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, as illustrated in Figure using a Cisco CallManager: A VoIP packet consists of the voice payload, RTP header, UDP header, IP header, and Layer 2 encapsulation. The IP header is 20 bytes, the UDP header is 8 bytes, and the RTP header is 12 bytes. The link layer overhead varies in size according to the Layer 2 media used; Ethernet requires 18 bytes of overhead. The voice payload size and the packetization period are device dependent. Coder-Decoders (codecs) convert the analog voice to a digital signal format. This technology has been used for years to convert a telephone signal into a 64,000 bps digital signal (DS0) for use on TDM-based systems. Today, an IP phone uses a G.711 codec for normal voice digitization. G.711 is the only type supported for the Cisco Conference Connection and Personal Assistant applications. G.729 is another supported codec that provides compression of the voice traffic down to 8 kbps. Cisco VoIP equipment supports G.711 and G.729, along with several other common industry standards.
Content 7.1 Planning for Implementation of Voice in a Campus 7.1.5 Auxiliary VLANs Some Cisco Catalyst switches offer a unique feature called an “auxiliary VLAN” or a “voice VLAN.” Auxiliary VLANs allow you to overlay a voice topology onto a data network. You can segment phones into separate logical networks, even though the data and voice infrastructure are physically the same. Auxiliary VLANs place the phones into their own VLANs without any end-user intervention. Furthermore, these VLAN assignments can be seamlessly maintained, even if the phone is moved to a new location. The user simply plugs the phone into the switch, and the switch provides the phone with the necessary VLAN information. By placing phones into their own VLANs, network administrators gain the advantages of network segmentation and control. Furthermore, network administrators can preserve their existing IP topology for the data end stations. IP phones can be easily assigned to different IP subnets using standards-based DHCP operation. With the phones in their own IP subnets and VLANs, network administrators can more easily identify and troubleshoot network problems. Additionally, network administrators can create and enforce QoS or security policies. Auxiliary VLANs enable Cisco network administrators to gain all the advantages of physical infrastructure convergence while maintaining separate logical topologies for voice and data terminals. This creates the most effective way to manage a multiservice network.
Content 7.1 Planning for Implementation of Voice in a Campus 7.1.6 QoS Almost any network can take advantage of QoS for optimum efficiency, whether it is a small corporate network, an Internet service provider (ISP), or an enterprise network. QoS utilizes features and functionality to meet the networking requirements of applications sensitive to loss, delay, and delay variation (jitter). QoS allows preference to be given to critical application flows for the available bandwidth. The Cisco IOS implementation of QoS software provides these benefits:
Content 7.1 Planning for Implementation of Voice in a Campus 7.1.7 Importance of High Availability for VoIP The traditional telephony network strives to provide 99.999 percent uptime to the user. This corresponds to 5.25 minutes per year of downtime. Many data networks cannot make the same claim. To provide telephony users the same, or close to the same, level of service as they experience with traditional telephony, the reliability and availability of the data network takes on new importance. Reliability is a measure of how resilient a network can be. Efforts to ensure reliability include choosing hardware and software with a low mean time between failure, or installing redundant hardware and links. Availability is a measure of how accessible the network is to the users. When a user wants to make a call, for example, the network should be accessible to that user. Efforts to ensure availability include installing proactive network management to predict failures before they happen, and taking steps to correct problems in the design of the network as it grows. When the data network goes down, it may not come back up for minutes or even hours. This delay is unacceptable for telephony users. Local users with network equipment, such as voice-enabled routers, gateways, or switches for IP phones, now find that their connectivity is terminated. Administrators must provide an uninterruptible power supply (UPS) to these devices in addition to providing network availability. Previously, users received their power directly from the telephone company central office or through a UPS that was connected to a keyswitch or PBX in the event of a power outage. Today, the network devices must continue to function, provide service to the end devices, and possibly (as with Power over Ethernet [PoE]) supply power to end devices. Note: Cisco has the option of using DC power with many of its routers, which allows power to be distributed from a “battery bank” that is continuously being charged. When a power outage occurs, the batteries supply DC to the equipment. Battery banks are very common in the telephone industry. Network reliability comes from incorporating redundancy into the network design. In traditional telephony, switches have multiple redundant connections to other switches. If either a link or a switch becomes unavailable, the telephone company can easily re-route calls. This is why telephone companies can claim a high availability rate. High availability encompasses many areas of the network. In a fully redundant network, the following components need to be duplicated: In some data networks, a high level of availability and reliability is not critical enough to warrant financing the hardware and links required to