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Chapter 18
Virtual-Circuit Networks:
Frame Relay and ATM
18.1
18-1 FRAME RELAY
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18.2
Packet switching can use two approaches: the virtual circuit
approach and the datagram approach.
Frame Relay is a virtual-circuit wide-area network that was
designed in response to demands for a new type of WAN in
the late 1980s and early 1990s.
Frame Relay is a relatively high-speed protocol that can
provide some services not available in other WAN
technologies such as DSL, cable TV, and T lines.
Frame relay, the outgrowth of the older(1970’s), slower
(64Kbps), more careful, error-correcting X.25, is packet
technology designed to carry variable-length frames over
high-quality connections such as fiber, which was just
coming into its own in the early 1990s.
WANs Based on T-1 and T-3 Lines
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18.3
T-1 and T-3 Lines are leased from public service providers.
If an organization has n branches spread over an area, it
needs n(n-1)/2 lines. Very costly especially when an
organization uses them 10% of the time.
The services provided by T-1 and T-3 Lines assume that the
user has a fixed data rate at all times. E.g. a T-1 line is
designed for a user who wants to use the line at a consistent
1.544 Mbps. This type of service is not suited for the many
users today that need to send bursty data.
E.g. A user may want to send data at 6Mbps for 2s, 0Mbps
for 7s, and 3.44Mbps for 1s : Total 15.44Mbits during 10s
i.e. average is still 1.544Mbps…but the T-1 line cannot
accept this type of demand.
Bursty data require what is called bandwidth on demand.
FRAME RELAY vs. ATM
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18.4
Data link layer of OSI model defines the ways of
encapsulating data for transmission between two
endpoints and the techniques of transferring the
frames.
Both Asynchronous Transfer Mode (ATM) and
Frame relay are data link layer technologies and
they have connection oriented protocols.
Each technique has its own application dependent
advantages and disadvantages.
FRAME RELAY vs. ATM
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ATM is a network switching technology that uses a cell based methodology to quantize data. ATM
data communication consists of fixed size cells of 53 bytes. An ATM cell contains a 5 byte header
and 48 bytes of ATM payload. This smaller size, fixed-length cells are good for transmitting voice,
image and video data as the delay is minimized.
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ATM is a connection oriented protocol and therefore a virtual circuit should be established between
sending and receiving points. It establishes a fixed route between two points when the data
transfer starts.
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Another important aspect of ATM is its asynchronous operation in time division multiplexing.
Therefore cells are transmitted only when data is available to be sent unlike in conventional time
division multiplexing where synchronization bytes are transferred if there data is not available to be
sent.
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ATM is designed to be convenient for hardware implementation and therefore processing and
switching have become faster. Bit rates on ATM networks can go up to 10 Gbps. ATM is a core
protocol used over the SONET/SDH backbone of the ISDN.
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ATM provides a good quality of service in networks where different types of information such as
data, voice, and are supported. With ATM, each of these information types can pass through a
single network connection.
18.5
FRAME RELAY vs. ATM
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Frame relay is a packet switching technology for connecting network points in Wide
Area Networks (WAN). It is a connection oriented data service and establishes a
virtual circuit between two end points. Data transfer is done in packets of data known
as frames. These frames are variable in packet size and more efficient due to flexible
transfers. Frame Relay was originally introduced for ISDN interfaces though it is
currently used over a variety of other network interfaces as well.
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In frame relay, connections are called as ‘Ports’. All the points which need to connect
to the frame relay network needs to have a port. Every port has a unique Address. A
frame is made of two parts which can be called as ‘actual data’ and the ‘frame relay
header’. Frame architecture is same as defined for LAP-D (Link Access Procedures
on the D channel) which has a variable length for information field. These frames are
sent over Virtual Connections.
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Frame relay can create multiple redundant connections among various routers,
without having multiple physical links. Since frame relay is not specific for media, and
provides means to buffer speed variations, it has the possibility to create a good
interconnect medium between different types of network points with different speeds.
18.6
Difference between ATM and Frame Relay
1. Although both techniques are based on end to end delivery of quantized data, there
are many differences in terms of sizes of the data quanta, application network types,
controlling techniques etc.
2. Although ATM uses fixed size packets (53 bytes) for data communication, frame relay
uses variable packet sizes depending on the type of information to be sent. Both
information blocks have a header in addition to data block and transfer is connection
oriented.
3. Frame Relay is used to connect Local Area Networks (LAN) and it is not implemented
within a single area network contrast to ATM where data transfers are within a single
LAN.
4. ATM is designed to be convenient for hardware implementation and therefore, cost is
higher compared to frame relay, which is software controlled. Therefore frame relay is
less expensive and upgrading is easier.
5. Frame relay has a variable packet size. Therefore it gives low overhead within the
packet which results it an efficient method for transmitting data. Although fixed packet
size in ATM, can be useful for handling video and image traffic at high speeds, it leaves a
lot of overhead within the packet, particularly in short transactions.
18.7
Frame Relay Features
• FR operates at a higher speed (1.544Mbps and recently 44.376 Mbps)
• FR operates in just the physical and data link layers. This means it can be
used as a backbone network to provide services to protocols that already
have a network layer protocol, such as the Internet.
• FR allows bursty data
• FR allows a frame size of 9000bytes, which can accommodate all local area
network frame sizes.
• FR is less expensive than other traditional WANs.
• FR has error detection at the data link layer only. No flow control or error
control. FR was designed in this way to provide fast transmission capability
for those protocols that have flow and error control at the higher layers.
18.8
Figure 18.1 Frame Relay network
• FR provides permanent virtual circuits and
switched virtual circuits.
• The FR WAN is used as one link in the global
Internet.
Switch Table matches an
incoming port-DLCI combination
with an outgoing port-DLCI
combination. (as in Chap.8 VCIs
are replaced by DLCIs.)
18.9
Note
VCIs in Frame Relay are called DLCIs.
Data Link control identifiers
18.10
Permanent vs. Switched Virtual Circuits
(PVC vs. SVC)
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In PVC the connection setup is very simple. The corresponding table entry
is recorded for all switches by the administrator (remotely and
electronically). An outgoing DLCI is given to the source, and an incoming
DLCI is given to the destination.
PVCs have 2 drawbacks:
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18.11
Costly…pay for connection all the time
A connection is created from one source to one single destination. If a source
needs connections with several destinations, it needs a PVC for each
connection.
SVC creates a temporary, short connection that exists only when the data
are being transferred between source and destination. SVC requires
establishing and terminating phases (Chapter 8).
Figure 18.2 Frame Relay layers
No flow or error
control, only an
error detection
Mechanism.
18.12
Note
Frame Relay operates only at the
physical and data link layers.
18.13
Figure 18.3 Frame Relay frame
18.14
Note
Frame Relay does not provide flow or
error control; they must be provided
by the upper-layer protocols.
18.15
Figure 18.4 Three address formats
18.16
Figure 18.5 FRAD (Frame Relay Assembler Disassembler)
FRAD assembles and disassembles frames coming from other protocols
to allow them to be carried by FR frames. A FRAD can be implemented as
a separate device or as part of a switch.
18.17
Congestion Control and Quality of
Service
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18.18
One of the nice features of FR is that it provides
Congestion Control and Quality of Service (QoS),
two important aspects of networking.
18-2 ATM
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18.19
ATM was designed in the 1980s to deliver five distinct levels of QoS, so
users could send traffic with greater or less delay.
Asynchronous Transfer Mode (ATM) is the cell relay protocol designed
by the ATM Forum and adopted by the ITU-T.
Key to ATM’s charm was that it could emulate direct circuits and
guarantee bandwidth, a shortcoming of frame relay.
Frame relay won in the WAN. ATM lived on, though in carrier core
networks, where it is slowly being decommissioned.
ATM has been accepted universally as the transfer mode of choice for
Broadband Integrated Services Digital Networks(BISDN). ATM can
handle any kind of information i.e. voice, data, image, text and video in
an integrated manner.
Design Goals
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18.20
The need for a transmission system to optimize the use of highdata-rate transmission media, in particular optical fiber.
The system must interface with existing systems and provide widearea interconnectivity between them.
The design must be implemented inexpensively. If ATM is to
become the backbone of international communications, it must be
available at a low cost.
The new system must be able to work with and support the existing
telecommunications hierarchies (local loops, local providers, longdistance carriers, etc.)
The new system must be connection-oriented to ensure accurate
and predictable delivery.
One objective is to move as many of the functions to hardware as
possible (for speed) .
Figure 18.6 Multiplexing using different frame sizes
The variety of frame sizes makes traffic unpredictable.
Switches, multiplexers, and routers must incorporate elaborate software systems to
manage various sizes of frames.
Internetworking among the different frame networks is slow and expensive.
Problem: Providing consistent data rate delivery when frame sizes are unpredictable
and can vary dramatically.
To get the most out of broadband technology, traffic must be time-multiplexed onto
shared paths. E.g. :
18.21
Note
A cell network uses the cell as the basic
unit of data exchange.
A cell is defined as a small, fixed-size
block of information.
Because each cell is the same size and all are small, the problems associated
With multiplexing different-sized frames are avoided.
18.22
Figure 18.7 Multiplexing using cells
The cells are interleaved so that
none suffers a long delay.
A cell network can handle real-time transmissions, such as a phone call,
without the parties being aware of the segmentation or multiplexing at all.
18.23
Figure 18.8 ATM multiplexing
ATM uses asynchronous time-division multiplexing.
It uses fixed-size slots (size of a cell).
18.24
Figure 18.9 Architecture of an ATM network
ATM is a cell-switched network. The user access devices, called endpoints, are connected
thru a user-to-network interface UNI. to the switches inside the network.
Network-to-network Interface
18.25
Virtual Connection
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Between two endpoints is accomplished thru transmission paths
(TPs), virtual paths (VPs), and virtual circuits (VCs).
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18.26
A TP is the physical connection (wire, cable, satellite,..etc.) between an endpoint
and a switch or between two switches.
A TP is divided into several VPs.
A VP provides a connection or a set of connections between two switches.
Cell networks are based on VCs.
All cells belonging to a single message follow the same virtual circuit and remain
in their original order.
Figure 18.10 TP, VPs, and VCs
18.27
Figure 18.11 Example of VPs and VCs
18.28
Note
Note that a virtual connection is defined
by a pair of numbers:
the VPI and the VCI.
18.29
Figure 18.12 Connection identifiers
• In a UNI, the VPI is 8 bits, whereas in an NNI
the VPI is 12 bits. The length of the VCI is the
same in both interfaces (16 bits).
• Hence a virtual connection is identified by
24 bits in a UNI and by 28 bits in an NNI.
• The idea behind dividing a VCI into 2 parts
is to allow hierarchical routing.
18.30
Figure 18.13 Virtual connection identifiers in UNIs and NNIs
18.31
Figure 18.14 An ATM cell
The basic data unit in an ATM network is called a cell.
18.32
Connection Establishment and Release
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18.33
Like FR, ATM uses two types of connections : PVC and SVC
PVC : a permanent virtual-circuit is established between two endpoints by
the network provider. The VPIs and VCIs are defined for the permanent
connection and the values are entered for the tables of each switch.
SVC : In a switched virtual-circuit connection, each time an endpoint wants
to make a connection with another endpoint, a new virtual circuit must be
established. ATM cannot do the job by itself, but needs the network layer
addresses and the services of another protocol (such as IP).
Figure 18.15 Routing with a switch
ATM uses switches to route the cell from a source endpoint to the destination endpoint.
18.34
Figure 18.16 ATM layers
Application Adaptation Layer
18.35
Figure 18.17 ATM layers in endpoint devices and switches
SONET : The original design of ATM was based on SONET as the physical layer carrier:
• First, the high data rate of SONET
• Second, in SONET, the boundaries of cells can be clearly defined.
Other Physical Technologies : ATM does not limit the physical layer to SONET.
Other technologies such as wireless may be used. Problem : cell boundaries !.. but there is a solution
18.36
Figure 18.18 ATM layer
The ATM layer provides : routing, traffic management, switching, and multiplexing services.
It processes outgoing traffic by accepting 48-byte segments from the AAL sublayers and
transforming them into 53-byte cells by the addition of a 5-byte header.
18.37
Figure 18.19 ATM headers
18.38
Figure 18.20 AAL1
Supports applications that transfer information at constant bit rates such as video and voice.
It allows ATM to connect existing digital telephone networks such as voice channels and T lines.
18.39
Figure 18.21 AAL2
It is used for low bit rate traffic and short-frame traffic such as audio, video, or fax.
Ex. In mobile telephony
18.40
Figure 18.22 AAL3/4
18.41
Figure 18.23 AAL5
18.42
18-3 ATM LANs
ATM is mainly a wide-area network (WAN ATM);
however, the technology can be adapted to local-area
networks (ATM LANs). The high data rate of the
technology has attracted the attention of designers
who are looking for greater and greater speeds in
LANs.
18.43
Figure 18.24 ATM LANs
18.44
Figure 18.25 Pure ATM LAN
18.45
Figure 18.26 Legacy ATM LAN
18.46
Figure 18.27 Mixed architecture ATM LAN
18.47
Figure 18.28 Client and servers in a LANE
18.48
Figure 18.29 Client and servers in a LANE
18.49
ATM Applications
There are several practical applications using ATM Technology.
ATM is the Backbone Network for many broadband
applications including Information SuperHighway.
Some of the key applications can be mentioned as follows:
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18.50
Video Conferencing
Desktop Conferencing
Multimedia Communications
ATM Over Satellite Communications
Mobile Computing over ATM for Wire-less Networks
Appendix
T-1
lines
T-3
lines
OC-1
lines
OC-3
lines
OC-12 lines
OC-48 lines
OC-192 lines
1.544 Mbps
43.232 Mbps
51.48 Mbps
155 Mbps
622 Mbps
2.5 Gbps
9.6 Gbps
(24DS0)
(28 T-1 lines)
(100 T-1 lines)
(4 OC-3 lines)
(4 OC-12 lines)
(4 OC-48 lines)
OC : optical carrier
Classifications are based on the abbreviation OC followed by a number
specifying a multiple of 51.84 Mbit/s: n × 51.84 Mbit/s => OC-n.
For example, an OC-3 transmission medium has 3 times the transmission
capacity of OC-1.
18.51
Appendix
DS0
64Kbps
1/24 of T-1
1 Channel
DS1
1.544Mbps
1 T-1
24 Channels
DS1C
3.152 Mbps
2 T-1
48 Channels
DS2
6.312 Mbps
4 T-1
96 Channels
DS3
44.736 Mbps
28 T-1
672 Channels
DS3C
89.472 Mbps
56 T-1
1344 Channels
DS4
274.176 Mbps
168 T-1
4032 Channels
18.52
OC
Specifica
tion
Data
Rate
(Mbps)
OC-1
51.84
OC-3
155.52
OC-9
466.56
OC-12
622.08
OC-18
933.12
OC-24
1244.16
OC-36
1866.23
OC-48
2488.32
OC-96
4976.64
OC-192
9953.28