A definition is the amount of calls that could have problems to get a free voice channel, is used mainly for trunks, for example GOS = 0.01 mean 1 call per each 100 could have problems to get free trunks.
Service level agreement, is the time your service will be up, normally it is beyond 97.00 % (includes, acces, transmision, platform service etc), also is used to apply to the service provider some charge in case the service is below of the SLA signed.
1. Grade of service (GOS)
The number of calls that allows to fail during busy hour.
For example GOS of long distance call will be 0.1% and for local call will be 0.5%. But it will depend on types of calls in your network.
2. Quality of Service is a set of service requirements to be met by the network while transporting a flow or packet. QoS is a meaurable level of service delivered to network users, which can be characterized by packet loss probability, available bandwidth, end to end delay. Such QoS can be provided by network service providers in term of some agreement – service level agreement SLA between network users and service providers.
3. Service Level Agreement SLA
It is very important parameter to set between the service provider and network users. In other words, that is the agreement provided by service provider to agree providing which level of service to network users.