ISDN and ATM

munchdrabNetworking and Communications

Oct 30, 2013 (4 years and 14 days ago)

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Chapter 5

ISDN and ATM

Switching network


Circuit switching


Feature


Used in PSTN, ISDN, leased line


A dedicated path is established between 2
stations for communication


Handle voice (analog) and digital data


Communication involves 3 phases


Circuit establishment


Data transfer


Circuit disconnection

Switching network


Packet switching


Feature


Used in TCP/IP, ATM, X.25


No dedicated path, data are transmitted in
packets


Two types of packet switching:


Datagram


packets are treated independently


Virtual circuit


a route is defined between two
endpoints


ISDN


Overview


Integrated Services Digital Network


A very good example that employs circuit
switching technology


What is the major applications over ISDN ?


Voice service using digital telephone


Data transmission


Voice/data are carried by


64Kbps voice channels (B
-
channels)


Signalling information for connection setup
and release are carried by


a 16Kbps or a 64Kbps data channel (D
-
channel) depending on the type of service
subscribed


Two basic types of ISDN service


Basic Rate Interface (BRI)


2B+D (2*64Kbps +
16Kbps

= 144Kbps)


service targeted for most individual users


Primary Rate Interface (PRI)


Intended for corporate users


23B+ D (23*64Kbps +
64Kbps

= 1.536Mbps)


In Europe, it is 30B+D (1.984Mbps)


H
-
channel services provide a way to
aggregate multiple B
-
channels


H0=384 kbps (6 B channels)


H10=1472 kbps (23 B channels)


H11=1536 kbps (24 B channels)


H12=1920 kbps (30 B channels)


Targeted for applications such as


Video telephone


High bit rate dedicated communication
channels, etc.


ISDN terminal adapters are used to
communicate with local ISDN local switch
for accessing these services

Advantages of ISDN


Speed


Theoretically, analog modem for old PSTN
can only transmit data at maximum rate
around 33.6Kbps


c.f. 144Kbps data rate offered by BRI ISDN
service


For telephone company,


Instead, digital signal(not the analog one) is
transmitted over the line



Multiple devices


In PSTN


One analog telephone line for one service,
e.g. standard telephone, fax, computer etc.


In ISDN


One telephone line supporting multiple
devices



Signalling


“Out
-
of
-
band” signalling channel in ISDN

ISDN Interface


U interface (for BRI)


1 pair ( 2
-
wire ) telephone cable is used.


Full
-
duplex digital data transfer


connect to only one ISDN device called
Network Termination 1 (NT1)


The ownership of NT1 is different for
different countries


Convert the 1 pair full duplex telephone
cable to 2 pairs of cables, one for transmit
and one for receive


S/T interface


NT
-
1 converts the U interface into 4
-
wire
S/T interface.


multiple devices ( up to 7 devices can be
connected to the bus )


Still full
-
duplex, a pair of wires for receive,
another pair of wires for transmit.


S/T interface (Cont’d)


S interface provides higher level of network
service (ISO layer 2 and 3 protocols)


Network Termination 2 (NT2)


provide the S interface to Terminal
Equipments (TE1) by handling layer 2 and 3
protocols


It is included in virtually all ISDN devices


R interface


for non
-
ISDN devices that are used over
the existing telephone network


e.g. analog telephones, FAX machines,
computers


They are called Terminal Equipment 2 (TE2)


Terminal Adapter (TA) provides the R
interface on top of the S interface


TA in U.S. generally have a built
-
in NT1
and attach via a two
-
wire “U” interface.


TA in Europe and Japan, the NT1 is
installed by the telephone company and
attaches to the TA via four
-
wire “T”
interface.


Layer 1
-

Physical Layer


Specified by ITU I
-
series and G
-
series
documents


U interface for BRI


2
-
wire telco cable


160Kbps digital connection


line coding


2B1Q (2 Binary 1 Quarternary) in North
America


4B3T in Europe


Both codes belongs to the “mBnL codes”


m consecutive bits are represented by n
pulses each of L levels


2B1Q


2 bits are represented by 1 pulse (symbol)
with 4(Quarternary) levels


With bit rate = 160Kbps


Pulse(Symbol) rate = 80 baud/s


Frame format


Each U interface frame is 240 bits long






With 160Kbps, each frame lasts for 1.5ms


Sync field has 9 Quaternaries in the pattern
+3 +3
-
3
-
3
-
3 +3
-
3 +3
-
3 for frame
synchronization


(B
1
+B
2
+D) is 18 bits long with (8+8+2)
arrangement


Maintenance field is for error correction, and
transfer of ISDN operator’s network control
commands

Sync

18bits

12*(B
1
+B
2
+D)


216 bits

Maintenance


6 bits

Perspective on ISDN


ISDN was a massive attempt to
replace the analog PSTN


But, have we seen this happens ? No


Why ?


Standardization of ISDN tooks years in the
1980’s


An obsolete standard in the 1990’s


ISDN only provide data and voice services


Never consider multimedia applications


Circuit switching
-

Inefficient use of network
resources


Potential killer applications


Video
-
on
-
demand


At least T1 speed (~1.5Mbps) is needed


BRI has only a bit rate of 144Kbps


PRI is too expensive


ATM

Broadband
-
ISDN (B
-
ISDN)


Reasons of failure of ISDN


Obosolete assumptions on


bandwidth demand


type of network services provided


Voice and data only


Broadband
-
ISDN


No this assumption


Any kind of service can be provided with
QoS guarantee ?


Voice


Data


Video


Small or large bandwidth requirements


Real
-
time / non
-
real
-
time


How this can be done ?


Conceptually, circuit switching can provide
services with QoS guarantee


A circuit is dedicated to the application and
not shared by other applications


But, think about this application


No traffic for 99.9% of the time


Very high instantaneous traffic burst at
peak bit rate of 2.048Mbps


Small end
-
to
-
end delay requirement


With circuit switching


It should allocate 2.048Mbps to guarantee
the QoS of the bursty traffic


2.048Mbps * 99.9% of the network
resource is wasted


Circuit switching may not be a good
choice


What if we use packeting switching for
B
-
ISDN ?


Statistically multiplexing gain


QoS of service is not guaranteed


Packet drop / delay due to potential network
congestion

ATM


B
-
ISDN employs an innovative
technology called ATM (Asynchronous
Transfer Mode) which


is a cell
-
switching technology


can emulate the QoS guarantee provided
by circuit switching


Comparison of STS(Synchronous
Transfer Signal) in SONET and ATM


STS






ATM

32

1

2


. . . . . . .

3

31

32

1

1 frame = 125 micro sec. = 32 time slots = 2.048Mbps

17


10

1

7

4

23

9

15


8

4


. . . . . . .

transmission direction

transmission direction

4

ATM


Benefits


High performance via hardware switching


Dynamic bandwidth for bursty traffic


Class
-
of service support for multimedia


Scalability in speed and network size


Common LAN/WAN architecture


Opportunities for simplification via VC
architecture


International standards compliance



ATM cell header


Fixed
-
size packets (53 bytes) called cells





ATM


ATM fills the time slots with


Fixed
-
size packets (53 bytes) called cells






Information is transferred by a stream of
cells from the source to the destination

Header

User data

5 bytes

48 bytes

ATM Switch


ATM logical connection


Virtual Channel Connection (VCC)


Basic unit of switching in ATM network


Virtual Path Connection (VPC)


A collection of VCC bundled together








ATM classes of services


Real
-
time service


Constant bit rate (CBR)


Real
-
time variable bit rate (rt
-
VBR)


Non
-
real
-
time service


Real
-
time variable bit rate (nrt
-
VBR)


Available bit rate (ABR)


Unspecified bit rate (UBR)


ATM


Like packet switching


A virtual connection is first established from
the source, through a number of ATM
switches, to the destination


Routing across the network


Cells for the VC carry a VCI at their 5
-
byte
header


ATM switches along the path of the VC
route the cell based on their VCIs


Why small and fixed packet size ?


Flexible in representing different traffic


Constant bit rate traffic


Video stream






Variable bit rate traffic


Voice


Data

ATM Protocol Reference Model

Physical Layer

ATM Layer

ATM Adaptation Layer

Data

Voice

Video
-
on

-
demand

Video

Conferencing

Transmission of cell bits

SAR

CS

PMD

TC

(AAL)


Physical layer has two separate Sub
-
layer:


TC ( Transmission Covergence )


Convert cells into bits or vice versa.


Add idle cells into the channel
when it is in synchronous
transmission.



PMD ( Physical Medium Dependent )


For Bit timing, handle both wire and
fiber.

QoS guarantee by ATM Layer


ATM layer


Determine how ATM cells are switched
over the network


Congestion control inside the network



SAR (Segmentation And Reassembly)


Responsible for breaking the message into
cells or vice versa.



CS ( Convergence Sublayer )


Allow ATM systems offer different kinds of
services to different application.


(e.g. file transfer and video on demand
have different requirements concerning
error handling, timing, etc.)


Application
-
oriented AAL & SAR


AAL
-
1


Serve real
-
time constant bit rate
applications


Uncompressed video, Emulated LAN circuit


Transfer of Voice signal.


JPEG running at rates around 10Mbps


AAL
-
2


Serve real
-
time variable bit rate applications


Compressed video, voice


AAL
-
3/4


Non
-
real
-
time variable bit rate applications


Data transmission


AAL
-
5


Serve similar purpose as AAL
-
3/4


But, it is a simpler and more efficient
adaptation layer


MPEG2 for Video
-
On
-
Demand application.