Accommodating Voice Traffic on Campus Switches
Network design considerations for voice

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.
NOTE:

Plan to work with a voice specialist to complete traffic engineering analysis for the network.

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.