Virtual-Circuit Networks:

1
Chapter 18 Virtual-Circuit Networks Frame Relay
and ATM
2
18-1 FRAME RELAY

• 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(1970s),
  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.

3
WANs Based on T-1 and T-3 Lines

• 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
  1.544Mbpsbut the T-1 line cannot accept this
  type of demand.
• Bursty data require what is called bandwidth on
  demand.

4
FRAME RELAY vs. ATM

• 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.

5
FRAME RELAY vs. ATM

• 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.
• 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.
• 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.
• 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.
• 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.

6
FRAME RELAY vs. ATM

• 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.
• 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.
• 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.

7
Difference between ATM and Frame Relay

• 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.

8
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.

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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.)
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VCIs in Frame Relay are called DLCIs.
Data Link control identifiers
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Permanent vs. Switched Virtual Circuits (PVC vs.
SVC)

• 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
• Costlypay 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).

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Figure 18.2 Frame Relay layers
No flow or error control, only an error
detection Mechanism.
13
Frame Relay operates only at the physical and
data link layers.
14
Figure 18.3 Frame Relay frame
15
Frame Relay does not provide flow or error
control they must be provided by the upper-layer
protocols.
16
Figure 18.4 Three address formats
17
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.
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Congestion Control and Quality of Service

• One of the nice features of FR is that it
  provides Congestion Control and Quality of
  Service (QoS), two important aspects of
  networking.

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18-2 ATM

• 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 ATMs 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.

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Design Goals

• The need for a transmission system to optimize
  the use of high-data-rate transmission media, in
  particular optical fiber.
• The system must interface with existing systems
  and provide wide-area 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,
  long-distance 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) .

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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.
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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.
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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.
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Figure 18.8 ATM multiplexing
ATM uses asynchronous time-division
multiplexing. It uses fixed-size slots (size of
a cell).
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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
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Virtual Connection

• Between two endpoints is accomplished thru
  transmission paths (TPs), virtual paths (VPs),
  and virtual circuits (VCs).
• 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.

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Figure 18.10 TP, VPs, and VCs
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Figure 18.11 Example of VPs and VCs
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Note that a virtual connection is defined by a
pair of numbers the VPI and the VCI.
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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.

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Figure 18.13 Virtual connection identifiers in
UNIs and NNIs
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Figure 18.14 An ATM cell
The basic data unit in an ATM network is called a
cell.
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Connection Establishment and Release

• 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).

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Figure 18.15 Routing with a switch
ATM uses switches to route the cell from a source
endpoint to the destination endpoint.
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Figure 18.16 ATM layers
Application Adaptation Layer
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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

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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.
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Figure 18.19 ATM headers
39
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.
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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
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Figure 18.22 AAL3/4
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Figure 18.23 AAL5
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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.
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Figure 18.24 ATM LANs
45
Figure 18.25 Pure ATM LAN
46
Figure 18.26 Legacy ATM LAN
47
Figure 18.27 Mixed architecture ATM LAN
48
Figure 18.28 Client and servers in a LANE
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Figure 18.29 Client and servers in a LANE
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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
• Video Conferencing
• Desktop Conferencing
• Multimedia Communications
• ATM Over Satellite Communications
• Mobile Computing over ATM for Wire-less Networks

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Appendix
T-1 lines 1.544 Mbps (24DS0) T-3
lines 43.232 Mbps (28 T-1 lines) OC-1
lines 51.48 Mbps OC-3 lines 155 Mbps
(100 T-1 lines) OC-12 lines 622 Mbps
(4 OC-3 lines) OC-48 lines 2.5 Gbps
(4 OC-12 lines) OC-192 lines 9.6 Gbps
(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 gt OC-n. For
example, an OC-3 transmission medium has 3 times
the transmission capacity of OC-1.
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Appendix
OC Specification 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

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