------ -------Actual rate in Software

Drawer Code 39 Full ASCII in Software ------ -------Actual rate

T(s)

The ATM Forum defines four service classes: CBR, VBR, ABR, and UBR (see Figure 2430)

CBR The constant-bit-rate (CBR) class is designed for customers who need realtime audio or video services The service is similar to that provided by a dedicated line such as a T line VBR The variable-bit-rate (VBR) class is divided into two subclasses: real-time (VBR-RT) and non-real-time (VBR-NRT) VBR-RT is designed for those users who need real-time services (such as voice and video transmission) and use compression techniques to create a variable bit rate VBR-NRT is designed for those users who do not need real-time services but use compression techniques to create a variable bit rate ABR The available-bit-rate (ABR) class delivers cells at a minimum rate If more network capacity is available, this minimum rate can be exceeded ABR is particularly suitable for applications that are bursty UBR The unspecified-bit-rate (UBR) class is a best-effort delivery service that does not guarantee anything Figure 2431 shows the relationship of different classes to the total capacity of the network

ATM defines two sets of attributes User-related attributes are those attributes that define how fast the user wants to send data These are negotiated at the time of contract between a user and a network The following are some user-related attributes: SCR The sustained cell rate (SCR) is the average cell rate over a long time interval The actual cell rate may be lower or higher than this value, but the average should be equal to or less than the SCR PCR The peak cell rate (PCR) defines the sender's maximum cell rate The user's cell rate can sometimes reach this peak, as long as the SCR is maintained MCR The minimum cell rate (MCR) defines the minimum cell rate acceptable to the sender For example, if the MCR is 50,000, the network must guarantee that the sender can send at least 50,000 cells per second CVDT The cell variation delay tolerance (CVDT) is a measure of the variation in cell transmission times For example, if the CVDT is 5 ns, this means that the difference between the minimum and the maximum delays in delivering the cells should not exceed 5 ns

The network-related attributes are those that define characteristics of the network The following are some network-related attributes: CLR The cell loss ratio (CLR) defines the fraction of cells lost (or delivered so late that they are considered lost) during transmission For example, if the sender sends 100 cells and one of them is lost, the CLR is

= _1_ = 10-2

CTD The cell transfer delay (CTD) is the average time needed for a cell to travel from source to destination The maximum CTD and the minimum CTD are also considered attributes CDV The cell delay variation (CDV) is the difference between the CTD maximum and the CTD minimum CER The cell error ratio (CER) defines the fraction of the cells delivered in error

For more details about subjects discussed in this chapter, we recommend the following books and sites The items in brackets [] refer to the reference list at the end of the text

Congestion control and QoS are discussed in Sections 53 and 55 of [Tan03] and in 3 of [Sta9S] Easy reading about QoS can be found in [FH98] The full discussion of QoS is covered in [BlaOO]

Integrated Services (IntServ) jitter leaky bucket load maximum burst size multiplicative decrease open-loop congestion control Path message peak data rate per-hop behavior (PHB) priority queuing quality of service (QoS) reliability Resource Reservation Protocol (RSVP) Resv message slow start throughput token bucket traffic shaping unspecified bit rate (UBR) variable bit rate (VBR) weighted fair queuing

access rate additive increase available bit rate (ABR) average data rate backward explicit congestion notification (BECN) bursty data choke packet closed-loop congestion control committed burst size Be committed information rate (CIR) congestion congestion avoidance congestion control constant bit rate (CBR) delay Differentiated Services (DS or Diffserv) effective bandwidth excess burst size Be first-in, first-out (FIFO) queuing forward explicit congestion notification (FECN)