Previous Table of Contents Next


ITU-T QoS ATM Forum QoS Timing Between Source/Destination Bit Rate Connection Mode
1 A Necessary Constant Connection
2 B Necessary Variable/Constant Connection
3 C Not Necessary Variable Connection
4 D Not Necessary Variable Connectionless

Table 6-3. Adaptation service classes

Table 6-3 lists the four service classes defined by ITU-T and the ATM Forum. Class A and B require a timing relationship between the source and destination, while class C and D do not. Table 6-4 provides more specific details concerning the attributes of each service class. Notice Class B will be used for real-time audio and video and packetized audio and video.

ATM Forum QoS QoS Category Cell Rates Uses
A ABR Available Bit Rate SCR guaranteed SCR<PCR Intelligent network control, LAN emulation
B CBR Constant Bit Rate PCR=SCR Real-time audio and video, private line emulation
C,D VBR-nrt Variable Bit Rate - not real time SCR<PCR Burst LAN
B VBR-rt Variable Bit Rate - real time SCR<PCR Packet audio and packet video
C UBR Unspecified Bit Rate SCR,PCR Any application not requiring time dependencies between source and destination

Table 6-4 Quality of Service classes

The ATM service class categories, listed in order of decreasing cost to the user, are:

  ABR    Available Bit Rate
ABR class of service specifies a minimum bandwidth that is guaranteed by the service provider. Each service provider has specific bandwidths that are sold ABR. The bandwidths available from each carrier are dependent upon the ATM network backbone capacities of the carrier and the carrier’s order entry, provisioning, and billing systems. ABR is ideally suited for intelligent network congestion control and LAN Emulation (LANE). The user can burst above the SCR to the PCR.
  CBR    Constant Bit Rate
CBR is just what the name says. The peak cell rate always equals the sustainable cell rate. Any burst above the SCR results in dropped cells and the need to either retransmit the data or accept lost data. CBR is used primarily for private line emulation and real-time video and voice applications.
  VBR-nrt    Variable Bit Rate - not real time
VBR-nrt is used for connection-oriented data applications. The timing relationship between source and destination is not important. The transfer of data does not rely upon any timing dependencies between source and destination. The bit rate is not constant and may burst up to some predetermined PCR. Bursty LAN traffic is well suited to this class of service. PCR and SCR are not equal for this class of service.
  VBR-rt    Variable Bit Rate - real time
VBR-rt is used for connection-oriented audio and video applications. Another name for connection-oriented audio and video is packet audio and packet video. The timing relationship between source and destination is very important to the successful transfer of data. The bit rate is not constant and may burst up to some predetermined peak cell rate (PCR). PCR and SCR are not equal for this class of service.
  UBR    Unspecified Bit Rate
UBR is just like flying standby. You get on only when there is an empty seat. There is no guarantee by the service provider that your data will get from source to destination. This service class is the least expensive way to transmit ATM cells. (PCR and SCR can be anything negotiated with the carrier providing the carrier order entry, provisioning, and billing systems can accommodate the desired data rates.)

The service classes are subsets of the ATM Adaptation Layer. A CBR can be either AAL1 or AAL5.

Latency

Latency is a fancy way of saying "wait your turn." Cells are multiplexed onto some physical medium for transport around the country or globe. Multiplexing requires all good cells (and bad ones too at this stage of the transmission path) to queue up and wait for their turn to be multiplexed. How long is the wait? Assuming (and we know about assume, don’t we?) all other things are perfect, the latency of a cell is 53 bytes times 8 bits/byte divided by the transmission rate. More formally, latency = (53 bytes x 8 bits / byte) / (transmission rate). Assuming (there we go again) the transmission rate is DS-1, then latency = (53x8)/(1.544 x 10^6), or latency = 27 milliseconds.


Previous Table of Contents Next