QoS and Voice
| Venti Systems will implement IP telephony in the head office only. Head-Office QoS and VoiceBecause IP telephony is included in the Venti Systems design, you must also consider managing the quality of service that the traffic experiences. Recall that although you typically think of applying queuing only to slow WAN links, LAN links can also be congested, so queuing should be deployed on any link that could potentially experience congestion, to provide the needed services to the network traffic. Queuing policiesin other words, how each traffic class is handledshould be consistent across the enterprise. Venti Systems will use IP phones to digitize and packetize the voice traffic. As illustrated earlier in Figure 12-6, IP phones with a built-in switch will be used; one port will connect to the access switch and another will connect to the user's laptop. Access switches will provide inline power for the IP phones, using power over Ethernet (PoE). The voice and data traffic will be on separate VLANs to allow easier implementation of QoS tools; the connection to the access switch is an 802.1q trunk, with the laptop on the native VLAN. Classification and marking of voice traffic will be done by the IP phone. Recall that the point within the network where markings are accepted is known as the trust boundary; any markings made by devices outside the trust boundary can be overwritten at the trust boundary. A Cisco IP phone could be considered to be a trusted device because it marks voice traffic appropriately, while a user's laptop would not usually be trusted because users could change markings (which they might be tempted to do to attempt to increase the priority of their traffic). Therefore, the access switches will mark the laptop traffic appropriately.
As described in Chapter 6, "Quality of Service Design," Cisco created a QoS Baseline that provides recommendations to ensure that both its products, and the designs and deployments that use them, are consistent in terms of QoS. The baseline includes an 11-class scheme, and companies can evolve to this level of classification. Venti Systems will start with a 5-class scheme, as illustrated in Figure 12-10. Figure 12-10. Venti Systems' QoS Classes After traffic has been classified and marked and sent on its way through the network, other devices will then read the markings and act accordingly. Tools such as weighted random early detection (WRED) and low latency queuing (LLQ) will be used. Cisco Automatic QoS (AutoQoS) provides a simple, automatic way to enable QoS configurations in conformance with the Cisco best-practice recommendations. Venti Systems will use AutoQoS to create configuration commands to perform such things as classifying and marking VoIP traffic and then applying strategies for that traffic. The configuration created by AutoQoS becomes part of the normal configuration file and can therefore be edited if required. The call-processing function previously performed by the PBX will now be handled by a call-processing manager, CCM, a software-based system that provides functions such as setting up and terminating calls, routing to voice mail, and so forth. CCM is installed on a Cisco Media Convergence Server (MCS) and selected third-party servers. Because this is a relatively small IP telephony network (with less than 2500 phones), two CCM servers will be deployed: one to act as a publisher (and store the master copy of the database of configurations) and the other to act as a subscriber (the device with which phones register). The subscriber also acts as a backup to the publisher. The Cisco Unity unified messaging solution will be deployed to deliver e-mail, voice mail, and fax messages to a single inbox so that users can, for example, listen to their e-mail over the telephone, check voice messages from the Internet, and forward faxes to wherever they might be. The complete voice network architecture, with the redundant components removed for clarity, is illustrated in Figure 12-11. Figure 12-11. Venti Systems' IP Telephony Architecture The voice gateway to the PSTN can be implemented with a variety of devices. For example, Cisco multiservice access routers communicate directly with CCM and support a wide range of packet telephonybased voice interfaces and signaling protocols. Alternatively, voice gateway modules can also be installed in Cisco switches. If voice traffic travels across WAN links, the traffic should be compressed to reduce the required bandwidth. Currently, voice traffic is not going over WAN links in this design; if it does in the future, G.729 compression is recommended. RTP header compression, cRTP, would also be considered on these WAN links. Another way that could be used to reduce the bandwidth required by voice calls is voice activity detection (VAD). Recall that, on average, about 35 percent of a call is in fact silence; when VoIP is used, this silence is packetized along with the conversation. VAD suppresses the silence, so instead of sending IP packets of silence, only IP packets of conversation are sent. The network bandwidth is therefore being used more efficiently and effectively. Because voice is being transported in IP packets, security of voice traffic is inherent in the security provided for other data, as described earlier. Branch-Office QoS and VoiceThe Seattle office will have a PBX system installed to replace the outsourced Centrex solution. The PBX will be the one that was previously installed in the Grandics Corporation office. Remote User QoS and VoiceRemote users will use their own phone and will use a company calling card for long-distance calls. |
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