CS 635, Lecture 7, Part 2 ATM
In order for ATM networks to deliver guaranteed quality of service on demand while
maximizing the utilization of availble network resource, effective traffic management
mechanisms are neded. Almost every aspect of ATM network operation, from signaling
requests and routing to network resource allocation and policing, contains some traffic
Quality of Service Attributes
While setting up a connection on ATM networks, users can specify the following
parameters related to the desired quality of service:
- Peak Cell Rate (PCR): The maximum instantaneous rate at which the user will transmit.
For bursty traffic, the inter-cell interval and the cell rate varies considerably. The PCR
is the inverse of the minimum inter-cell interfval.
- Sustained Cell Rate (SCR): This is the average rate as measured over a long time
- Cell Loss Ratio (CLR): The percentage of cells that are lost in the network because of
error or congestion and are not delivered to the destination, i.e,
CLR = #Lost Cells/ #Transmitted Cells
Each ATM cell has a cell loss priority (CLP) bit in the header. During
periods of congestion, the network will first discard cells with CLP=1. Since the loss of
cells with CLP=0 is more harmful to the operation of the application, the CLR can be
specified separately for cells with CLP=1 and for those with CLP=0
Cell Transfer Delay (CTD): The delay experienced by a cell between
network entry and exit points is called the cell transfer delay. It includes propagation
delays, queueing delays at various intermediate switches, and service times at queueing
Cell Delay Variation (CDV): This is a measure of variance of CTD. High
variation implies larger buffering for delay sensitive traffic such as voide and video.
Burst Tolerance (BT): This determines the maximum burst size that can be
sent at the peak rate. This is the bucket size parameter for the leaky bucket algorithm
that is used to control the traffic entering the network. The algorithm consists of
putting all ariving cells in a buffer (bucket) which is drained at the sustained cell rate
(SCR). The maximum number of back to back cells that can be sent at the peak cell rate is
called maximum burst size (MBS). BT and MBS are related as follows:
BT = (MBS - 1)(1/SCR -
Minimum Cell Rate (MCR): This is the minimum rate desired by the user.
The first six of the above parameters were originally specified in UNI
version 3.0. MCR has been added recently and will appear in the next version of the
traffic management document.
- To provide a guaranteed QoS, a traffic contract is established during connection setup,
which contains a connection traffic descriptor and a conformance definition.
- It is not necessary for every ATM virtual connection to have a specified QoS.
Unspecified QoS contracts can be supported by an ATM network on a "best-effort"
basis. Such best effort services are sufficient for supporting most of the existing data
In general, a traffic contract specifies one of the following classes of traffic:
- Constan Bit Rate (CBR): This class is used for emulating circuit switching, where the
bit rate is constant. Cell loss ratio is specified for cells with CLP=0 and may or may not
be specified for cells with CLP=1
- Variable Bit Rate (VBR): This class allows users to send at a variable rate.
Statisticall multiplexing is used and so there may be small nonzero random loss. Depending
upon whether or not the application is sensitive to cell delay variation, this class is
subdivided into two categories: real-time VBR (VBR-RT) and nonreal-time VBR (VBR-NRT).
While cell transfer delay is specified for both categories, CDV is specified only for
real-time VBR. An example of real-time VBR is interactive compressed video while that of
nonreal-time VBR is mutlimedia email.
- Available Bit Rate (ABR): This class is designed for normal data traffic such as file
transfer and email. Although the standard does not require the cell transfer delay and
cell loss ratio to be guaranteed, it is desirable for switches to minimize the delay and
loss as much as possible. Depending upon the congestion state of the network, the source
is required to control its rate. The users are allowed to declare a minimum cell rate
(MCR), which is guaranteed to the VC by the network. Most VCs will ask for an MCR of zero.
Those with higher MCR may be denied connection of sufficient bandwidth is not available.
- Unspecified Bit Rate (UBR): This class is designed for those data applications that want
to use any left-over capacity and are not sensitive to cell loss or delay. Such
connections are not rejected on the basis of bandwidth shortage (i.e., no connection
admission control) and not policed for their usage behavior. During congestion, the cells
are lost but the sources are not expected to reduce their cell rate. Instead, these
applications may have their own higher-level cell loss recovery and retransmission
mechanisms. Examples of applications that use this service are email and file transfer.
Congestion Control Techniques
- One method to avoid network congestion is to accept a new ATM connection during
connection setup pahse only when sufficient network resources are available to provide the
acceptable QoS. This is called connection admission control (CAC), which is needed for
connections where the QoS must be guaranteed. The "busy" tone on telephone
networks is an example of CAC. Mechanisms for CAC are currently not standardsized.
- UNI 3.0 allows traffic shaping using a generic cell rate algorithm (GCRA) and binary
(EFCGI0 feedback congestion control.
Generic Cell Rate Algorithm (GCRA)
- Also called the "leaky bucket" algorithm, which converts a bursty stream into
a more regular pattern. This algorithm works by putting all arriving cells into a bucket,
which is drained at the sustained cell rate.
- If too many cells arrive at once, the bucket may overflow. The overflowing cells are
called non-conforming and may or may not be admitted into the network.
- If admitted, the cell loss priority (CLP) bit of the non-conforming cells be be
set so that they will be first discarded in case of overload
- Leaky bucket is used by the netowkr to ensure that the input meets the prenegotiated
parameters such as the sustained and peak cell rates. Such "traffic shaping"
algorithms are open loop in the sense that the parameters cannot be changed dynamically if
congestion is detected after negotiation.
- In a closed loop (feedback) scheme, however, sources are informed dynamically about the
congestion state of the network and are asked to increase or decrease their input rate.