Traffic Control in ATM Networks

There are many functions involved in the traffic control of ATM networks which are given as follows.

1. Connection Admission Control: This can be defined as the set of actions taken by the network during the call set-up phase to establish whether a VC/VP connection can be made. A connection request for a given call can only be accepted if sufficient network resources are available to establish the end-to-end connection maintaining its required quality of service and not affecting the quality of service of existing connections in the network by this new connection.

There are two classes of parameters which are to be considered for the connection admission control. They can be described as follows.

A. Set of parameters that characterize the source traffic i.e. Peak cell rate, Average cell rate, burstiness and peak duration etc.

B. Another set of parameters to denote the required quality of service class expressed in terms of cell transfer delay, delay jitter, cell loss ratio and burst cell loss etc.

2. Usage Parameter Control(UPC) and Network Parameter Control(NPC): UPC and NPC perform similar functions at User-to-Network Interface and Network-to- Node Interface respectively. They indicate the set of actions performed by the network to monitor and control the traffic on an ATM connection in terms of cell traffic volume and cell routing validity. This function is also known as "Police Function". The main purpose of this function is to protect the network resources from malicious connection and to enforce the compliance of every ATM connection to its negotiated traffic contract. An ideal UPC/NPC algorithm meets the following features. 1. Capability to identify any illegal traffic situation. 2. Quick response time to parameter violations. 3. Less complexity and much simplicity of implementation.

3. Priority Control: CLP(Cell Loss priority) bit in the header of an ATM cell allows users to generate different priority traffic flows and the low priority cells are discarded to protect the network performance for high priority cells. The two priority classes are treated separately by the network Connection Admission Control and UPC/NPC functions to provide two requested QOS classes.

4. Network Resource Management: Virtual Paths can be employed as an important tool of traffic control and Network resource management in ATM networks. They are used to simplify Connection Admission Control(CAC) and Usage/ Network parameter control(UPC/NPC) that can be applied to the aggregate traffic of an entire virtual path. Priority control can also be implemented by segregating traffic types requiring different quality of service(QOS) through virtual paths. VPs can also be used to distribute messages efficiently for the operation of particular traffic control schemes like congestion notification. Virtual paths are also used in statistical multiplexing to seperate traffic to prevent statistically multiplexed traffic from being interfered with othe types of traffic, for example guaranteed bit rate traffic.

5.Traffic Shaping: Traffic shaping changes the traffic characteristics of a stream of cells on a VPC or VCC by properly spacing the cells of individual ATM connections to decrease the peak cell rate and also to reduce the cell delay variation. Traffic shaping must preserve the cell sequence integrity of an ATM connection. Traffic shaping is an optional function for both network operators and end users. It helps the network operator in dimensioning the network more cost-effectively and it is used to ensure conformance to the negotiated traffic contract across the user-to network interface in the customer premises network.

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