1. ATM By
Kauleshwarnath Pandey
Maravar Kannan

2. Asynchronous Transfer Mode
• ATM is the cell relay protocol designed by
ATM Forum and adopted by ITU-T.
• The combination of ATM & B-ISDN will allow
high-speed interconnection of all the world’s
networks.
• In fact, ATM can be thought of as the
“highway” of the information superhighway.
• ATM operates at a speed of 155 & 622 Mbps &
is a connection oriented network

3. Design Goals
• Need for a transmission system to optimize the use of high-
data-rate transmission media, in particular optical fiber.
• A system that can interface with the existing systems, such
as various packet networks, & to provide wide area
interconnectivity between them without lowering their
effectiveness or requiring their replacement.
• design that can be implemented inexpensively
• able to work with & support the existing telecommunications
hierarchies
• must be connection oriented to ensure accurate &
predictable delivery.
• to move as many of the functions to hardware as possible
and eliminate as many software functions as possible

4. Packet Networks
• Data communications today are based on
packet switching & packet networks. A
packet is a combination of data & overhead
bits that can be passed through the
network as a self contained unit. The
overhead bits, in the form of a header &
trailer, act as an envelope that provides
identification & addressing information as
well as the data required for routing, flow
control, error control, …

5. ATM LANs
Issues to be resolved
• Connectionless Vs Connection-oriented
• Physical Addresses Vs Virtual
Connection Identifier
• Multicasting & Broadcasting delivery
• varying size & intricacy
• To improve utilization

6. Cell Networks
• Many of the packet internetworking problems are solved
by adopting a concept called cell networking.
• A cell is a small data unit of fixed size. In a cell network,
all data are loaded into identical cells that can be
transmitted with complete predictability & uniformity.
• As packets of different sizes & formats reach the cell
network from a tributory network, they are split into
multiple small data units of equal length & loaded into
cells.
• Because of the same size, the problems associated with
multiplexing different sized packets are avoided.

7. Advantages of Cells
• High speed of the link coupled with the
small size of the cells
• A cell network can handle real time data
• To a cell network, the smallest unit is a
cell, not a bit.
• Switching & multiplexing can be
implemented in hardware rather than
software

8. Asynchronous TDM
• ATM uses asynchronous TDM (that’s why
called as ATM) to multiplex cells coming from
different channels. It uses fixed size slots.
• ATM mux 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.
• At the first tick of the clock, the multiplexer fills
the slot with a cell from each channel. When all
the cells from all the channels are multiplexed.
The output slots are empty.

9. ATM Switching:
Contentionless Time Division
• Advantages
• Non-blocking
• Deterministic performance—probability of cell loss = 0
• Flexible port speeds
– (DS-1, E-1, DS-3, E-3, 100M, 155M, 622M)
• Hardware multicast without increasing fabric cell traffic
• Low transit delays
• Disadvantages
• Limited scalability on single TDM fabric
– Use time-space-time to expand

10. ATM Architecture
 ATM is a cell
switched network.
 The user access
devices, called the
end points, are
connected through a
user-to-network
interface (UNI) to the
switches inside the
network.
 The switches are
connected through
network-to-network
interfaces.

11. ATM Protocol Stack
Upper Layers
ATM Adaptation Layer
ATM Layer
Physical Layer

12. The ATM Layer
header payload
Fixed length packet = cell

13. ATM Layers in end-point
devices & switches
The end points use all three layers while the
switches use only the two bottom layers.

14. ATM Process
Conventional Conventional
ATM
Telecom LAN
Traffic Type Data, Voice, Video Voice Data
Transmission Fixed Fixed Variable
Unit Cell Frame Packet
Switching Cell Circuit Packet
Connection Type Connection-oriented Connection-oriented Connectionless
Delivery Defined Classes Guaranteed Best Effort
Access Dedicated Dedicated Shared
Rate & Media Application Dependent Channel Dependent Protocol Dependent

15. ATM Process
Conventional Conventional
ATM
Telecom LAN
Traffic Type Data, Voice, Video Voice Data
Transmission Fixed Fixed Variable
Unit Cell Frame Packet
Switching Cell Circuit Packet
Connection Type Connection-oriented Connection-oriented Connectionless
Delivery Defined Classes Guaranteed Best Effort
Access Dedicated Dedicated Shared
Rate & Media Application Dependent Channel Dependent Protocol Dependent

16. ATM is packet switching!
• Switched or permanent connections
• Traffic type independent (voice, data, interactive video)
• Fixed length packet - 53 bytes (cell)
header payload
Fixed length packet = cell

17. Anatomy of an ATM Cell
8 7 6 5 4 3 2 1
Byte 1 GFC (UNI) or VPI (NNI) VPI
Byte 2 VPI VCI
Byte 3 VCI Header
Byte 4 VCI PTI CLP
Byte 5 HEC
48 Bytes Payload
VPI: Virtual Path Identifier CLP: Cell Loss Priority
VCI: Virtual Channel Identifier HEC: Header Error Check
PTI: Payload Type Indicator GFC: Generic Flow Control

18. Thank You