Variable Bandwidth Allocation Bandwidth Allocation in ATM Networks

Figure 6-3 Monitoring of VBR: cells above the PCR line or exceeding MBS limit above SCR line are marked for possible discard.

V. Unspecified Bit Rate

The Unspecified Bit Rate UBR service category was primarily defined for handling LAN traffic to take advantage of the remaining available bandwidth. UBR does not guarantee any level of bandwidth. It allows a source to use max remaining available bandwidth. Therefore, during a low traffic period, a LAN source can take advantage of the full line speed. However, during very high bandwidth usage, it is possible that the UBR source will not receive any bandwidth at all. For UBR traffic class, ATM network does not guarantee any of the QoS parameters CLR, CTD. The UBR service is, in a sense, considered as the Best Effort attempt by the ATM network. As one can realize, this behavior is very similar to the Ethernet LAN environment. In fact, UBR was defined to match this type of traffic pattern. In addition to LAN traffic, UBR can also be used to take advantage of the available background bandwidth for applications such as transferring news and weather pictures, file transfer and email. ATM does not consider congestion control for the UBR traffic. It is assumed that it is done by a higher layer on a end- to-end basis. Figure 6-4 Bandwidth allocation for UBR traffic.

VI. Available Bit Rate

The Available Bit Rate ABR service class has been defined as a refinement to the UBR class. It provides some degree of control over the traffic flow as well as guaranteeing a minimum bandwidth. Through the traffic flow control mechanism, the traffic source adjusts its traffic generation according to the network condition. ABR takes advantage of all the remaining bandwidth during normal traffic conditions up to the specified PCR level. The PCR parameter is used to make sure that the remaining bandwidth is shared fairly among ABR type users. However, when the congestion occurs, the ABR source is expected to reduce its bandwidth usage. Unlike UBR, ABR is guaranteed a minimum bandwidth specified by the traffic parameter Minimum Cell Rate MCR. The ABR class is typically used for LAN traffic. Figure 6-5 Bandwidth allocation according to ABR service class. Previous Table of Contents Next Copyr ight © CRC Pr ess LLC