System information

Expert Displays 303
• Show Reference Lines—displays a “reference line” indicating the speed of the
network/WAN from the initial capture data. This will only be displayed if the option
to “Show Reference Lines” is enabled in the Expert Global Settings, under the What-
If tab. See “Expert Global Settings – What-If Analysis Tab” on page 288.
• Full Duplex—toggles off and on the interpretation of data as full-duplex.
• Reset Values—resets all values to the initial settings for the analyzed pair. This has
the same effect as selecting the Restore Original Values button.
• Change Pair Direction—changes the view of the “direction” of the pair (i.e., swaps
the client and server).
Voice Over IP Expert
With the increasing importance of real-time communications—such as audio and video
conferencing—over networks, the International Telecommunications Union (ITU)
developed the H.323 standard, for real-time communications over networks that, like
Ethernet, Token Ring, and FDDI, do not provide a guaranteed Quality of Service (QoS).
Prominent among the uses of H.323 is Voice over IP, or VoIP. VoIP uses RTP (Real-time
Transport Protocol), a UDP-based protocol for the transmission of real-time data, for use
in such applications as audio and video conferencing. While RTP packets contain the
actual real-time data, the protocol is augmented by RTCP (Real-time Transport Control
Protocol), which is used to send information about the data being transferred: the number
of packets sent and received, the identities of the stations involved in the conversation, and
so forth. By analyzing an RTCP conversation and using it to interpret the RTP data, VoIP
Expert can identify and diagnose problems in a VoIP or other RTP/RTCP session.
The VoIP Expert displays H.323 conversational data in three separate graphs. Each display
is designed to help identify why a connection may be experiencing problems, or at what
level of network load are H.323 conversations exhibiting acceptable quality behavior.
To access the VoIP Expert, right-click on a RTCP (Real-Time Control Protocol)
connection in the “UDP Events” display and select “VoIP Expert” item. The RTCP part of
the conversation is the “control” aspect of the RTP (Real-Time Protocol) conversation.
When selecting the RTCP conversation, you will see that its port number is always one
more than its associated RTP data stream.
The first display shows the conversations lost packets and jitter in the direction of the
arrow. The second display shows the other direction.
• Lost Packet % (fraction lost)—The fraction of RTP data packets from a particular
source lost since the previous Sender Report (SR) or Receiver Report (RR) packet
was sent.
• Jitter—An estimate of the statistical variance of the RTP data packet arrival time,
measured in timestamp units and expressed as an unsigned integer.