IP telephony places strict requirements on the network
infrastructure. The network must provide sufficient bandwidth and quick
convergence after network failures or network changes. Most IP telephony
installations are built on an existing network infrastructure, therefore the
infrastructure typically requires enhancement with priority given to voice
traffic.

General Design Considerations
To determine if an infrastructure
can support the addition of voice, evaluate these considerations:

-
Features required for each device in the campus network – IP phones
require power and most enterprises put IP telephony applications on a separate
VLAN with priority handling.
-
Physical plant capable of supporting IP telephony – The wiring and
cabling plant must be adequate for IP telephony needs. At a minimum, Category 5
cabling is required and consideration should be made for increased wall jacks
and switch ports required to support phone and PC connections.
-
Provision switches with inline power to support IP phones – Within a
wiring closet, deploy a Catalyst Inline Power Patch Panel or an in-line power
from the switch to provide in-line power to the IP phones. This may increase
the power requirements of the switch itself.
-
Network bandwidth adequate for data, voice and call control traffic
– Along with data traffic, consider both voice and call control traffic loads.
Bandwidth provisioning requires careful planning of the LAN infrastructure so
that the available bandwidth is always considerably higher than the load. There
should be no steady-state congestion or latency over the LAN links. This is
critical for voice operations over a LAN infrastructure.
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NOTE:
Plan to work with a voice specialist to complete traffic engineering
analysis for the network.
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Bandwidth Provisioning
Properly provisioning the network
bandwidth is a major component of designing a successful IP telephony network.
The required bandwidth can be calculated by adding the bandwidth requirements
for each major application, including voice, video, and data. This sum then
represents the minimum bandwidth requirement for any given link, and it should
not exceed approximately 75 percent of the total available bandwidth for the
link.

From a traffic standpoint, an IP telephony call consists of two traffic
types:
-
Voice carrier stream – This consists of Real-Time Transport Protocol
(RTP) packets that contain the actual voice samples.
-
Call control signaling – This consists of packets belonging to one
of several protocols; those used to set up, to maintain, to tear down, or to
redirect a call depending upon call endpoints. Examples are H.323 or Media
Gateway Control Protocol (MGCP).
A Voice over IP (VoIP) packet consists of the voice payload, IP header,
UDP header, RTP header, and Layer 2 link header. Coder-decoder (codec) type
(G.711, G.729, etc.) is configurable by device. However, G.729 does not support
fax or modem traffic. The IP header is 20 bytes, the UDP header is 8 bytes, and
the RTP header is 12 bytes. The link header varies in size according to the
Layer 2 media used; Ethernet requires 14 bytes of header. The voice payload
size and the packetization period are device-dependent.
To calculate the
bandwidth that voice streams consume, use this formula:
- (Packet payload + all headers in bits) * Packet rate per second; for
example, 50 packets per second (pps) when using a 20-ms packet period
Power Considerations
Accurate calculations of power
requirements are critical for an effective IP telephony solution. Power can be
supplied to the IP phones directly from Catalyst switches with inline power
capabilities or by inserting a Catalyst Inline Power Patch Panel. In addition
to IP phones, failover power and total load must be considered for all devices
in the IP telephony availability definition, including Distribution and
Backbone submodules, gateways, CallManager and other servers and devices. Power
calculations, therefore, must be network rather than device based.
Providing highly available power protection requires an uninterruptible
power supply (UPS) with a minimum battery life to support one hour and a four
hour response for power system failures, or a generator with an onsite service
contract. This solution must include UPS or generator backup for all devices
associated with the IP Telephony network. In addition, consider UPS systems
that have auto-restart capability and a service contract for four-hour support
response.

IP
telephony high-availability power and environment include these
recommendations:
- UPS and generator backup
- UPS systems with auto-restart capability
- UPS system monitoring
- A 4-hour service response contract for UPS system problems
- Maintain recommended equipment operating temperatures 24/7
Intelligent Network Services
Network management, high
availability, security, and quality of service (QoS) intelligent network
services must extend to incorporate voice-specific attributes.

-
Network Management – The merging of network management tasks
associated with both voice and data networks is one of the key benefits of
using a converged network as opposed to a voice only network. However, it is
still necessary to understand the traditional voice-only management concepts to
relate the features available in that technology to the converged network
management techniques.
-
High Availability – As with any network capability, plan redundancy
for critical voice network components such as the Cisco CallManager and the
associated gateway and infrastructure devices.
-
Security – The subject of securing voice communications has received
more visibility recently as network convergence becomes an accepted design
model. With the advent of IP telephony traffic traversing the LAN
infrastructure, the potential exists for malicious attacks on call-processing
components and telephony applications. As with all network devices, there
should be a predefined security policy for all devices, applications, and users
associated with the voice network that is appropriate for the level of caution
required. Consider security measures for voice call-processing platforms,
applications and telephony traffic.
-
QoS – The goal of QoS is to provide critical applications a higher
priority for service so that they are the least likely to be delayed or to be
dropped in times of congestion. When a network becomes congested, some traffic
will be delayed or lost. Voice traffic has strict requirements concerning delay
and delay variation (also known as "jitter") and compared to most
data traffic, voice traffic is relatively intolerant of loss. To establish
priority processing for voice traffic, a wide range of IP QoS features can be
employed, such as classification, queuing, congestion detection, traffic
shaping, and compression.