Atm( Asynchronous Transfer mode )

1. ATM
Asynchronous Transfer Mode

2. ATM
Asynchronous Transfer Mode
Cell relay protocols
Designed by ATM forum (formed in Oct. 1991 )and
adopted by ITU-T
Combination of ATM and SONET allows high speed
interconnection of the world network.
Also thought as “High way”
Signaling (connection setup) Protocol: Q.2931–
Discovering routes and allocating resources at switches

3. Design Goals
Optimize use of high data rate transmission media e.g
fiber optics and new equipments are less susceptible to
noise degradation. A technology is needed to take
advantage of both factors.
The system must interface with existing systems and
provide wide area interconnectivity B/W them without
lowering their effectiveness.
The design must be implemented inexpensively so that
cost would not be a barrier to adopt.
The new system must be able to work with existing
telecom hierarchies(local loop, local provider, long haul
and so on).

4. Design Goals
The new system should be connection oriented to
ensure accurate and predictable delivery.
One objective is to move as many of functions to
hardware as possible (for speed) and eliminate as many
software functions as possible (again for speed).

5. Problems
Problems associated with existing systems;
Frame networks:
Before ATM,DC at DDL based on frame switching and
frame networks. Different protocols use frames of varying
size. As network become more complex, the information
that must be carried in header become more expensive.
The result larger and larger headers relative to the size of
data unit. In response, some protocols have enlarged the
size of the data unit to make header use more efficient.
Unfortunately, large data fields create waste if there is not
much information to transmit. To improve utilization, some
protocols provide variable frame size to users.

6. Mixed network traffic:
The variety of frame sizes makes traffic
unpredictable.switches, multiplexes and router must
incorporate software systems to manage the various
sizes of frames. A great deal of header information
must be read and each bit counted and evaluated to
ensure the integrity of every frame. Internetworking
among different frame works is slow and expensive.

7. Another problem is that of providing consistent data rate
delivery when frame sizes are predictable and can vary so
dramatically. To get the most out of broadband technology,
traffic must be TDM onto shared paths. Imagine onto one
link. What happen when line 1 uses large frames(data
frames) while line 2 uses small frames(audio or video). In
fig. Large size of X create unfair delay for frame A,B &
C.This makes shared medium unusable and create
unacceptable delays for audio and video traffic.The traffic
must travel among different paths, like automobile and
train traffic.but , to fully utilize broad bandwidth links, we
need to be able to send all kinds of traffic over same link.

8. ATM
Asynchronous Transfer Mode
A cell is basic unit of data exchange
It fixed size block of information
Provides predictability and uniformity
Provides continuous stream (movies or series of still
photos)
Handle real time transmission (phone call)
Cell size is 53 bytes(5 header plus 48 payload)

9. Frames of diff. Sizes are split into smaller data unit of
equal size and are loaded into cells. The cells are
multiplexed with others are routed through the cell
network.

10. ATM uses asynchronous time division multiplexing, that is
why it is called asynchronous Transfer Mode to multiplex
cells coming from different channels. It uses fixed size
slot(size of cell). ATM multiplexers fill a slot with a cell from
any input channel that has a cell; the slot is empty if none
of the channels has a cell to send.

11. Architecture of an ATM network
UNI- user to network interface
NNI- network to network interface

12. TP, VPs, and VCs
TP- transmission path
VPs- virtual paths
VCs- virtual paths

13. Example of VPs and VCs

14. a virtual connection is defined by a pair
of numbers: the VPI and the VCI.
Connection identifier:

15. Virtual connection identifiers in UNIs and NNIs

16. An ATM cell

17. Routing with a switch

18. ATM layers

19. Physical layer
ATM cells can be carried by any physical layer carrier like
Ethernet& wireless LAN.
SONET is original design on which ATM based
Reason for using SONET
High data rate & boundaries of cell can be clearly
defined by pointer
Other physical technologies can also be used
The receiver is used to guess the end of cell and
apply CRC to 5 byte of header, if there is no error then count 52
bytes back to find begging of cell.
Transmission Convergence (TC) handles error detection,
framing & Physical medium dependent (PMD) sublayer
handles encoding

20. ATM layers in end points & switches

21. ATM layer
It provides routing, traffic management, switching &
multiplexing services. Also Handles virtual circuits, cell
header generation, flow control
It process outgoing traffic by accepting 48 bytes
segment from AAL sub layer & transform them into 53
byte cell by addition of a 5 bytes header.

22. ATM headers
GFC(4 bit provides flow control at UNI,no need in NNI),VPI(8 bit in UNI& 12
bit in NNI), VCI(16 bit in both frame headers),PT(3 bit, define user data/
managerial info.)CLP(1 bit),HEC(1 byte)

23. Application Adaptation layer AAL
AAL was designed to accept any types of payload, both
data frames and streams of bits.
It has two sub layers:
CS(convergence sub layer)& SAR(segmentation & Reassembly)
These two layers break the data into chunks that are
encapsulated into a cell at ATM layer.
CS checks the integrity of data and SAR segmented the
data at source & reassembly the data at the designation
There many version of AAL:AAL1,AAL2,AAL3/4,AAL5

24. AAL1
Support Apps. That transfer info. At constant bit rate
such as video and voice
It allows ATM to connect existing digital telephone
network such as voice channels and T lines.
In this CS divides bit stream into 47 bytes segments and
pass them to SAR. CS adds no header here but SAR
adds 1 byte header and passes the 48 bytes segment to
ATM layer.
SN(sequence number)- 4 bit used to order the bits
SNP(sequence number protection)-4bit protect the first
field

25. AAL1

26. AAL2
Original design was to support a variable data rate bit
stream
But it has been redesigned for low bit rate traffic and
short frame such as audio, video or fax.
It is also uses in mobile telephony.
The process of encapsulating short frames from the
same source(the same user of mobile phone) or from
several sources(several users of mobile telephones)
into one cell is shown.

27. AAL2

28. AAL3/4
AAL3 support connection oriented data services
AAL4 support connection less data services
The fundamental issues of the two were same therefore
they have been combined into single format AAL3/4.
It provides compressive sequencing and error control
mechanism.
The encapsulation process is shown.

29. AAL3/4

30. AAL5
AAL 5 designed as replacement for AAL3/4
No PDU information in ATM header
Only uses 1 bit of ATM header to mark end of PDU
Does not require additional per-cell headers/trailers
CS-PDU contains data with only 8-byte trailer
Uses stronger error correction at PDU level (CRC- 32)
Protection against lost, corrupt and misordered cells is provided by
CS-PDU

31. AAL5

32. Applications
• Government
• Long distance provider
• Bell telephone company
• ISP
• Large banks
• Frame relay
• Large universities

33. ATM support many interfaces
FDDI100 Mbps
SONETOC-3—155 Mbps, OC-12—622 Mbps
DS-11.544 MBPS
DS-344.736 Mbps
E12.048 Mbps
E334.368 Mbps
UTP &STP25.6 Mbps

34. QoS in ATM
CBR: Designed for real-time audio or video services
VBR: Variable-bit-rate
–VBR-RT [Real-time]
»For real time services
»Uses compression techniques to create a variable bit rate.
–VBR-NRT [Non-real-time]
»Uses compression but for non real time.
ABR: Available bit rate
–Delivers cells at a minimum rate.
–If more network capacity is available, this minimum rate can be exceeded.
–Suitable for applications that are bursty.
UBR: Unspecified bit rate; Best effort delivery that does not guarantee
anything.

35. Figure 23.28 Relationship of service classes to the total capacity
User-Related attributes: Defines how fast the user
want to send data.
–Sustained cell rate: Average cell rate over a long time interval.
Actual cell rate can be higher or lower but average should be
equal or less than SCR
–Peak cell rate: Sender’s maximum cell rate.
–Minimum cell rate: Sender’s minimum cell rate.
–Cell Variation delay tolerance: Measure of variation in cell
transmission times. This is the difference between the
minimum and maximum delays in delivering the cells.

36. Network related attributes are those that define
characteristics of the network.
–Cell loss ratio (CLR): Fraction of cells lost (or delivered so
late that they are considered lost) during transmission.
–Cell transfer delay (CTD): average time needed for a cell to
travel from source to destination.
–Cell delay variation (CDV): difference between CTD
maximum and CTD minimum
–Cell error ratio (CER): Fraction of the cells delivered in
error.
ATM Network

37. Quality of Service: Flow Demands
Reliability: Lack of reliability means losing a packet or
acknowledgement, which entails retransmission. Different application
programs need different levels of reliability.
Delay: Source-to-destination delay. Delay tolerance varies between
applications.
Jitter: Variation in delay for packets belonging to the same flow. Real-
time audio and video applications cannot tolerate high jitter.
Bandwidth: bits per second
Flow classes: Depend on flow characteristics, we can classify flow into
groups e.g., CBR, UBR, etc.