trailer. This tends to reduce fragmentation overhead (e.g., an IPv4 datagram containing a TCP ACK-
only packet with neither IP nor TCP options exactly fits into a single cell). The chief disadvantage of
such a scheme lies in duplication of virtual circuits: a host must create a separate virtual circuit for
each high-level protocol if more than one protocol is used. Because most carriers charge for each
virtual circuit, customers try to avoid using multiple circuits because it adds unnecessary cost.
It is commonly used in conjunction with PPPoE and PPPoA which are used in various xDSL
implementations
LLC/SNAP Encapsulation—The user multiplexes multiple protocols over a single ATM VC. The
protocol of a carried protocol data unit (PDU) is identified by prefixing the PDU with a Logical Link
Control (LLC)/Sub network Access Protocol (SNAP) header.
ATM Service Categories:
Enabling packet level QoS for PVC improves performance for selected classes of
applications. QoS cannot be set for CBR and Real-time VBR. QoS consumes system
resources; therefore the number of PVCs will be reduced. Use Advanced Setup/Quality of
Service to assign priorities for the applications
Within 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. UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.
2. CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
3. VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for
a maximum interval before being problematic. VBR has real-time and non-real-time variants,
and serves for "bursty" traffic. Non-real-time is sometimes abbreviated to vbr-nrt.
4. ABR - Available bit rate: a minimum guaranteed rate is specified
Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT), which
defines the "clumping" of cells in time.
To maintain traffic contracts, networks usually use "shaping", a combination of queuing and marking
of cells. "Traffic policing" generally enforces traffic contracts.
Traffic Parameters (Cell Rates)
Each ATM connection contains a set of parameters that describes the traffic characteristics of the
source. These parameters are called source traffic parameters. They are :
Peak Cell Rate (PCR). The maximum allowable rate at which cells can be transported along a
connection in the ATM network. The PCR is the determining factor in how often cells are sent in
relation to time in an effort to minimize jitter. PCR generally is coupled with the CDVT (Cell Delay
Variation Tolerance), which indicates how much jitter is allowable.
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