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TopBackground
In the context of service provisioning, the Internet supports the guaranteed and controlled-load services. While the guaranteed services class is designed for real-time traffics that need guaranteed maximum end-to-end delays, the controlled-load services class is designed for traffics that does not require this guarantee, but are, nevertheless, sensitive to, overloaded networks, and to the danger of losing packets. A file transfer session is an example of traffic that does not need real guarantee, while real-time audio and video transfers are examples of traffic that needs real guarantees. Both classes of services operate on the principle of ‘admission control’, in which a flow is set-up, which must conform to the IETF’s (Internet Engineering Task Force’s) T-SPEC. The admission control principle operates as follows: (Le Boudec & Thiran, 2004, p.75)
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In order to receive either type of service, a flow must first perform a reservation during a flow set-up.
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A flow must conform to an arrival curve of the form σ(t) = min (M + pt, rt + b). In Intserv (Integrated Services) jargon, the 4-uple (p, M, r, b) is called a T-SPEC or traffic specification. In order words, the T-SPEC is declared during the reservation phase. Here, M = maximum packet size, p = peak rate, b = burst tolerance, r = sustainable rate.
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All routers along the path accept or reject the reservation. With the guaranteed service, routers accept the reservation only if they are able to provide a service curve guarantee and enough buffer for loss-free operation. The service curve is expressed during the reservation phase.
Key Terms in this Chapter
Excess Information Rate (EIR): Specifies the average rate up to, which, excess Service Frames (frames whose average rate > CIR or Committed Information Rate) are admitted into the provider’s network. These Service Frames are not CIR-conformant, and are hence, delivered without any performance objectives.
Excess Burst Size (EBS): Specifies the maximum number of bits allowed for incoming Service Frames to be EIR-conformant; for every connection, an EBS is defined. It is also defined as the maximum number of bits in excess of the Committed Burst Size (CBS) that a user can send during a predefined time period.
Maximum End-To-End Delay: This is the sum of the maximum delays experienced by a packet at each hop (node) on its path from origin node to destination node.
Maximum Burst Size: The maximum amount of traffic in bits that can be sent or transmitted in a burst.
Access Rate: (in bits per second) The definition for every connection depends on the bandwidth of the channel connecting the user to the network; the user can never exceed this rate. For example, if the user is connected to a Frame Relay network by T-1 line, the access rate is 1.544 Mbps and can never be exceeded.
Communication Subnet: This is the aspect of a MAN/WAN that consists of transmission lines, interconnected by switching elements – essentially routers and layer-3 switches.
Carrier Ethernet Network: This is the communication subnet for providing Ethernet services to customers.