What is ATM?
Asynchronous transfer mode (ATM) is a network switching technology
used by Broadband Integrated Service Digital Network (BISDN). It uses a
technique called cell switching, breaking all data into cells, or
packets and transmits them from one location on the network to another,
connected by switches (Carr and Snyder p.640). The small, constant cell
size allows ATM equipment to transmit video, audio, and computer data
over the same network, and assure that no single type of data hogs the
line.
The latest implementations of ATM support data transfer rates of from
25 to 622 Mbps (megabits per second). This compares to a maximum of 100
Mbps for Ethernet, the current technology used for most LANs or WANs(3).
What are the advantages of ATM?
ATM will provide a single network for all traffic types-voice, data,
and video. ATM allows for the integration of networks improving
efficiency and manageability.
Due to its high speed and the integration of traffic types, ATM will
enable the creation and expansion of new applications such as multimedia
to the desktop (2).
Because ATM is not based on a specific type of physical transport, it
is compatible with currently deployed physical networks. ATM can be
transported over twisted pair, coax and fiber optics (2).
ATM is evolving into a standard technology for local, campus/backbone
and public and private wide area services. This uniformity is intended
to simplify network management by using the same technology for all
levels of the network (2).
The information systems and telecommunications industries are
focusing and standardizing on ATM. ATM has been designed from the onset
to be scalable and flexible in:
- Geographic distance
- Number of users
- Access and trunk bandwidths (as of today, the speeds range from
Megabits to Gigabits)
What are the disadvantages of ATM?
- It is too expensive
- Overhead of cell header (5 bytes per cell)
- Complex mechanisms for achieving Quality of Service(QOS)
- Congestion may cause cell losses (1)
There are three layers in the B-ISDN protocol reference model (1).
- Physical layer
- Various transmission media
- Kilobits per second to gigabits per second
- ATM Layer
- short fixed-length cells (53 bytes (48 bytes payload + 5 bytes
header))
- multiplex logical channels within a physical channel
- fixed length cells very-high-speed switching hardware
- problems with traffic management
- ATM adaptation layer
- AAL Type 1 --- constant bit rate services
- AAL Type 2 --- variable bit rate services
- AAL Type 3/4 --- connectionless services/data protocols
- AAL Type 5 --- high-speed data protocol
ATM Protocol Architecture Seven layer of OSI Model
Figure1: Comparison of ATM Protocol Architecture to Seven-Layer
OSI Model shows that ATM adaptation layer and ATM layer of ATM
Architecture are equivalent to Transport, Network, and Data Link
layers of OSI Model.
There are four different types of ATM service.
- Constant Bit Rate (CBR) specifies a fixed bit rate so that data is
sent in a steady stream. This is analogous to a leased line.
- Variable Bit Rate (VBR) provides a specified throughput capacity
but data is not sent evenly. This is a popular choice for voice and
videoconferencing data.
- Unspecified Bit Rate (UBR) does not guarantee any throughput
levels. This is used for applications, such as file transfer, that
can tolerate delays.
- Available Bit Rate (ABR) provides a guaranteed minimum capacity
but allows data to be bursted at higher capacities when the network
is free.
The figure below shows implementations of the ATM User Network
Interface accessing Public ATM Network through Private User Network
interface and/or Public User Network Interface.
Figure2: Implementations of the ATM UNI (2)
UNI = User Network Interface
Public UNI - which will typically be used to interconnect an ATM
user with an ATM switch deployed in a public service provider's
Network
Private UNI - which will typically be used to interconnect an ATM
user with an ATM switch that is managed as part of the same corporate
Network
Some people think that ATM holds the answer to the Internet bandwidth
problem, but others are skeptical. ATM creates a fixed channel, or
route, between two points whenever data transfer begins. This differs
from TCP/IP, in which messages are divided into packets and each packet
can take a different route from source to destination. This difference
makes it easier to track and bill data usage across an ATM (3).
Reference:
Houston H. Carr, and Charles A. Snyder. "The Management of
Telecommunication." McGral-Hill Companies, 1997.
Web Page Reference:
http://www.npac.syr.edu/users/mahesh/homepage/atm_tutorial/
http://www.atmforum.com/atmforum/market_awareness/casestudies/1.html
