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Two keys features lie at the heart of the Intserv architecture:
- Reserved resources-A router is required to know what amount of its resources are already reserved fro ongoing sessions.
- Call setup-A session requiring QoS guarantees must first be able to reserve sufficient resources at each network router
on its source-to-destination path to ensure that its end-to-end QoS requirement is met. Ths call setup process requires
the participation of each router on the path.
The steps involded in call admission in more detail:
- Traffice characterization and specifiaction of the desired QoS. In order for a router to determine whether
or not its resources are sufficient to meet the QoS requirements of a session, that session must first declare its QoS requirement,
as well as characterize the traffic that it will be sending into the network, and for which it requires a QoS guareantee.
- Signaling for call setup. A session's Tspec and Rspec must be carried to the routers at which resources
will be reserved for the session.
- Per-element call admission. Once a router receives the Tspec and Rsepc for a session requesting a QoS guarantee,
it can determine whether on not it can admit the call.
The Intserv architecture defines two major classes of service: guaranteed service and controlled-load service.
Guaranteed Quality of Service
The guaranteed service specification provides firm bounds on the queuing delays that a packet will experience in a router.
While the details behind guaranteed service are rather complicated, the basic idea is really quite simple. To first
approximation, a source's traffic characterization is given by a leaky bucket with parameters (r,b) and the requested service
is characterized by a transmission rate, R, at which packets will be transmitted. A session requesting guarnateed service
is requiring that the bits in its packet be guaranteed a forwarding rate of R bits/second. Given that traffic is specified
using a leaky bucket characteization, and a guaranteed rate of R is being requested, it is also possible to bound the maximum
queuing delay at the router.
The actual delay bound guaranteed under the guaranteed service definition is slightly more complicated, due to packetization
effects, the fact that the traffic arrival process is subject to the peak-rate limitation of the input link, and possible
additional variations in a packet's transmission time.
Controlled-Load Network Service
A session receiving controlled-load service will receive "a quality of service closely approximating the QoS that same
flow would receive from an ulnoaded network element". The Session may assume that a "very high percentage" of its packets
will successfully pass through the router without being dropped and will experience a queuing delay in the router that is
close to zero. Control-load service makes no quantitative guarantees about performance--it does not specify what constitutes
a very high percentage of packets not what quality of service closely approximates that of an unloaded network element.
The controlled-load service targets real-time multimedia applications that have been developed for today's Internet.
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