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- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
20
Frame Relay
For datatransfer, 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
so
modern data communications networks, and it is beginning to be supplanted new techniques,
by
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
Thereliability of X.25 packet switching resulted worldwide
has from accepted
standards and the hugeavailability 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 universal
first data
communication protocol and it stimulated rapid growth in data communication traffic
volumes, because of its reliability and its robustness. Paradoxically, its robustness is
nowleading to thedemise of X.25,becauseone of the main limitations of packet
switchingbased on X.25 is itsunsuitabilityforcarriage of highspeedinformation
channels andits relative inefficiency when used in conjunctionwithhighquality
transmission networks.
When X.25 was developed in the late 1970s, the relative speed of the communicating
9600
devices was very low (in comparison with today’s devices, typically under bit/s) and
the quality of wide area digital lines was comparatively poor. As a result (and to their
credit) X.25 packet networks are highly robust against poor line quality. X.25 networks
are able to survive and even recover from even extensive bit errors on digital lines. The
problem is that the cost of this robustness is the very limited linespeeds which are
possible, and the relative inefficiency of line utilization in the case higher quality lines.
of
The problems which arise when attempting to operate X.25 protocol at high speeds
areduetothe windowing techniqueemployed by X.25to helpavoid errors. To
379
- 380 FRAME RELAY
illustrate the problemwe consider trying to use a 2 Mbit/s line to carry X.25 dataover a
distance of 1000 km.
As Figure 20.1 illustrates, on a high speed data transmission line there are always a
large number of bits in transit on the line any point in time (because of its length), in
at
our example around 000 bits or 2500 bytes (line lengthX bitrate/speed of light). (That
20
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
withouterrors.One of themethods used to ensuresafearrival is the use of an
acknowledgement window. Only so manypackets(asdefined by the window size,
typically 7) may be transmitted by the sending device before an acknowledgement is
received confirmingsafearrival. As thetypicalmaximumpacket size is defined as
256 bytes, this means that a maximum of 1792 bytes (7 X 256) may be transmitted by
thesenderbefore an acknowledgement is returned by the receiver to confirmsafe
arrival. This compares with the2500 bytes actually on the line, so that even before con-
sidering the inefficiencies caused by packet overheads (the protocol control information in
the X.25 header) the X.25 window will constrain the efficiency of the line of Figure 20.1
to a maximum of 1792/2500 (maximum bits allowed in transit/available bits in transit)
or around 70%!
Simple, you might say: increase the maximum window size! Unfortunately this only
generates new problems. First, the end devices need to provide much greater storage
buffers forretainingcopies of thesentbutunacknowledgedinformation.Second,
because the window size is greater, so is the likelihood of errors within a window. The
probability of the need for a retransmission of the informationeliminate the errorsis
to
thus also greater. Also, because of the increased window size, the time required for
retransmission is
longer. So increasing the window size may actually
reduce
throughput!
Today's digital transmission is several orders of magnitude better quality than thatof
the 1970s, so that the heavy duty error detection and correction techniques used by X.25
(a) n ~ pulse
travels at around 10' m/s
4-
1 bit
length = speedm/S
in
bitrate
=
2X 1
0'
50 m
sender network (2048 kbitls) receiver
number of bits in transit on the line = line length I bit length = 1000 km I 5 0 m
= 20 000 bits
= 2500 bytes
Figure 20.1 Bits in transit in an X.25 packet-switched data network
- FOR NEED RESPONSE
THE FASTER DATA NETWORKS 381
have become redundant. Windowing and acknowledgement are now largely super-
fluous. Framerelay (or frame relaying) was one of the first techniques designed to
dispense with heavy duty error detection and correctiontechniques. Instead of it being
undertaken by the network, thejob of error detection and correction or recovery is left
to higher layer protocols (i.e. the end user’s device, computer or software) to sort out.
The frame relay network, meanwhile, may concentrate on the raw information carriage
and is thus more efficient and also more capable of higher information throughput.
Thusfor widearea computerandLAN-to-LANconnection needs of 64 kbit/sor
greater, frame relay is today’s preferred method. However, for bitrates above2 Mbit/s,
native ATM (see Chapter 26) should be considered.
20.2 THE NEED FORFASTER RESPONSE DATANETWORKS
Althoughthe basic need tocarry high bandwidth signals drovethe need forthe
development of the frame relay protocols, so did the need for faster responding net-
works. It may not immediately obvious, but the time required to propagate even low
be
bandwidth information across a digital network dependent on thebitspeed employed:
is
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 havepreviously already been available.
The following discussion gives two reasons why.
Let us imagine two rainwater conduits, one small bore and oneof large bore. Let us
of
assume that the first has throughput capacity of 5litres/s and the second of 10litres/s.
Now let us assume that therainfall rate is 4 litre+. Why should I bother with the large
bore conduit? The answer that therainfall rate is not constant.Over the courseof time
is
the rate may vary between, say 2 and 6 litres per second, so during moments time
that in
when the rate of rainfall exceeds 5 litres/s, water will be accumulating in the roof gutter
rather than flowing down the conduit. The accumulation clears when the rainfall rate
drops momentarilybelow 5 litresis. As aresult of the momentary accumulation, some of
the rainwateris delayed slightly, thereby increasing the propagation delay. The analogy is
relevant to data transmission, where the rate of generation of typed characters or other
data to be transferred is not constant, but can fluctuate wildly. The first reason why high
speed lines give better performance is that they cope with short high speed bursts better.
The second reason is that frames can be conveyed more quickly, as we explain next.
Figure 20.2 illustrates a more detailed example of data transmission across a tele-
communicationtransmission line. In this case, thecarriage of the electrical signals
(i.e. the waveform pulses representing individual bits of a digital signal pattern) is at a
speed close to thespeed of light. Thus theleading edge of an individual pulse traverses the
network at around108metres/s (Figure 20.2(a)). The time required, however, to transmit
an entire frameof 1 byte (8 bits) is sensitive to thetransmission bitspeed.The propagation
time in this case is equal to the sumof 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-, pulsetravelsataround 10’ m h
sender network receiver
travels (b)
pattern
signal at 108m l S
l+----+
signal duration = number of bitslbitspeed
Figure 20.2 Signal propagation time across a data network
0 single bit (pulse) propagation time = 105 m/lOs ms-’= 10-3 S = 1 ms
0 byte propagation time =pulse
propagation time + signal duration
= 1 ms + 8 bits/9600 bitS-’
= 1.8ms
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 a response inaround 4seconds, where previously it
see
was around 2 seconds. Such intercomputer dialogues are the main cause of delays for
modern computer software. (Typical dialogues run ‘please send first character’ - ‘first
character’ - ‘received firstcharacter OK, please next
send character’ - ‘second
character’ - ‘received second character OK. . .’ etc.)
The perhaps surprisingreality is that it may indeed make sense to use a network with
64 kbit/stransmissionlinksratherthan 9600 bit/slinks even though theaverage
throughput is only 4000 bit/s ! (Reduction in byte propagation time from 1.S ms to
1.1 ms). The following points are thuscritical in the response time performance of data
networks and associated computer applications software
0 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-computerinteractions
(request response
and 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
frame relay
switch
wide-area
frame relay
G-
Figure 20.3 Typicaluseofframerelay to improvethewideareaefficiencyof LAN/router
networks
20.3 THEEMERGENCEAND USE OF FRAME RELAY
The recent explosion in the number LANs (local area networks,networks connecting
of
personal computers withinoffice buildings) and theneed to interconnectLANs, as well
as the growing numberof clientlserver computing (UNIX) environments, have created
the demand for high speed networking in data networks. Frame relay provides for
relatively cheap wide area data communication at rates between 9600 bit/s and 2 Mbit/s,
and has proved a viable alternative to leaselines, particularly in router networks.
Initially, frame relay networks only supported 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 shared across
be
a number of users and therefore costs to be saved by each of them (Figure 20.3). The
pricing strategyof public network operators has also encouraged the of frame relay
use
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) become the industry standard.
has
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 usingsingle connection, typically
a
of 64 kbit/s, employing the Frame relay VNI (user-network interface). Over this single
physical connection, up to1024 (21°) logical channels(PVCs, in the correct frame relay
terminology, data links) may be connected, each to a separate end destination. These
- 384 FRAME RELAY
channels are available on a permanent basis, but the capacity of the wide area part of
the network is only used when there are actually frames requiring to be relayed from
one end of the data link to the other.
In theframe header (like the HDLC header of X.25) is a numbered value identifying
the logical connection to which the frame belongs. This is called the data link channel
identifier (DLCZ). Unlike X.25,there are
no real
end-to-end
networkfunctions
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 theneed for the routers themselves act as transit nodes
to
for inter-router traffic, and so improves the overall performance perceived by the LAN
users without having mucheffect on the overall cost of either the network hardware or
the transmission lines needed in the wide area network ( W A N ) .
20.5 FRAME
RELAY SVC SERVICE
The main drawback of the arrangement shown in Figure 20.3 the management effort
is
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
driventhedevelopment of an enhancement of the frame relay UN1 to includethe
capability for on-demand establishmentof datalinks (i.e. switchedvirtualcircuits,
SVCs) using a dial-up procedure. Although this adds layer 3 functions to the frame
relay protocol stack, these are only connection and clearing functions, which do not
affect thesubsequentend-to-endcarriagecharacteristics(highspeed, low delay as
discussed).
The main benefit of an SVC network is that individual data links need only be
established when needed and may be cleared afterwards. This simplifies the network
and its management, and in addition has the effect of automatically optimizing the
routing of connections each time they are newly established.
20.6 CONGESTION CONTROL IN FRAME RELAYNETWORKS
The high speed of the computer devices using frame relay networks leads the need for
to
special measures to control network congestion, because the layer 3 protocol is almost
non-existent. Figure 20.4 illustrates a case in which oneof the intermediate links within
the wide area or backbone part of the network is in congestion. As a result, frames are
accumulating rapidly (and unabated) the buffer immediately preceding the congested
in
link, as they wait to be transmitted over the link. Ultimately, the buffer will overflow
and frames will be lost, so affecting all thedata links sharing the congested link. Worse
still,oncetheenduser 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.
Any time a frame underway within a frame relay network encounters congestion (the
exceeding of a given waiting time or a given buffer queue length) then the frame header
is tagged with a notification message, called the forward explicit congestion notijica-
tion ( F E C N ) . This notifies the receiving device and the switch to which it is connected
of the congestion, which may then be communicated back to the source end by means
of a backward explicit congestion notijication( B E C N ) message. The sending device may
voluntarily respond to receipt of the BECN by reducing its transmitted output, or may
be forced by the network to do so. By reducingtheframetransmissionratefrom
relevant causal sources, the congestion will ease, and the transmission rate restriction
may be removed. Meanwhile, further service degradation within the networkas a whole
is avoided.
A further refinement of the congestion control procedure of framerelay is
provided by the committed information
rate ( C I R ) and excess informationrate
( E I R ) parameters.The committedinformationrate ( C I R ) is theagreedminimum
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 theCIRata time when the BECN message is received, then the
network may first request reduction in the rate of frametransmission to the CIR.
Shouldthereductionnot be undertaken(forexample,becausethesending 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 periodsof
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 E I R
(excess information rate). The EIR is always greater or equal to theCIR. Itis a manage-
ment decision for the network operator how high EIR and CIR may be set for a given
connection, and usually these values are included in the contract or order for the user’s
connection (for a PVC)or negotiated at connection set-up time foran SVC. The ability
- 386 FRAME RELAY
to handle short bursts of high speed information (above theCIR) is what makes frame
relaynetworksattractivetodataapplicationsrequiringfastresponsetimes,as we
discussed earlier in the chapter.
20.7 FRAME RELAY NNI
As in packet-switched data networks, it is common for all the switches within a given
operator’s network to be purchased from and supplied by a single manufacturer. The
leadingmanufacturers of framerelaynetworkcomponents arethe Stratacom and
Cascade companies and Northern TeIecom (NorteI). As the switches are supplied by a
single manufacturer, it is not necessary to use a standardized interface between the
nodes within the network. As a result, the individual manufacturers have tended to
develop extra congestion controls, network management and service features over and
above those required by the frame relay standards, to try to improve the market value
of their products. All very well; except, of course when a given frame relay connection
needs to be switchedacrosstwodifferentnetworks or sub-networks,supplied by
different manufacturers (as in Figure 20.5). For this a standardized interface, the NNZ
(network-network interface) is required.
Although frame
the relay NNI allows for interconnection of sub-networks of
switchessupplied by different manufacturers, and although reliable data transfer is
possible, it is true that the congestioncontrol 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 formatof 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
A
frame relay network frame relay networkB
(manufacturerA) (manufacturer B)
UN1 NNI UN1
Figure 20.5 Frame relay NNI (network-networkinterface)
- ADDRESS FIELD FORMAT 387
Flag Address Field Control Information field F r a m e check
s e q u e n c e (FCS)
Figure 20.6 Frame format for frame relay
the DLCI (data link connection identifier), the equivalent the logical channel number
of
( E N ) 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
congestionnotification (FECN and B E C N ) discussedpreviouslyinthis chapter, the
discard eligibility ( D E ) indication and some extra fields used for extended addressing.
The controlfield contains supervision indication for the connection like receiver ready
( R R ) ,receiver not ready ( R N R ) , etc. For user information frames, this
field indicates the
length of the frame. Such controls discussed morefully in chapter 18 on X.25. These
were
enable the two end devices to coordinate one another for the communication.
The informationJield contains the user information. This may be up 65 536 bytes in
to
length. Finally, the frame check sequence ( F C S ) is a cyclic redundancy check ( C R C )
code providing for error detection. We discussed CRC codes in Chapter 9.
20.9 ADDRESS FIELD
FORMAT
Figure 20.7 illustrates the two octet (i.e. two byte) address field used in frame relay. The
main function of the address field is to carry the data link connection identifier (DLCZ),
which identifies the end device to which the frame is to be sent (OS1 layer 2 address).
The DLCI is a 10 bit field, allowing up to 1024 separate virtual connections to share the
same physical connection.
1 2 3 4 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
- 388 FRAME RELAY
Higher layer information Higher layer information
network layer 4.933 (signalling) 4.933
link layer 4.922 (core aspects) 4.922 (Core aspects
physical layer physical layer
Note: 4.933 protocol is the network layer protocol used for establishing switched connections (SVCs). It is
not used in the PVC (permanent virtual connection) service.
Figure 20.8 Protocol stack for frame relay
Should congestion be encountered by a given frame during its transit through the
network, then the affected intermediate switch will toggle the FECN (forward explicit
congestion notijication) as we discussed earlier in the chapter. This alerts the receiving
device of the congestion. In response, returned frames are marked using the BECN
(backward explicit congestion notijication). This allows the flow control procedures to be
undertaken. Should congestion become so serious that frames need to be discarded, then
frames marked with thediscard eligibility ( D E ) set to ‘1’ will be discarded first. The DE
bit is set to ‘1’by the firstframerelayswitch(neartheorigin) on excess frames
(i.e. those causing the informationrate to exceed the committed information rate( C I R ) ) .
20.10 ITU-T RECOMMENDATIONSPERTINENTTO FRAME RELAY
The following ITU-T recommendations define frame relay.
0 Recommendation 1.233 describestheframerelayservice.
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. Itdefines the core aspects of
frame relay, specifically the data link procedure (i.e. frame format, address
field etc.).
0 Recommendation 1.370 defines the congestion management procedures.
e Recommendation 4.933 defines the signalling procedures to set up switched virtual
connections. Thisrecommendation is not relevant for permanent virtual circuit
( P V C ) service.
Figure 20.8 shows the layered protocol structure of frame relay.
20.11 FRAD (FRAME RELAY ACCESS DEVICE)
Frame relay has historically been offered by public telecommunication. operators as a
cheapalternativetoaleaselineincaseswherehighbitrates were desirablebutthe
To
number of hours of usage per day was relatively low. access a frame relay network, a
- ICE) FRAD (FRAME RELAY
ACCESS 389
leaseline eauivalent
frame relay
UN1
Figure 20.9 A frame relay access device (FRAD)
DTE (suchas a router)musteitherincorporateaframe relayinterfacecard,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 requiredby an X.25 network. AFRAD may thus be used to convert
acontinuousdatastreamfromanend 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 a data leaseline, created using two
of frame relay
access devices (FRADs) and a frame relay network. This is usually more economical
than the equivalentleaseline where the CIR (committed information rate) lower than
is
the maximumspeed of the leaseline. Usually the EIR setto the maximum speed the
is of
equivalent leaseline, and CIR is set somewhat above average
the actual user
information throughput rate. This virtually guarantees throughput of the user data
(at the CIR). Bursts up to the maximum leaseline speed (the EIR) are still possible for
short periods, but at more ‘quiet’ other users may share the trunk resources within
times
the network on a ‘statistical’ basis. This is reflected in the lower costs of frame relay
service compared to ‘transparent’ leaseline service.
Also shown in Figure 20.9 is the local management interface (LMI). This allows
status andother network,management informationtobesharedbetweentheend
terminal and the network in an SNMP-like format Chapter 27), so enabling overall
(see
monitoring and management of the network.
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