: Frame Relay and ATM

uptightexampleNetworking and Communications

Oct 24, 2013 (3 years and 9 months ago)

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18.
1

Chapter 18


Virtual
-
Circuit Networks
:

Frame Relay and ATM

18.
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

1980
s

and

early

1990
s
.


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

(
64
Kbps),

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

1990
s
.



18.
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

6
Mbps

for

2
s,

0
Mbps

for

7
s,

and

3
.
44
Mbps

for

1
s

:

Total

15
.
44
Mbits

during

10
s

i
.
e
.

average

is

still

1
.
544
Mbps

but

the

T
-
1

line

cannot

accept

this

type

of

demand
.


Bursty

data

require

what

is

called

bandwidth

on

demand
.



18.
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.






18.
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.







18.
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.







18.
7

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.
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.



18.
9

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.
10

VCIs in Frame Relay are called DLCIs.

Note

Data Link control identifiers


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:


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).

18.
11

Permanent vs. Switched Virtual Circuits
(PVC vs. SVC)

18.
12

Figure 18.2
Frame Relay layers

No flow or error
control, only an


error detection

Mechanism.

18.
13

Frame Relay operates only at the
physical and data link layers.

Note

18.
14

Figure 18.3
Frame Relay frame

18.
15

Frame Relay does not provide flow or
error control; they must be provided

by the upper
-
layer protocols.

Note

18.
16

Figure 18.4
Three address formats

18.
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.




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


18.
18

Congestion Control and Quality of
Service

18.
19

18
-
2 ATM


ATM

was

designed

in

the

1980
s

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
.


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) .


18.
20

Design Goals

18.
21

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.
22

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.

Note

Because each cell is the same size and all are small, the problems associated

With multiplexing different
-
sized frames are avoided.

18.
23

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.
24

Figure 18.8
ATM multiplexing

ATM uses asynchronous time
-
division multiplexing.


It uses fixed
-
size slots (size of a cell).

18.
25

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

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.


18.
26

18.
27

Figure 18.10
TP, VPs, and VCs

18.
28

Figure 18.11
Example of VPs and VCs

18.
29

Note that a virtual connection is defined
by a pair of numbers:

the VPI and the VCI.

Note

18.
30

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.
31

Figure 18.13
Virtual connection identifiers in UNIs and NNIs

18.
32

Figure 18.14
An ATM cell

The basic data unit in an ATM network is called a cell.

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).

18.
33

18.
34

Figure 18.15
Routing with a switch

ATM uses switches to route the cell from a source endpoint to the destination endpoint.

18.
35

Figure 18.16
ATM layers

Application Adaptation Layer

18.
36

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.
37

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.
38

Figure 18.19
ATM headers

18.
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.

18.
40

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.
41

Figure 18.22
AAL3/4

18.
42

Figure 18.23
AAL5

18.
43

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.
44

Figure 18.24
ATM LANs

18.
45

Figure 18.25
Pure ATM LAN

18.
46

Figure 18.26
Legacy ATM LAN

18.
47

Figure 18.27
Mixed architecture ATM LAN

18.
48

Figure 18.28
Client and servers in a LANE

18.
49

Figure 18.29
Client and servers in a LANE

18.
50


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


18.
51


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 => OC
-
n
.

For example, an OC
-
3 transmission medium has 3 times the transmission

capacity of OC
-
1.

18.
52


Appendix





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

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