6 NGN services
6.6 QoS

Multimedia services require from networks to ensure QoS (Quality of Service) parameters. IP networks have been designed with a best-effort delivery model which doesn’t meet these requirements. Therefore some QoS mechanisms must be implemented in IP transport networks:

Unfortunately, these mechanisms are not aware of communication sessions (e.g., VoIP sessions) started by higher layer protocols such as SIP.

NGN networks provide a lot of services (applications) which generate many different types of traffic in the network and which require a more controlled way of traffic handling. QoS enabled networks are based on a concept which divides all network traffic into different classes with different characteristics.

An end-to-end packet delay is referred as a time necessary to transfer a packet from its source to its destination. A packet jitter is defined as variations in the packet delays. Packet error ratio (PER) is defined as the percentage of sent packets that were dropped or lost. Based on the PER we can divide applications into two groups error tolerant and intolerant. Table 1 summarizes these applications in relation to the packet delay parameter.

Table 1 Application categorisation based on packet delays and PER

Error tolerant

Error intolerant

Interactive (delay <<1 s)

Conversational voice and video

Command/control (e.g. Telnet, interactive games)

Responsive (delay ~2 s)

Voice/video messaging

Transactions (e.g. E- commerce, WWW browsing, Email access)

Timely (delay ~10 s)

Streaming audio and video

Messaging, Downloads (e.g. FTP, still image)

Non-critical (delay >>10 s)

Fax

Background (e.g. Usenet)

RSVP (defined in RFC 2205 in 1997) is used by the IntServ model and more importantly MPLS to realize a resource reservation. It provides applications with the means to let the network know what and how much resources it requires. This process is called signalling.

In order to support QoS policy in NGN, ITU has recommended an infrastructure with a Resource and Admission Control Function (RACF). The resource allocation and admission control is realized in the transport layer. At the same time ETSI developed a functional architecture for resource management called a Resource Admission Control Subsystem (RACS) for access and aggregation networks. Both systems show a lot of similarities and only a few minor differences.