This includes description about what is ATM, its definition, layers, applications, working procedure, format type, available data bit rates, necessity of ATM, benefits & difference between Internet & ATM Network.

1. ASYNCHRONOUS
TRANSFER MODE
(ATM)
…Details about ATM..
ABSTRACT
This includes description about what is ATM, its
definition, layers, applications, working
procedure, format type, available databit rates,
necessity of ATM, benefits & difference between
Internet & ATM Network.
Supervised By
Pranab Bandhu Nath
(Senior Lecturer)
CSE Department
City University, Dhaka
Submitted By
Shamima Akther| ID - 1834902616
CSE 317 : Computer Networks

2. “ATM”
ATM stands for “Asynchronous Transfer Mode”. It is a specific packet oriented mechanism for
the transfer of digital based on the use of “cells” which are of a constant length, having a payload
field and a header. Cells are transmitted asynchronously and the network is connection oriented.
Each cell is total 53 Bytes long- 5 Bytes is for header & 48 Bytes is for payload.
The technique is asynchronous in the sense that the cells carrying a particular address (within a
particular virtual channel) may appear at irregular intervals within the cell stream. That means
Each cell is processed at its own time. When one is finished, the procedure calls for the next cell
to process. This is why it's called asynchronous; none of the cells go off at the same time relative
to the other cells. And it is said as connection oriented because a sender and a receiver on a
network set up a fixed path between each other before sending data and the information arrives
in the order it was sent.
It is basically cell switching and multipplexing technique that is a merger of cercuit switching as
well as packet switching. Sometimes referred to as a protocol of packet switching.
Necessity of ATM:
Telephone networks support a single quality of service and is expensive to boot.
Internet supports no quality of service but is flexible and cheap.
ATM networks were meant to support a range of service qualities at a reasonable cost- intended
to subsume both the telephone network and the Internet.
ATM offers a flexible transfer capability common to a broad range of services with widely
varying traffic patterns and can be employed on various transmission media operating at widely
varying rates.
Format types of ATM:
Asynchronous Transfer Mode can be of two format types which are as follows:
1. UNI Header: which is used within private networks of ATM for communication between
ATM endpoints and ATM switches. It includes the Generic Flow Control (GFC) field.
2. NNI Header: is used for communication between ATM switches, and it does not include the
Generic Flow Control(GFC) instead it includes a Virtual Path Identifier (VPI) which occupies
the first 12 bits.
Working of ATM:
ATM differs from other data link technologies such as Ethernet in several ways. ATM uses zero
routing. Instead of using software, dedicated hardware devices known as ATM switches establish
point-to-point connections between endpoints, and data flows directly from source to destination.

3. Ethernet and Internet Protocol (IP) use variable-length packets. ATM uses fixed-sized cells to
encode data. These ATM cells are 53 bytes in length, which consists of 48 bytes of data and five
bytes of header information. Each cell is processed at its own time. When one is finished, the
procedure calls for the next cell to process. This is why it's called asynchronous; none of the cells
go off at the same time relative to the other cells. The ATM connection can be preconfigured by
the service provider to make a dedicated or permanent circuit, or it can be switched or set up on
demand and then terminated at the end of its use.
Available Data Bit Rates for ATM:
Four data bit rates are usually available for ATM services:
• Available Bit Rate,
• Constant Bit Rate,
• Unspecified Bit Rate, and
• Variable Bit Rate (VBR).
Difference between ATM & Internet:
ATM is a “virtual circuit” based: the path is reserved before transmission. While, Internet
Protocol (IP) is connectionless and end-to-end resource reservations not possible. RSVP is a new
signaling protocol in the internet.
Addressing: ATM uses 20-byte global NSAP addresses for signaling and 32-bit locally assigned
labels in cells. While, IP uses 32-bit global addresses in all packets.
Ethernet and Internet Protocol (IP) use variable-length packets. ATM uses fixed-sized cells to
encode data.
ATM Layers:
1. ATM Adaption Layer (AAL) –
This layer corresponds to network layer of OSI model. It provides facilities to the existing packet
switched networks to connect to ATM network and use its services. It accepts the data and
converts them into fixed sized segments. The transmissions can be of fixed or variable data rate.
This layer has two sub layers − Convergence sub layer and Segmentation and Reassembly sub
layer.
It is meant for isolating higher layer protocols from details of ATM processes and prepares for
conversion of user data into cells and segments it into 48-byte cell payloads. AAL protocol
excepts transmission from upper layer services and help them in mapping applications, e.g.,
voice, data to ATM cells.
2. Physical Layer –
This layer corresponds to physical layer of OSI model. At this layer, the cells are converted into
bit streams and transmitted over the physical medium. This layer has two sub layers: PMD sub
layer (Physical Medium Dependent) and TC (Transmission Convergence) sub layer.

4. It manages the medium-dependent transmission and is divided into two parts physical medium-
dependent sublayer and transmission convergence sublayer. Main functions are as follows:
• It converts cells into a bit stream.
• It controls the transmission and receipt of bits in the physical medium.
• It can track the ATM cell boundaries.
• Looks for the packaging of cells into appropriate type of frames.
3. ATM Layer –
This layer is comparable to data link layer of OSI model. It accepts the 48 byte segments from
the upper layer, adds a 5 byte header to each segment and converts into 53 byte cells. This layer
is responsible for routing of each cell, traffic management, multiplexing and switching.
It handles transmission, switching, congestion control, cell header processing, sequential
delivery, etc., and is responsible for simultaneously sharing the virtual circuits over the physical
link known as cell multiplexing and passing cells through ATM network known as cell relay
making use of the VPI and VCI information in the cell header
Benefits of ATM Networks :
• It provides the dynamic bandwidth that is particularly suited for bursty traffic.
• Since all data are encoded into identical cells, data transmission is simple, uniform and
predictable.
• Uniform packet size ensures that mixed traffic is handled efficiently.
• Small sized header reduces packet overload, thus ensuring effective bandwidth usage.
• ATM networks are scalable both in size and speed.
ATM Applications:
1. ATM WANs – It can be used as a WAN to send cells over long distances, router serving as a
end point between ATM network and other networks, which has two stacks of protocol.
2. Multimedia virtual private networks and managed services – It helps in managing ATM, LAN,
voice and video services and is capable of full service virtual private-networking, which includes
integrated access of multimedia.
3. Frame relay backbone – Frame relay services are used as a networking infrastructure for a
range of data services and enabling frame relay ATM service to Internetworking services.
4. Residential broadband networks – ATM is by choice provides the networking infrastructure
for the establishment of residential broadband services in search for highly scalable solutions.

5. 5. Carrier infrastructure for telephone and private line networks – To make more effective use of
SONET/SDH fiber infrastructures by building the ATM infrastructure for carrying the telephonic
and private-line traffic.