Asynchronous Transfer
Mode or ATM is a high-speed networking standard designed to support both
voice and data communications. ATM is normally utilized by Internet service
providers on their private long-distance networks. ATM operates at the data
link layer (Layer 2 in the OSI) over
either fiber or twisted-pair cable. ATM is a statistical time−division multiplexed (TDMed) form of traffic that is designed to
carry any form of traffic and enables the traffic to be delivered
asynchronously to the network. The system also uses preassigned slots for
transmission for the traffic flow. It is also a connection-oriented protocol
but uses virtual circuits like permanent virtual circuits and switched virtual circuits.
ATM can also be used
across group of networks and also any other network like LAN, CAN, WAN and MAN.
ATM also emerged as the basis for future broadband communications with high
speed data rate and larger bandwidths. For basis of speed and performance a
table is shown below.
ATM differs from more
common data link technologies like Ethernet in several ways. For
example, ATM utilizes no routing. Hardware devices known as ATM switches establish point-to-point connections
between endpoints and data flows directly from source to destination.
Additionally, instead of using variable-length packets as Ethernet
does, ATM utilizes fixed-sized cells. ATM cells are 53 bytes in length, that includes
48 bytes of data and five (5) bytes of header information.
Why cells?
When speech, data and
video pushed through into one network they encounter queuing or jitter that
would produce traffic. Because packet size are not constant, this produces
jitters or traffic. That is why all packets, or
"cells," should have the same small size. In addition the fixed cell
structure means that ATM can be readily switched by hardware without the
inherent delays introduced by software switched and routed frames.
Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch on
the circuit is informed of the traffic class of the connection.
ATM traffic contracts form part of the
mechanism by which "quality of service"
(QoS) is ensured. There are four basic types (and several variants) which each
have a set of parameters describing the connection.
1. CBR - Constant bit rate: a Peak Cell
Rate (PCR) is specified, which is constant.
2. VBR - Variable bit rate: an average
cell rate is specified, which can peak at a certain level for a maximum
interval before being problematic.
3. ABR - Available bit rate: a minimum
guaranteed rate is specified.
4. UBR - Unspecified bit rate: traffic is
allocated to all remaining transmission capacity.
ATM defines two different cell formats: UNI (User-Network Interface) and NNI (Network-Network Interface). Most ATM links use UNI
cell format.
UNI ATM CELL NNI ATM CELL
ATM Layered Architecture
ATM layered architecture falls into the 1st two OSI
layers which are the DATA link layer and the physical link layer. The Physical Layer defines the
medium for transmission, any medium-dependent parameters (e.g., rate, quality
of service required), and framing used to find the data contained within the
medium. The ATM Layer provides the basic 53-byte cell format, by defining the
5-byte ATM header for each 48-byte payload segment handed down by the AAL. The
ATM Adaptation Layer (AAL) adapts the higher-level data into formats compatible
with the ATM Layer requirements, i.e., this layer segments the data and adds
appropriate error control information as necessary. It is dependent on the type
of services (voice, data, etc.) being transported by the higher layer. Several
AAL protocols have been defined for specific types of data. These are loosely
associated with various classes of data. However, no AAL is restricted to a
specific data class or type; all types of data could conceivable be handled by
any of the AALs.
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AAL 1
Constant bit rate, connection-oriented,
synchronous traffic (e.g., uncompressed voice)
AAL 2
Definition never completed undefined, but
envisioned to be for variable bit rate, connection-oriented, synchronous
traffic (e.g., compressed video)
AAL 3/4
Variable bit rate, connection- oriented,
asynchronous traffic (e.g., X.25 data) or connectionless packet data (e.g.,
SMDS traffic) with an additional 4-byte header in the information payload of
the cell
AAL 5
Similar to AAL 3/4 with a simplified information
header scheme that requires only one header per data unit and uses the PTI bit
(see below) to indicate the last cell in a transmission Examples of services
that use AAL 5 are Classic IP over ATM, and LAN Emulation (LANE). AAL 5 is the
most widely used ATM Adaptation Layer Protocol.
ATM
Applications
ATM is an enabler of basic and advanced applications such as
remote sensing, 3-D interactive simulations, tele-instruction, biological
teleresearch, and medical teleconsultations. Edge devices at the boundary of an
ATM network convert non-ATM traffic streams into standard ATM cells. ATM
technology is implemented in backbone, enterprise, and edge switches as well as
hubs, routers, bridges, multiplexer, servers, servers farms, and NICs (Network
Interface Cards) in high-end Internet appliances. The ATM data Exchange Interface(DXI) enables fast acces to public network services. A flexible and extendible networking solution.
