Wimax standards and Security The Wimax

Asynchronous Transfer Mode

Asynchronous transfer mode (ATM) is a connection-oriented service. Each con- nection is called an ATM virtual circuit that inherits many essential features of

a circuit-switched network. The QoS parameters are specified for each virtual circuit rather than on individual ATM cells. The ATM adaptation layer (AAL) specifies the following QoS schemes for ATM virtual circuits:

1. 
   Constant bit rate (CBR): A CBR connection has a guaranteed band- width and it supports CES that is to emulate DS0 or T1 circuit. There is no need for QoS of a CBR connection as the services are guaran- teed by the nature of circuit-switched technology. Many carriers are using ATM CBR to carry voice traffic on their ATM backbone.
   
2. 
   
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   Variable bit rate (VBR): This is the most flexible QoS scheme. A VBR connection has three parameters—peak cell rate (PCR), sus- tained cell rate (SCR), and maximum burst size (MBS). A VBR connection guarantees the service at the SCR level. If there is available bandwidth after SCR, it would burst the traffic to the PCR level. For example, if a customer subscribes to the service of SCR 512 Kbps and PCR 1024 Kbps, the customer is guaran- teed to have a data rate up to 512 Kbps. If the network is not busy, the customer would enjoy up to 1024 Kbps. The VBR service includes real-time VBR (rtVBR) and nonreal-time VBR (nrtVBR), where rtVBR provides assurance of delay for real-time applications such as video conference.
   
3. 
   Unspecified bit rate (UBR): This is also known as the best effort service. In other words, there is no guarantee of this service. If the network has a bandwidth, it will serve UBR connections. If the network is congested, the UBR cells would be put on a waiting queue. When the queue buffer is full, the cells are dropped.
   

There are more AAL services (such as ABR), but this chapter covers only four—CBR, rtVBR, nrtVBR, and UBR—as they are related to the WiMAX QoS services to be discussed later. An ATM switch with QoS capability supports admission control that is configured via either permanent virtual circuit (PVC) or switch virtual circuit (SVC). PVC is a manual provision and SVC uses the Q.2931 protocol to establish a virtual circuit. If an ATM switch does not have the capacity to support a connection request, it rejects this request. If an ATM switch accepts a connection request, it guarantees the QoS until the connection is terminated manually or via the Q.2931 protocol.

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