Voice over IP. Demonstration 1: VoIP Protocols. Network Environment

Transcription

1 Voice over IP Demonstration 1: VoIP Protocols Network Environment We use two Windows workstations from the production network, both with OpenPhone application (figure 1). The OpenH.323 project has developed free VoIP applications for UNIX and Windows platforms, including H.323 client, gatekeeper, gateway and answering machine. The IP telephone call will be made between two VoIP clients on the same LAN without a gatekeeper. Network traffic will be monitored and analysed from one of the workstations. Ethernet, TCP/IP Open phone LAN Advisor Open phone Figure 1: Point-to-point IP telephone connection. OpenPhone Application The OpenPhone is distributed with a separate dll-library and an application. No installation is needed, the library files are copied into the working directory or any directory on the PATH. The user interface of the application is minimal, but giving good control over the VoIP components. When a call is not waiting, the menus give the following choices: Call: Make a Call, Send Information Options, Speed Dial number manipulation Options, General: username, aliases, bandwidth, max. recent calls, ring sound, auto-answer, some H.323 parameters Options, Forwarding: call forwarding - all, on busy, no answer Options, Gatekeeper: H.323 gatekeeper information, if available (not used in this demonstration) Options, Audio: sound card, code selection, packet size, dejitter buffer, silence detection on/off and silence detection parameters Options, Video: receive size, transmit size, device, format, source, quality, local video, flip VANHA MAANTIE 6 VOICE OVER IP 1

2 Options, Trace Options: enable tracing, output file, instances Help: Content, Search for Help, How to Use Help, About... Aligent LAN Advisor Network Analyser The LAN Advisor is a professional network and protocol analyser software, that can use local or remote probes. This application offers, for example, possibility to record network loading, protocol distribution, Ethernet statistics, to discover LAN nodes, to capture, filter and encode Ethernet frames and many other additional features. First we use the analyser to measure network utilisation by selecting the Protocol Vital Stats toolbar and starting the measurement (figure 2). The analyser will scale the utilization graph to the low loading on an unused network. Figure 2: Measuring the network utilization. G.711 Audio without Silence Detection On both workstations we put the European G.711 A Law with 20 ms framing on top and disable all other coding methods (figure 3). Also the silence detection will be disabled, so the terminals will generate an eight bit voice sample every 125 us. VANHA MAANTIE 6 VOICE OVER IP 2

3 Figure 3: Audio Settings. When receiving a call, we must answer. The log window on the OpenPhone application will record the events. Using the network monitor we see, that a full duplex G.711 voice call uses about 175 kbit/s. The G.711 coding should generate 64 kbit/s on both stations. Additional bandwidth is consumed by the protocol headers. To make sure, we stop monitoring and shut down the protocol vital stats window, open the protocol analysator (Decode/F5) and start capturing frames without any filters. After a while we have enough data and we can stop capturing. While our monitor is on a live network, we have captured also unnecessary traffic. To see only the VoIP frames, we can filter the frame capture and display based on the following frame details: VLAN Id and 802.1p priority value Various protocol field content Source and destination IP address. First, we maybe should filter out other packets but the one exchanged between our VoIP terminals (Station Filters/Source, Destination, Reverse Direction, see figure 4). VANHA MAANTIE 6 VOICE OVER IP 3

4 Figure 4: Display filtering based on source and destination IP addresses. Then we look at any RTP message and pay attention on the following information, that is found on the frame: RTP protocol stack: PCM data, RTP, UDP, IP, Ethernet Source and Destination TCP Port number: 5000, 5000 (even) RTP Payload Data size: 20 ms/125 us = 160 bytes RTP data contents and data variation Sum of header sizes: 18 B + 20 B + 8 B + 12 B = 58 B RTP Header fields, their contents and meaning. Looking at the RTP Timestamp values on consecutive frames, we see it is incremented by 20 ms from frame to frame. The RTP transmission is controlled by RTCP. To display the RTCP messages, we first have to disable the station filter and only then filter the frames carrying RTCP (Protocol Filters/XoIP/RTCP). Please pay attention to the following information: RTCP Protocol Stack: RTCP, UDP, IP, Ethernet Source and Destination TCP Port number: 5001 and 5001 are odd Time interval between RTCP messages: about 2 seconds Length of the frame: 78 bytes RTCP Packet Type: Sender Report SR format: Header, Sender Info, Report Block Report Block details NTP Time Stamp contents and decoding. VANHA MAANTIE 6 VOICE OVER IP 4

5 Signalling At the previous example, we started capturing after the connection setup. To be able to discover the messages at the call setup, we first disable all filters, then close the existing call, start frame capturing and make a new call. After a short time we can brake the call and stop capturing. H.323 signalling between two terminals use Q.931 and H.245 over TCP, so we can filter out other frames but TCP to see the signalling (Protocol Filters/IP/TCP). The call setup happens in phases, including the following: TCP connection setup for port 1720 using the three way handshake: SYN, SYNACK, ACK (figure 5) Q.931 signalling: Setup, Call Proceeding, Connect. All segments are acknowledged by the other end TCP connection setup for H.245 H.245 signalling, including agreement on the TCP ports for media and media control channels, and the coding details. The same way, the call is closed in phases, including: H.245 End Session Q.931 Release Complete. Figure 5: TCP Connection Setup for the Q.931 port. Why are the TCP ports for RTP and RTCP sessions important? VANHA MAANTIE 6 VOICE OVER IP 5