration,
B,)
to transmit at bitrates higher
than the CIR. The maximum bitrate at which the device may send is termed the
EIR
(excess information rate).
The EIR is always greater or equal to the CIR. It is a manage-
ment decision for the network operator how high EIR and CIR may be set for a given

20.5).
For this a standardized interface, the
NNZ
(network-network interface)
is required.
Although the frame relay NNI allows for interconnection of sub-networks of
switches supplied by different manufacturers, and although reliable data transfer is
possible, it is true that the congestion control and management capabilities of the
combined network are much more restricted than the capabilities available within each
of the sub-networks independently. This reflects the relative youth of the frame relay
NNI standard.
20.8
FRAME FORMAT
Figure
20.6
illustrates the format of a single frame. It consists of five basic information
fields, much like the data link layer format of
X.25
(i.e.
HDLC).
The flag marks the
beginning
of
the frame, delineating it from the previous frame. The address field carries
frame relay network
A
frame relay network
B
(manufacturer
A)

of HDLC (i.e. is an OS1 layer
2
address). In addition, the address field also
contains control information
(command/response),
the
forward
and
backward explicit
congestion notification (FECN
and
BECN)
discussed previously in this chapter, the
discard eligibility (DE)
indication and some extra fields used for
extended addressing.
The control field contains supervision indication for the connection like
receiver ready
(RR), receiver not ready (RNR),
etc. For user information frames, this field indicates the
length of the frame. Such controls were discussed more fully in chapter
18
on
X.25.
These
enable the two end devices to coordinate one another for the communication.
The
informationJield
contains the user information. This may be up to
65

5
6
l
8
(transmitted
first
BECN
=
backward explicit congestion notification
C/R
=
command/response bit
DE
=
discard eligibility
DLCI
=
data link connection identifier
EA
=
extended addressing
lsb
=
least significant bits
msb
=
most significant bits
Figure
20.7
Address field format for frame relay

serious that frames need to be discarded, then
frames marked with the
discard eligibility
(DE)
set to
‘1’
will be discarded first. The
DE
bit is set to ‘1’ by the first frame relay switch (near the origin) on
excess
frames
(i.e. those causing the information rate to exceed the
committed information rate
(CIR)).
20.10 ITU-T RECOMMENDATIONS PERTINENT TO FRAME RELAY
The following
ITU-T
recommendations define frame relay.
0
Recommendation 1.233 describes the frame relay service.
0
Recommendation 1.122 defines the framework of recommendations which specify
frame relay, referring
to
the complete list of relevant recommendations.
e
Recommendation Q.922 is perhaps the most important. It defines the
core aspects
of
frame relay, specifically the

DTE (such as
a
router) must either incorporate a frame relay interface card, or
alternatively use some other standard interface and an external conversion device. The
most important of the available external conversion devices is the FRAD.
Frame relay
access devices (FRADs)
provide for the conversion
of
continuous bit stream oriented
(CBO)
data signals into a
frame format,
in much the same way that
a
PAD
(packet
assembler/disassembler)
converts the data stream from a ‘dumb’ computer terminal into
the packet format required by an X.25 network. A FRAD may thus be used to convert
a continuous data stream from an end user device normally expecting to use a
‘transparent’ leaseline-like connection into a format suitable for carriage by a frame
relay network. Figure 20.9 illustrates the principle.
Figure 20.9 illustrates the equivalent of
a
data leaseline, created using two
frame relay
access devices (FRADs)
and
a