Following
the subject which we talked in the last post, in this entry we will talk about
mechanisms to provide QoS.
In order of
developing those mechanisms, we must know some characteristics of the traffic
source: it can be defined as Constant or Variable Bit Rate, depending if traffic
rate is fixed, such as 64Kbps voice, or not, such as MPEG coded video. Other
interesting parameters which characterize flow are peak rate, average rate and
burst size.
QoS mechanisms
can be defined into Static and Dynamic Functions. Static Functions are based on
providing QoS in a constant way, meanwhile Dynamic Functions allow providing
QoS in a specific moment and application.
- Static Functions:
- Admission control:Before accepting a new connection, these functions must examine both traffic and QoS parameters in order of maintaining QoS and available resources.
- Resource reservation: In order of guaranteeing QoS, network resources can be reserved in advance. The usual resources reserved are bandwidth and buffer space.
- QoS signaling: It is a mechanism used by end users to communicate QoS related information to the network. Signaling is often used to work on a dynamic way with admission control and reservation resources.
- Dynamic Functions
- Traffic policing: In order of protecting the net from users violating the traffic contract (which can compromise the QoS of other connections), all entering traffic is therefore subject to policing. Depending on the “policer”, the decision maker element, traffic will be admitted or not into the network. The decision will be done depending on the parameters of the traffic defined on top: Peak Rate, Average Rate, Burst Size and its combinations. Some examples of this kind of functions are Frame Relay, Leaky Bucket and Token Bucket.
A similar function of traffic policing is Shaping
Policing: it regulates a flow to make sure it does not violate traffic
contract. The difference from traffic policing remains in a buffer, in order of
not dropping all the packets. Token Bucket Shaper is an example of those
mechanisms.
Other
dynamic functions will be presented in future posts.
