ATM
Asynchronous Transfer Mode (ATM) is a multiplexed packet switching technique
that uses packets (cells) with a fixed 53-octet length. Each cell contains a 48-octet
information field and a 5-octet header that identifies the cell as belonging to a
certain virtual path or channel.
The 53-octet cell size resulted from a compromise between the U.S. and Japan and
the rest of the world.
The U.S. and Japan wanted a 64-octet payload size to minimize overhead issues.
Europeans wanted a 32-octet payload size to minimize packetization delay and
ensure that it would accommodate all traffic types.
The 53-octet size splits the difference down the middle.
Benefits of ATM
ATM provides several advantages:
Fixed length cells require lower processing overhead and produce higher
transmission speeds than traditional packet switching methods.
ATM transmits asynchronous data in a synchronous network. It can also
prioritize time-sensitive traffic ahead of non-time-sensitive traffic to ensure
that quality of service is maintained.
ATM allocates bandwidth on demand, making it suitable for high-speed
connection of voice, data, and video services. It also employs statistical
multiplexing techniques to efficiently utilize resources.
ATM is application-independent, meaning it can be used as a common
infrastructure for many types of networks, including public, private, Local
Area Network (LAN), and campus backbone.
ATM is designed for high-performance multimedia networking. It has been
implemented in a very broad range of networking devices:
– PC, workstation, and server network interface cards
– Switched-Ethernet and token ring work group hubs
– Work group and campus ATM switches
– Enterprise network switches, multiplexers, edge, and backbone
switches
International standards compliance in central office and customer premise
environments allows multi-vendor operation and interoperability.
1.3 ATM Standards
ATM standards have been established for the industry through several standards
organizations:
The International Telecommunication Union – Telecommunication
Standardization Sector (ITU-T) is the leader in defining Integrated Services
Digital Network (ISDN), Broadband ISDN (B-ISDN), and ATM
specifications on an international level. The organization formerly was
known as the CCITT or the Consultative Committee for International
Telegraph and Telephone.
The American National Standards Institute (ANSI) is the formal standards
body guiding the development of ATM in the U.S.
ATM Protocol Model
ATM is a Layer 2 protocol. The ATM layer adds five octets of header to each cell.
The header carries sufficient information to route the cells.
The OSI Layer 1—Physical Layer transmits and receives cells. Layer 1 also
encodes and decodes bits into electrical/optical systems.
The OSI Layer 2—Data Link Layer, subdivided into two ATM layers, contains an
ATM layer and an ATM Adaptation Layer (AAL).
The ATM layer:
Detects and corrects header errors
Generates and extracts the cell header
Translates Virtual Channel Identifiers (VCIs) and Virtual Path Identifiers
(VPIs) and places them into the header
Multiplexes and demultiplexes the cell to and from facilities
Inserts and removes idle cells used to pad the transmission rate
Routes and switches cells
Controls ATM traffic flow
The ATM Adaptation Layer (AAL) handles the Segmentation and Reassembly
(SAR), the payload error checking and correction, and end-to-end timing
responsibilities. This layer contains three fully-defined AALs.
The OSI Layer 3 – Network Layer identifies end-point, selects service, and
signals.
1.6 ATM Cell Size
The cell size consists of a 5-octet header that contains information for switching
cells. The header is examined and updated on a node-by-node basis and is
service-independent.
The cell size also consists of information or data that is 48 octets long. This part of
the cell contains payload or information relevant to a specific service. This
information is relevant only to end-user systems and is service-specific.
ATM encapsulates all services into cells, places them into a synchronous cell
stream, and routes them across an ATM network. ATM cells interleave when
there is an available slot. If a slot is not available, the cell is queued; therefore,
cells are said to be transferred asynchronously with guaranteed cell order.
1.7 ATM Cell Header
The ATM cell header consists of:
Generic Flow Control (GFC)—Has local significance only and can be used to
provide standardized local functions (e.g., flow control) on the customer site.
The value encoded is not carried end-to-end and is overwritten in the public
network.
Virtual Path Identifier (VPI)—Distinguishes data of one virtual path from
data of another.
Virtual Channel Identifier (VCI)—Distinguishes data of one virtual channel
from data of another.
Payload Type Indicator (PTI)—Indicates whether the cell contains Layer
Management Information. PTI also is used for indicating a network
congestion state or for network resource management.
Cell Loss Priority (CLP)—Indicates the loss priority of the cell to assist the
cell discarding process and to minimize the Quality of Service (QOS)
degradation.
Header Error Control (HEC)—Is used by the physical layer for detection/
correction of bit errors in the cell header. HEC may also be used for cell
delineation.
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