Pre-Deployment and Network Readiness Assessment Is Essential
IP Telephony is very different from conventional data applications in that
call quality is especially sensitive to IP network impairments. Existing network
and “traditional” call quality problems become much more obvious with the deployment
of VoIP service. For network managers, this means that LANs, access links and
network equipment may need to be upgraded and that more sophisticated management
and diagnostic tools are needed when deploying and maintaining VoIP networks.
Types of VoIP Performance Problems
There are three basic categories of performance-related problems that can occur
in IP Telephony: IP Network Problems, Equipment Configuration, and Signaling
Problems and Analog/TDM Interface Problems.
IP Network Problems:
- Jitter &$151;Variation in packet transmission time that leads to
packets being discarded in VoIP end systems; jitter is usually due to network
congestion - Packet Loss &$151;Packets lost during transmission due to network
errors, route changes, link failures or random early detection (RED) in routers - Delay &$151;Overall packet transmission “lag time” that leads to
two-way conversational difficulty.
Equipment Configuration and Signaling Problems:
- VoIP Endpoint Configuration —Performance impact of the type
of CODEC and packet loss concealment algorithm, or jitter buffer configuration - Router and Firewall Configuration —Firewalls or incorrectly
configured routers block VoIP traffic; routers need to be configured to deliver
RTP packets in a timely manner -
Bandwidth Allocation
- Network lacks sufficient bandwidth to support peak traffic volumes.
- Network lacks sufficient bandwidth to support peak traffic volumes.
Analog/TDM Interface Problems:
- Echo —"Echo” commonly occurs at the boundary between the
digital network (VoIP or TDM) and analog local loops. This becomes very obvious
and annoying with the additional delay introduced by the IP network problems
previously described. - Signal Level — Abnormally high or low voice signal levels, “clipping,”
excessive noise and “echo” occur due to incorrectly configured gateway signal
levels.
Network architects and managers should address call quality and performance
management problems when they plan and deploy their IP networks, but they
should be aware that these problems also frequently occur during normal day-to-day
network operation.
Many VoIP-related problems are transient in nature and can occur at many
places along the network path. For example, a single user accessing a file
from a server can cause a period of congestion lasting a few seconds.
This, in turn, can cause short-term degradation in call quality for other
users on the network. Given this, it is essential that network managers use
performance management tools that can detect and measure these types of network
impairments.
The transient nature of IP problems also means that they are not easily reproduced
for analysis once the call is terminated. Unlike traditional POTS, once an end
user completes and disconnects an IP call, vital diagnostic information about
that call and its packet stream is lost. Network managers can use packet loss
and jitter metrics to determine how bad the call quality was; however, these
metrics alone do not provide enough diagnostic information to determine why
the call was bad.
Six Steps To Getting Your Network Ready For VoIP
Step 1
Define High-level VoIP Requirements
Your ability to deliver good quality VoIP performance will depend on patterns
of traffic and usage, existing network capacity, existing data bandwidth and
many other factors. The first step is to define what your VoIP deployment
will look like:
- What utilization do you expect?
- Where will gateways be located?
- How will internal calls be routed?
- How will external calls be routed?
- What CODECs do you plan to use (e.g. G.711, G.729A, iLBC..)? And what
bandwidth do these take (including IP headers – 96kbps for G.711 and typically
24kbps for G.729A)?
