LAN/WAN Design Guidelines for VoIP

There is no standard definition for QoS as it applies to real-time voice communications carried over an Ethernet LAN or IP WAN. As applied to a circuit switched PBX, QoS means consistent, reliable service delivery of control and communications signals in support of customer needs. This definition also can be used for LAN QoS in support of IP telephony. To enable LAN QoS requires all network elements, at all network layers, to work together to support a required level of traffic and service.

An IP-PBX by definition is not a virtual circuit switched communications system, like a traditional PBX, but rather a system that uses an IP network infrastructure. An IP network makes more efficient use of available bandwidth resources than does a circuit switched PBX and is designed to support the “bursty” nature of data communications traffic rather than the continuous traffic flow of real-time voice communications. IP networks can adapt to changing traffic conditions, but the level of service can be unpredictable. When used to support an IP-PBX system, the IP network must be properly designed and engineered to support the unique real-time traffic requirements of voice as opposed to less stringent data communications requirements.

QoS techniques manage bandwidth according to different application demands and network management settings but cannot guarantee a service level if resources are not available and allocated. Reserving resources for voice communications can seriously affect other network traffic. A priority for QoS network designers has been to ensure that best-effort traffic is available after resource allocations have been made. QoS-enabled high-priority voice applications must not harm lower-priority data applications.

The Internet was based on a dumb network concept with intelligent endpoints to transmit and receive datagram packets flowing through a series of network routers. IP does not deliver reliable service over the Internet: packets can be dropped by routers and are retransmitted as necessary. The service mechanism can assure data delivery, but not timely delivery. This “best-effort” service may be adequate for data networking services, but it is not good enough for voice communications.

Audio and video traffic demands sufficient bandwidth with low-latency requirements when used in two-way communications. A major challenge for network planners is to design a LAN infrastructure that satisfies an acceptable QoS level that PBX system users have grown accustomed to for their voice communications applications. A newly installed IP-PBX system in a green field location provides an ideal situation, but if a network is already installed and operating, introducing IP telephony-grade QoS should not disrupt existing services and applications.

LAN QoS levels fluctuate over time due to unanticipated changes in customer usage patterns and traffic flow. If QoS is degraded for short periods, it may significantly affect IP telephony services in ways noticeable by all system users, even if data communications services appear satisfactory. There are several reasons QoS can change:

• Temporary excessive network usage

• Insufficient link capacity

• Insufficient switch/router resources

• Traffic flow peaks

• Traffic flow interference

• Improper use of resources

Several basic control methods can be employed to manage QoS levels to ensure the higher grade of service level required by real-time voice communications:

• Reserving fixed bandwidth for mission-critical voice communications applications

• Restricting network access and usage for defined users or user groups

• Assigning traffic priorities

• Designating which kinds of traffic can be dropped when congestion occurs

There are several high-level decisions facing network planners and managers regarding the type of QoS-based network to be designed and operated. The network planner must decide whether network users are involved in the QoS functions or whether the network is in total control of QoS functions. If a network user has knowledge of QoS functions and a limited degree of QoS control, the network QoS is said to be implicit. If network QoS functions are predetermined and only the network administrator can program changes when needed, the network QoS is said to be explicit.

Another planning issue is whether QoS is soft or hard. Network QoS is said to be soft when there is no formal guarantee that target service levels will be met, even if QoS functions are implemented. Hard network QoS is a guarantee of service at a predefined level of QoS. Hard network QoS is usually available only with connection-mode transport, such as ATM constant bit rate (CBR) service.

Network QoS is also manageable by network design, by installing the necessary physical resources to support target service levels. IP-PBX system voice quality and availability can be determined by the physical LAN infrastructure and available cable bandwidth. Cisco Systems, a leading IP-PBX system supplier and the dominant supplier of data communications systems, has developed and published an IP telephony network planning guide. The Cisco guide is a planning tool for its CallManager IP-PBX system customers, but it is also useful as a network design guide for customers who plan to install and operate any converged or client/server IP-PBX system.