Application Note Contents Title Six Steps To Getting Your Network Overview.......................................................... 1 Ready For Voice Over IP Pre-Deployment and Network Readiness Assessment Is Essential ..................... 1 Date January 2005 Types of VoIP Overview Performance Problems ...... ............................. 1 This Application Note provides enterprise network managers with a six step methodology, including pre- Six Steps To Getting deployment testing and network readiness assessment, Your Network Ready For VoIP ........................ 2 to follow when preparing their network for Voice over Deploy Network In Stages ................................. 7 IP service. Designed to help alleviate or significantly reduce call quality and network performance related Summary .......................................................... 7 problems, this document also includes useful problem descriptions and other information essential for successful VoIP deployment. Pre-Deployment and Network IP Telephony: IP Network Problems, Equipment Readiness Assessment Is Essential Configuration and Signaling Problems and Analog/ TDM Interface Problems. IP Telephony is very different from conventional data applications in that call quality IP Network Problems: is especially sensitive to IP network impairments. Existing network and “traditional” call quality Jitter -- Variation in packet transmission problems become much more obvious with time that leads to packets being the deployment of VoIP service. For network discarded in VoIP end systems; jitter is managers, this means that LANs, access links usually due to network congestion and network equipment may need to be upgraded and that more sophisticated management and Packet Loss -- Packets lost during diagnostic tools are needed when deploying and transmission due to network errors, maintaining VoIP networks. route changes, link failures or random early detection (RED) in routers Types of VoIP Performance Problems Delay -- Overall packet transmission “lag time” that leads to two-way There are three basic categories of conversational difficulty. performance-related problems that can occur in Application Note Six Steps To Getting Your Network Ready For Voice Over IP January 2005 1 Telchemy Application Note Equipment Configuration and For example, a single user accessing a file from a server Signaling Problems: can cause a period of congestion lasting a few seconds. VoIP Endpoint Configuration This, in turn, can cause short-term degradation in --Performance impact of the type of call quality for other users on the network. Given this, CODEC and packet loss concealment it is essential that network managers use performance algorithm, or jitter buffer configuration management tools that can detect and measure these types of network impairments. Router and Firewall Configuration -- Firewalls or incorrectly configured The transient nature of IP problems also routers block VoIP traffic; routers need means that they are not easily reproduced for to be configured to deliver RTP packets analysis once the call is terminated. Unlike in a timely manner traditional POTS, once an end user completes and disconnects an IP call, vital diagnostic information Bandwidth Allocation -- Network about that call and its packet stream is lost. lacks sufficient bandwidth to support Network managers can use packet loss and jitter peak traffic volumes. metrics to determine how bad the call quality was; however, these metrics alone do not provide Analog/TDM Interface Problems: enough diagnostic information to determine why the call was bad. Echo -- “Echo” commonly occurs at the boundary between the digital network (VoIP or TDM) and analog local loops. This becomes very obvious Six Steps To Getting Your Network and annoying with the additional delay Ready For VoIP introduced by the IP network problems previously described. Step 1 Define High-level VoIP Requirements Signal Level -- Abnormally high or low voice signal levels, “clipping,” Your ability to deliver good quality VoIP excessive noise and “echo” occur due performance will depend on patterns of traffic and to incorrectly configured gateway signal usage, existing network capacity, existing data levels. bandwidth and many other factors. The first step is to define what your VoIP deployment will look Network architects and managers should like: address call quality and performance management problems when they plan and deploy their IP What utilization do you expect? networks, but they should be aware that these problems also frequently occur during normal day- Where will gateways be located? to-day network operation. How will internal calls be routed? Many VoIP-related problems are transient in nature and can occur at many places along the network path. How will external calls be routed? 2 Application Note Six Steps To Getting Your Network Ready For Voice Over IP Januaary 2005 Telchemy Application Note What CODECs do you plan to use (e.g. Using low bandwidth CODECs helps reduce G.711, G.729A, iLBC..)? And what bandwidth requirements however due to the bandwidth do these take (including overhead of IP, UDP and RTP protocols, such IP headers - 96kbps for G.711 and reduction is limited. The table below shows typically 24kbps for G.729A)? the effective network bandwidth used by some common CODEC types for different voice frame This should allow you to estimate the amount of sizes. Note that using a longer frame can have network bandwidth required between each location. some impact on delay, hence it is common to use Consider the following example: 10-20mS frames. Scenario: A remote location has 50 Low bit rate CODECs also introduce some users, and no more than 10 users are voice distortion and may be unacceptable for expected to be on the phone at any one extended use. time. A gateway located on the central site will be used for connection to the Step 2 telephone network, and G.711 will be Map Existing Wide Area Network/VPN used. The remote location is connected Capabilities to the central site via IP VPN and has a single T1 to access the IP network. Many call quality-related problems occur in access links or on limited bandwidth WAN or Bandwidth requirements: 10xG.711 VPN links. If significant jitter or delay occurs on streams will require 10 x 96 kbits/s of inter-site connections, this is a strong indicator bandwidth or 960 kbits/s, with some that similar problems will occur during VoIP extra overhead for signaling. If the deployment. Budget bandwidth usage between number of streams increases to 15 then sites and verify that routers can prioritize RTP the bandwidth needed would be 1.44 traffic. Mbits/s (almost the entire T1). What is the bandwidth available between each site? How much data traffic is currently Figure 1: Effective Network carried, (both average levels and peak Bandwidth By CODEC Type For levels)? Different Voice Frame Sizes. CODEC Effective Network Bandwidth (kbits/s) Bandwidth CODEC Type (kbits/s) 5mS Frame 10mS Frame 20mS Frame 30 mS Frame G.711 64 131.2 97.6 80.8 75.1 G.729A 8 - 41.6 24.8 19.1 G.723.1 6.3 - - - 17.4 iLBC 15.2 - - 32 - Application Note Six Steps To Getting Your Network Ready For Voice Over IP January 2005 3 Telchemy Application Note Is sufficient bandwidth available to Examine Ethernet switch statistics for support both the current data traffic and evidence of packet errors or excessive the estimated bandwidth needed for collisions and upgrade equipment voice? accordingly. Is there any capacity for growth or for If Wireless LANs are being used, be aware that unexpected peaks in activity? these can introduce significant impairments into the packet stream and should be checked carefully Will the addition of voice traffic have before using them for Voice traffic. an adverse effect on data application performance, i.e. if there is significantly Step 4 less bandwidth for data applications, Verify Inter-Site Readiness then how will they be affected? Pre-deployment testing is essential; however, you Map VoIP bandwidth requirements onto need to perform Steps 1 to 3 first. Many problems available bandwidth. are predictable by simply verifying that sufficient bandwidth is available to carry the expected traffic 10xG.711 streams will require 10 x 96 kbits/s level. of bandwidth or 960 kbits/s, with some extra overhead for signaling. If the number of streams When conducting a pre-deployment test, it is increases to 15, then the bandwidth needed important that the test: would be 1.44 Mbits/s (almost the entire T1). This will leave approximately 500 kbits/s for Is conducted over a sufficient period data traffic. If more than the expected 10 users of time to assess network readiness are simultaneously on the phone, then problems under a variety of “typical” network may start to occur with data applications due to conditions. Conditions vary through the insufficient bandwidth. And if more than 15 users day and through the week, and network are on the phone, then voice quality will degrade problems are often transient in nature. very quickly. There is no substitute for a sufficient amount of testing time. Step 3 Verify LAN readiness Is conducted under network load conditions that are similar to those Use a switched 100BaseT Ethernet expected post-deployment. There is architecture, potentially using VLAN to separate little point in testing a single VoIP call voice and data. Even with the use of switched when you expect that there will be 20 Ethernet, problems can still occur due to duplex calls. mismatch, excessively long Ethernet