3. Implementing Qos - NEC UNIVERGE SV9100 Manual

SDSL
(Symmetric Digital Subscriber Line)
BT Baseband
- EPS 8 (4-wire)
- EPS 9 (2-wire)
Wireless LAN
ISDN
(Integrated Services
Digital Network)
Note: These services may not be available in your country
1.3

3. Implementing QoS

We have seen some of the problems associated with Voice Quality in Section 2. This section looks
at how QoS can be implemented on data networks to provide the "best case" for VoIP traffic.
It should be noted that not all network hardware supports QoS and each manufacturer will have their
own methods of implementing QoS. The explanations below are intended to be as generic as
possible. The installer/maintainer of the data network should be familiar with the QoS characteristics
of their equipment and should be able to configure the equipment accordingly.
The term Quality of Service is commonly used to describe the actual implementation of Prioritisation
on network hardware. This prioritisation (at Layer 2 and Layer 3 of the OSI model) is described
below.
3.1 What is Prioritisation?
When data is transmitted through a network it will typically encounter "bottlenecks". This is where
the amount of available bandwidth is reduced, or the amount of data increases. This means that the
packet delivery is affected.
Consider data communication between the two computers shown in the diagram below. The Hosts
are capable of transmitting data at 100Mbps. When a packet from Host A (destined for Host B)
reaches the router, the available bandwidth is reduced to 256Kbps and the packet flow must be
reduced. This is an example of a bottleneck.
1Mbps – 10Mbps - Symmetric bandwidth
Depends on cable - Inexpensive
- No contention
quality and
distance between
endpoints (up to
2Mbps)
Typically up - Inexpensive
to 54Mbps (running costs)
64Kbps per - Inexpensive (install)
"B" channel
- Guaranteed bandwidth
- No contention
- Reliable
Part 1: VoIP Reference Manual
- Availability – only
certain areas are
SDSL enabled
- Contention
(shared
bandwidth)
- No bandwidth guarantees
- Delay varies dependant
on Internet conditions
- Unreliable
- Both ends must be
located on same
Telephone Exchange
- Bandwidth depends on
distance between
endpoints – can be
relatively low
- Can be unreliable
- Limited distance
- Difficult to implement QoS
- Not an "always on"
solution
- Limited speed
- High call costs
11
cable quality and