Asynchronous Transfer Mode ATM is the cell relay protocol designed by ATM Forum and adopted by the ITU-T. Cell, a small fixed size block of information with asynchronous TDM ensures high speed real time transmission with efficient and cheaper technology. Instead of user addresses, it uses virtual circuit identifier and virtual path identifier, which can be repeated at unrelated locations. This technology ensures connectivity to much more users than normal packet switching networks. ATM and ISDN-B combination allows high-speed interconnection of world's network.

1. ATM
Asynchronous Transfer Mode

2. ATM
✓ Cell relay protocol
✓ Designed by ATM forum
✓ Adopted by ITU-T
✓ International Telecommunication Union-Telecommunications Standards
Section

3. ATM
✓ Cell-switching and multiplexing technology.
✓ Combines the benefits of –
✓ circuit switching
✓ (guaranteed capacity and constant transmission delay)
✓ packet switching
✓ (flexibility and efficiency for intermittent traffic).
✓ It provides scalable bandwidth from a few
megabits per second (Mbps) to many gigabits per
second (Gbps).

4. WHY ATM ?
• Optical fibre offered new transmission media less
susceptible to noise with very high data rates.
• Existing system not utilising its full potential.
• A new system is required which --
• Could interface with existing system of packet
transmission.
• Is inexpensive.
• Capable of supporting existing hierarchies as local
loop, local service provider etc.
• Is connection oriented / virtual circuit switched.
• Has more functions in H/W to get more speed.

5. PROBLEMS WITH EXISTING SYSTEM
• Different protocols use packets of different sizes
and different bit distribution.
• More complex network result in larger header.
• Data field should be increased to increase
Data : Header ratio.
• Field is wasted in absence of large data.
• VISIBLE REMEDY: Variable packet size

6. PROBLEMS WITH VARIABLE PACKET
SIZE
• Unpredictable traffic.
• Switches, multiplexers and routers need elaborate
S/W to manage variable packet sizes.
• Large header information.
• Inter-networking different N/Ws is slow, expensive
and sometimes impossible.
• Difficult to provide consistent data rate.
• TDM may prove detrimental to some packets.
• A smaller packet waiting on a larger packer will be
severely delayed…

7. Multiplexing Using Different Packet Sizes

8. ATM SOLUTION -- ATM Network
• Packet switched network.
• Supports multiplexing of multiple logical
connections over single physical channel.
• No error/flow control at intermediate stages.
• Can serve as LAN or WAN backbone to existing
system without major change.
• Other protocol packets can be transformed to ATM
cell.
• Can deliver voice, data and video.

9. ATM SOLUTION -- ATM Network
• Uses ATM switch in star.
• Communication via switch.
• Network is highly scalable.
• Extremely high data rates – 1.544Mbps to 155Mbps.
• 2 OFC between host and switch for full duplex line.
ATM
SWITCH
HOST
HOST
HOST
HOST
HOST

10. • ATM caters to all requirements of data, audio
and video .
• Audio – Real time and fast.
• Data – Very high accuracy
• Video – Very high bandwidth, to create
continuity.
ATM SOLUTION -- ATM Network

11. • Size can not be large and variable.
• Fixed size cells of 53 octets (bytes).
• 5 octet headers + 48 octets data.
• Achieves higher data rates.
• Avoids transmission delays.
ATM SOLUTION -- ATM Network

12. No Delay in Multiplexing
■ Small and Equal size cells.
■ Delay suffered is very small and uniform.
■ Due to high speed of link and small cell size,
packet appear to reach almost continuously.
■ Very suitable for video and audio.
■ Switching and MUX can be implemented in H/
W.

13. ATM Multiplexing (Asynchronous)

14. Architecture of an ATM Network
• UNI – user-to-network interface
• NNI – network-to-network interface

15. Architecture of an ATM Network
■ User access devices (End points) are
connected through UNI to the switches inside
network.
■ Switches are connected through NNIs.

16. TP, VPs, and VCs
TP – Transmission path
VP – Virtual path
VC – Virtual circuit

17. Architecture of an ATM Network
■ Connection accomplished through TPs, VPs and
VCs.
■ TP – Physical connection (wire, cable, satellite ..)
between an end point and a switch or between two
switches..
■ VP –set of connections between two switches.
( combination of VCs bundled together because
parts of their path are same)
■ VCs – All cells belonging to a single message follow
same virtual circuit, in original order till destination.
(logical connections)

18. ATM Network Analogy
■ Two switches as two cities.
■ TP - Set of all highways that directly connects two
highways.
■ VP – Highway that connects two cities.
■ VCs – Lanes of a highway

19. VPs and VCs

20. Architecture of an ATM Network
■ 8 end points communicating using 4 VCs.
■ Two VCs share same VP from switch I to switch
III, bundled together in one VC.
■ Other two VCs share same path from switch I to
switch IV, hence bundled together in one VP.

21. Connection Identifiers
VPI – Virtual path identifier
VCI – Virtual circuit identifier

22. Connection Identifiers
■ ATM gives hierarchical identifier for two levels.
■ Virtual path identifier (VPI)- defines specific VP
■ Virtual circuit identifier (VCI)- defines a particular
VC inside the VP.
■ Virtual connection is identified by pair of VPI and
VCI together.
■ UNI – VPI is 8 bits, VCI is 16 bits. (Total 24)
■ NNI – VPI is 12 bits, VCI is 16 bits (Total 28)

23. Virtual Connection Identifiers
in UNIs and NNIs

24. !
UNI Cell Format
!
!
!
■ GFC (Generic flow control) – Source of information voice, data
or video. Default – 0000
■ PT – Payload Type – 3 bits
0- User Data 0- No Congestion 0- Last cell of
Frame
1- Control Data 1- Congestion 1- More to follow
■ CLP (Cell loss priority) – In case of congestion:
■ 0 = High priority. Must not be ignored
■ 1 = cell can be discarded.
■ HEC (Header error control) –Checksum corrects 1 bit error.
GFC
4
VPI
8
VCI
16
PT
2 1
CLP
1
HEC
8
Data
48 bytes

25. NNI Cell Format
• 12 bit VPI.
•Chosen from separate list for audio, video or data.
!
VPI
12
!
VCI
16
!
PT
2
!
R
1
C
L
P
1
!
HEC
8
!
Data
48 bytes

26. SVC
Setup

27. SVC setup
■ A new virtual circuit is established whenever an
end point wants connection with another.
■ ATM needs network layer address and services of
protocol like IP or B-ISDN .
■ Signalling mechanism of this other protocol makes
connection request using network layer address of
the two end points using its network layer
protocol.

28. Routing with a VP Switch

29. Routing with a VP Switch
■ A VP switch routes the cell using only the VPI.
■ During connection establishment, Switch stores—
■ arrival interface number,
■ incoming VPI,
■ corresponding outgoing interface number,
■ New VPI
■ A cell with VPI 153 arrives at switch interface 1.
■ Using switching table, switch directs cell to
outgoing interface 3 with new VPI 140.
■ VPIs change but VCI remains same.

30. A Conceptual View of a VP Switch

31. Routing with a VPC Switch

32. Routing with a VPC Switch
■ VPC switch uses both VPIs and VCIs.
■ Routing requires whole identifier.
■ Switching table stores Interface, VPI and VCI of both
input and output as required during connection
establishment.
■ A cell with VPI 153 and VCI 67 arrives at input
interface 1.
■ As per table the cell is directed to output interface 3
with new VPI 140 and new VCI 92.
■ Switch to switch — Only VPI
■ Switch to end point — VPI and VCI both.

33. Prof. Madhumita Tamhane
A Conceptual View of a VPC Switch

34. ATM Layers
■ ATM standards define three layers:
■ Application Adaptation Layer- AAL
■ ATM Layer
■ Physical Layer
!
■ End point uses all three layers.
■ Switches use two bottom layers

35. Application Adaptation Layer- AAL
■ Allows existing network e.g. packet network to
connect to ATM.
■ Accepts transmission from upper layer services
e.g. packet data.
■ Converts them into fixed size cells.
■ Input -Voice, data, audio, video.
■ Input – variable or fixed rate.
■ Reverse process at receiver.

36. Data Types
■ ATM divides AAL into categories depending on
different applications.
■ Defines 4 types of data types.
■ Constant-bit-rate(CBR): Application that generates
and consumes bits at a constant rate.
■ Real time with minimum td. (AAL1)
■ Real time voice (telephone) and video (TV).
■ Variable-bit-rate(VBR): Application that generates and
consumes bits at variable rates.
■ Bit rate varies section to section within a limit. (AAL2)
■ Compressed voice, data and video.

37. Data Types
■ Connection oriented packet data: Conventional packet
application. (AAL3)
■ X.25, TCP protocol of TCP/IP that uses virtual circuits.
■ Connection less packet data: Applications that use
Datagram approach to routing. (AAL4)
■ IP protocol of TCP/IP
■ Point-to-point: Cuts across all above data types but
for Point-to-Point. (AAL5).
■ This sublayer is also called SEAL Simple and Efficient
Adaptation Layer.
■ AAL3 and AAL4 overlap hence combined to AAL3/4.

38. Convergence and Segmentation
■ ITU-T divides each categories of AAL in two Sub
layers.
■ Convergence sub layer CS
■ Segmentation and Reassembly SAR
■ Duties of sublayers vary for each AAL.

39. AAL1 -- Constant Bit Rate
■ Supports applications that transfer information at
constant rates.
■ Allows ATM to connect existing digital telephone
networks e.g. DS-3 or E-1.
■ Convergence sub layer CS – Divides incoming bit
stream into 47 byte segments.
■ Passes to SAR sub layer below.
■ Segmentation and Reassembly SAR – Receives 47
bytes from CS, adds 1 byte header to make 48 byte
data unit.
■ Passes to ATM layer for encapsulation into cell.

40. AAL1 -- Segmentation and Reassembly SAR
■ CSI– Convergence Sub layer Identifier
■ SC – Sequence count.
■ CRC – Cyclic Redundancy Check
■ P - Parity
1 Byte
Header
Payload
47 Bytes
CSI
1bit
SC
3 bits
CRC
3 bits
P
1 bit

41. AAL1 -- Segmentation and Reassembly SAR
■ CSI– 1 bit for Signalling, Not defined yet.
■ SC – 3-bit, gives modulo-8 sequence number for ordering and
identifying cells for end-to-end error and flow control.
■ CRC – 3-bit calculated on first 4 bits using x3
+ x + 1.
■ Detects single/multiple errors.
■ Corrects single error as real time transmission.
■ P - 1-bit Parity calculated over first 7 bits of header.
■ Only odd number of errors can be detected.
■ Can be used for error correction on first 4 bits.—-
■ 1 bit error-detected by both CRC and P bit, which CRC corrects.
■ 2 bits error- CRC detects but not P, correction invalid. Cell
discarded.

42. AAL2 -- Variable Bit Rate
■ Variable data rate e.g. news reader vis-a-vis
football match.
■ CS – Divides incoming bit stream into 45 byte
payload.
■ SAR – Adds 1 byte header and 2 byte trailer.
■ Makes 48 Byte data unit for ATM layer for
encapsulation.
■ CSI and SC are same as AAL1.

43. AAL2 -- Segmentation and Reassembly
■ IT – Information Type - Identifies data segments as falling at
beginning, middle or end of message.
■ LI– Length Indicator – 6 bits of trailer with final segment of message (if
IT in header indicates end of message) indicates how much of final cell is data/
padding.
■ If original bit stream not a multiple of 45, dummy bits added at end.
■ Indicates where in segment padding starts.
■ CRC – 10 bits, CRC for entire data unit. Also for Single error correction.
Header
1 Byte
Payload
45 bytes
Trailer
2 Bytes
CSI
1 bit
SC
3 bits
IT
4 bits
LI
6 bits
CRC
10 bits

44. AAL3/4 --
• AAL3 for Connection oriented and AAL4 for Connection
less services.
• Later combined into single format AAL3/4, fundamental
format being same.
!
• Convergence sub layer CS – 44 byte segment
■ CS accepts 65535 bytes from upper layer e.g. SMDS or Frame Relay.
■ Adds a header at beginning and a trailer at end of 65535 bytes,
indicating how much of final cell is data/padding.
■ Indicates where in segment padding starts.
■ If original bit stream not a multiple of 44, dummy bits added at end.
■ Then All control bits would fall at right place.
■ CS passes 44 bytes message to SAR.

45. AAL3/4
■ T- Type – all zero. (Holdover from previous format)
■ BT – Begin tag. Identifies first cell of segmented message.
■ Provides synchronisation
■ BA – Buffer Allocation.
■ Tells size of buffer needed for coming data.
■ AL – Alignment – Makes rest of trailer 4 bytes.
■ ET – End tag or ending flag for synchronisation.
■ L – Length of user data.
User Data ≤ 65,536 bytes
T
1 byte
BT
1 byte
BA
2 bytes
PAD
0-43 byte
AL
1 byte
ET
1 byte
L
2 byte

46. AAL3/4
■ PAD - 0-43 bytes added to last or last two segments.
1. No. of data bytes in final segment exactly 40—no padding required as 4
bytes of AL added to make 44.
2. No. of data bytes in final segment less than 40—padding added to make
total 40.
3. No. of data bytes in final segment between 41 and 44—padding added
to make total 84.
1. First 44 makes complete segment.
2. Next 40 bytes with trailer makes last segment.
User Data ≤ 65,536 bytes
T
1 byte
BT
1 byte
BA
2 bytes
PAD
0-43 byte
AL
1 byte
ET
1 byte
L
2 byte

47. AAL3/4 -- Segmentation and Reassembly
■ SAR adds 2 bytes header and 2 bytes trailer to each 44 byte
payload.
■ ST – Segment Type -2 bits- segment falling at beginning,
middle or end of message. or is a single segment message.
■ CSI– Convergence sub layer identifier- 1 bit-for signalling,not
defined yet.
■ SC -Sequence count - 3-bit, gives modulo-8 sequence
number for ordering and identifying cells for end-to-end
error and flow control.
Header
2 Byte
Payload
44 bytes
Trailer
2 Bytes
LI
6 bits
CRC
10 bits
ST
2
CSI
1
SC
3
MID
10

48. AAL3/4 -- Segmentation and Reassembly
■ MID – 10 bits- identifies cells coming from different data
flows and multiplexed on same virtual connection.
■ LI - 6 bits- Length Indicator- First 6 bits of trailer in
conjunction with ST indicates how much of last segment is
data/padding.
■ LI field is used only if ST indicates message as last.
■ CRC- 10 bits - for entire data.
Header
2 Byte
Payload
44 bytes
Trailer
2 Bytes
LI
6 bits
CRC
10 bits
ST
2
CSI
1
SC
3
MID
10

49. AAL5— SEAL
■ Point to point link. Hence..
■ No addressing, sequencing or other header info.
■ Because, cells belonging to a single message travel
sequentially.
■ Only padding and 4-field trailer added at CS.
■ Convergence sub layer CS –
■ CS accepts max 65535 bytes and adds 8 byte trailer
and padding at end of 65535 bytes, if required.
■ Passes 48 byte segments to SAR.
■ All segments are 48 bytes except last which is 40 byte
data with 8 byte trailer.

50. AAL5 -- Point to point link
No addressing, sequencing or other header info.
■ PAD - 0-47 bytes to make last segment 48 byte.
■ UU- User to user ID- 1 byte- at discretion of user.
■ T – Type – 1 byte- Reserved, not defined.
■ L – Length –2 bytes- How much is data / pad.
■ CRC - last 4 bytes- Error check for entire data unit.
■ No header/trailer added in SAR sub layer. 48 bytes passed
directly to ATM layer
User Data ≤ 65,536 bytes
PAD UU
1
T
1
L
2
CRC
4

51. ATM Layer
■ ATM layer provides routing, traffic management, switching
and multiplexing services.
■ Processes outgoing traffic by accepting 48 bytes segment
AAL sublayer.
■ Transforms them to 53 bytes cell by adding 5 byte header
as per UNI or NNI.

52. ATM Layer- Header Format
■ GFC- Generic Flow Control - 4 bits
■ UNI format—Flow control at UNI level.
■ NNI format - Not necessary. Bits added to VPI.
■ Longer VPI allows more virtual paths at NNI level.
■ VPI- Virtual Path Identifier
■ UNI Format- 8 bits
■ NNI Format- 12 bits

53. ■ VCI- Virtual Path Identifier- 16 bits in both formats.
■ PT-Payload Type- 3 bits in both formats-
■ bit 1- defines payload as user data/managerial info 0/1.
■ Two bits as above.
■ CLP Cell Loss Priority— 1bit - Congestion control
■ Low priority cell discarded during congestion (QOS).
■ CLP=1, High priority, must be retained.
■ EX- Dummy cells to (inserted to achieve desired data rate)
assigned CLP=0. Must be dropped first at congestion.
■ HEC- - 8 bits in both formats- on first 4 bytes of header.
■ x8
+x2
+ x + 1 used to correct single bit error and detect multiple
bit errors.
ATM Layer- Header Format

54. Physical Layer
■ Defines transmission medium, bit transmission, encoding,
E/O transformation etc..
■ Provides convergence with physical transport protocol as
SONET and T-3.
■ Provides mechanism to transform flow of cells into flow
of bits.
■ Transport medium can be twisted pair, coaxial, fiber optic
cable as per user.

55. ATM Switch Fabric -
Banyan Switch
■ Named after Banyan tree
■ Multistage switch with microswitches at each stage.
■ Route cells based on output port represented as a binary string.
■ For n inputs and n outputs- log2
(n) stages with n/2 microswitches
at each stage.
■ Second stage routes the cells based on second high order bit.
■ Above 8 inputs, 8 outputs, stages= log2
(8)=3.

56. ATM Switch Fabric -
Routing in Banyan
Switch
■ Note the interconnection of switched.
■ Cell at input port 1 to be routed to output port 6(110 in
binary).
■ Microswitch A-2 routes the cell based on first bit ‘1’
■ B-4 routes on second bit ‘1’ and
■ Finally C-4 routes to 6 based on third bit ‘0’ to 6.
■ Similarly cell from input 5 is routed to output 2 (010).
■ Problem: The possibility of internal collision even when two
cells are not heading for same output port.

57. ATM Switch Fabric -
Batcher-Banyan Switch- By K. E. Batcher
■ Batcher switch comes before Banyan switch and sorts the incoming
cells according to their final destination.
■ H/W trap module prevents cells with same output destination from
passing to Banyan switch simultaneously. Only one cell at each tick.

58. ATM -
Service Classes
■ CBR- for customers that need real time audio or video.
■ VBR-RT- for those users that need real time services (voice, video)
and use compression techniques to create variable bit rate.
■ VBR-NRT- for those users that do not need real time services but
use compression techniques to create variable bit rate.
■ ABR- delivers cells at minimum rate, which can be exceeded if
more network capacity is available. For applications which are
bursty.
■ UBR- Best-effort delivery service that does not guarantee anything.

59. Relationship of Service Classes to Total Capacity
of Network

60. ATM -
Quality of Service (QOS)
■ Defines set of attributes related to the performance of the
connection.
■ Each service class is associated with a set of the attributes.
■ Two Categories:
■ User-Related Attributes:
■ Defines how fast the user wants to send data.
■ Negotiated at the time of contract between a user and
a network.
■ Network-Related Attributes:
■ Defines characteristics of the network.

61. ATM -
Quality of Service (QOS)- User-Related Attributes
■ SCR (Sustained Cell Rate):-
■ Average cell rate over a long time interval.
■ Actual cell rate may be lower or higher than this value.
■ Average should be equal to or less than SCR.
■ PCR (Peak Cell Rate):-
■ Defines the sender’s maximum cell rate.
■ User’s cell rate can reach this rate as long as SCR is maintained.
■ MCR (Minimum Cell Rate):-
■ Defines minimum cell rate acceptable to the sender.
■ Network must guarantee that sender can send at least MCR.
■ CVDT (Cell Variation Delay Tolerance):-
■ Measure of variation in cell transmission time.
■ Difference between maximum and minimum delays in delivering cell
not to exceed CVDT.

62. ATM -
Quality of Service (QOS)- Network-Related Attributes
■ CLR (Cell Loss Ratio):-
■ Defines fraction of cells lost or delivered so late that they are
considered lost during transmission.
■ One cell lost out of total sent 100, CLR = 1/100
■ CTD (Cell Transfer Delay):-
■ Average time needed for a cell to travel from source to
destination.
■ Maximum CTD and Minimum CTD are also considered
attributes.
■ CDV (Cell Delay Variation):-
■ Difference between the CTD maximum and CTD minimum.
■ CER (Cell Error Ratio):-
■ defines the fraction of cells in error.

63. ATM Applications -
ATM WANs
■ ATM basically WAN to deliver cells over long distance.
■ Connects LANs and other WANs together.
■ Router between an ATM network and other network serves
as end-point.
■ Router has two stacks of protocol, for ATM and other N/W.

64. ATM Applications -
ATM LANs
■ ATM Originally designed for WAN.
■ Can be used for high speed LANs(with issues which need
attention).
1. Connectionless Vs Connection-oriented - Connection
oriented ATM needs connection establishment/termination
for sending packet while Ethernet can send directly.
2. Physical address Vs VCI - Connectionless Ethernet uses
source and destination addresses while ATM uses VCI.
3. Multicasting and broadcasting delivery - Ethernet can be
both while ATM is point-to-point.

65. ATM Applications -
LANE - Local Area Network Emulation
■ LANE enables ATM switch to behave like a LAN switch
■ Provides connectionless services
■ Lets stations use their traditional address
■ Allows broadcast delivery
■ Based on client/server approach
■ All stations use LANE client (LEC)
■ Two servers use LANE server (LES) S/W-LES and BUS.
■ LES installed on top of three ATM protocols.
■ Higher layer protocols send request to LES for LAN service
as connectionless delivery using MAC unicast, multicast or
broadcast address.
■ LEC interprets and uses services of LES or BUS…

66. ATM Applications -
LANE - Local Area
Network Emulation
■ During frame transmission through physical layer, LEC
sends special frame to LES server.
■ Server creates Virtual Circuit between source and
destination.
■ Source uses this VC and VCI to send frames to destination.
■ Multicast and broadcast requires Broadcast/Unknown
server (BUS) which has permanent VC connection to all.
■ Multicast and broadcast frame first goes to BUS.
■ BUS copies frame and sends to all required stations.

67. ATM Applications -
LANE - Local Area
Network Emulation
■ This can also be used for unicast by sending frame to all
and desired destination recognises its frame.
■ This unicast sometimes is more efficient.
■ Layers in each station, the LES server and the server can
be seen above.