Understanding IP Phone Behavior

1
Understanding IP Phone Behavior

• Skinny Client Control Protocol
• SCCP
• Prof. Yousif
• _at_
• Valencia Community College

2
Skinny Protocol

• Skinny Protocol is a lightweight master/slave
  protocol
• CallManager controls every function of the phone.
  
• CallManager facilitates call signaling to other
  devices using different protocols
• H.323 for routers
• TAPI/JTAPI for Soft Phones/IP Communicator

3
Master/Slave Example

• When the Phone goes off-hook, it notifies CM that
  the user has gone off-hook.
• CM sends a message to the phone notifying it play
  dial tone.
• The phone can not determine when to play dial
  tone on its own.
• It is important to understand the roles that both
  IP phones and CM play when troubleshooting IP
  phones
• Example Where do you troubleshoot the problem
  of delayed dial tone???

4
Call Processing Behavior

• The Call Processing Behavior can be best
  explained by examining The CM trace file
• CM Traces are configured from Cisco Service
  configuration.

5
From the Trace Menu, Select Trace configuration
6
Collect the Traces for a Certain Period of Time
7
A Sample Trace for a Call Between Two IP Phones

• IP phones use the Skinny protocol to communicate
  with CallManager
• All messages to and from a Skinny device are
  preceded by either the word StationInit or
  StationD.
• StationInit means that an inbound TCP message
  form a Skinny station reached CallManger.
• Example
• StationInit 000000004 OffHook

8
TCP Handle

• The TCP handle represents a unique value that
  identifies a specific IP pone registered to this
  CM Server.
• With The TCP handle, you can follow every message
  sent and received from a particular IP phone.
• The TCP handle of a device can also be found by
  looking up the IP phones MAC Address until you
  find a keep alive message to that phone.
• Example StationInit - InboundStim -
  KeepAliveMessage - Send KeepAlive to Device
  Controller. DeviceNameSEP000F8F59D304,
  TCPHandle000000004, IPAddr192.168.1.128,
  Port52952, Device Controller1,92,1ltCLIDStan
  dAloneClustergtltNID192.168.1.140gt

9
Skinny Protocol

• The OffHook message means that CallManger
  received a Skinny message indicating the phone
  went off-hook
• The next message in the trace file is StationD
  000000004 StationOutputDisplayText 40000
  
• StationD signifies that the CM server is sending
  a message to the IP phone.
• Notice that the same TCP handle is listed
• The number 40000 represents the directory number
  of the phone.

10
Other Skinny Messages

• StationD 000000004 StartTone
  tone33(InsideDialTone)? Tells the IP phone to
  start plying dial tone.
• CallReference16777225 The Call reference ID is
  created by CM for each participant in a call and
  can be used to track a particular call through a
  CM Trace

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Other Skinny Messages

• StationInit 000000004 KeypadButton kpButton4
• StationD 000000004 StopTone.
• CCMStationD 000000004 SelectSoftKeys
  instance1 reference16777225 softKeySetIndex6
• StationInit 000000004 KeypadButton kpButton0
• CCMStationInit 000000004 KeypadButton
  kpButton0.
• StationInit 000000004 KeypadButton kpButton0.
• StationInit 000000004 KeypadButton kpButton4
• Note digit shows as e and a shows as f
• StationD 000000004 SelectSoftKeys instance1
  reference16777225 softKeySetIndex8

12
Other Skinny Messages

• Call Manger is constantly analyzing the digits
  the user dials, and once it finds an exact match,
  digit analysis returns the results for the match.
  
• Now is the time to ring the CIPC phone which is
  configured with DN 40004.
• The Call Routing Chapter will provide additional
  details on how digit analysis wok in CM

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Other Skinny Messages

• Because CM has collected all the required digits,
  it is ready to notify the destination IP phone
  there is an incoming call.
• StationD 000000005 CallState callState4
  lineInstance1
• StationD 000000005 CallInfo
  callingPartyName'Wael Yousif' callingParty40000
  cgpnVoiceMailbox calledPartyName''
  calledParty40004
• callReference16777226
• StationD 000000005 SetRinger
  ringMode2(InsideRing)
• StationD 000000005 DisplayPriNotify
  timeOutValue10 pri5 notify'40000'
  content'From 40000'

14
Other Skinny Messages

• Notice that the CIPC phone has a different TCP
  handle (was assigned during registration)
• Also, notice that the CIPC phone has a different
  CallReference ID
• The CIPC phone now starts to ring displaying the
  calling party name and extension number

15
Other Skinny Messages

• The following messages perform two functions
  First, the display on the IPP changes to indicate
  that the call is in progress. Second, the phone
  is told to play the ringback tone which you hear
  when you place a call
• StationD 000000004 CallState callState12
  lineInstance1
• StationD 000000004 CallInfo
  callingPartyName'Wael Yousif' callingParty40000
  cgpnVoiceMailbox calledPartyName''
  calledParty40004
• StationD 000000004 StartTone
  tone36(AlertingTone),

16
The CIPC Answers the Call

• StationInit 000000005 SoftKeyEvent
  softKeyEvent11(Answer) lineInstance1
• The above message is equivalent to the
  StationInit OffHook message in the case where
  the called phone is another IPP.
• The preparation is now complete for the actual
  media connection

17
Audio Stream Setup

• Skinny uses Real Time Transport Protocol (RTP)
  over User Datagram Protocol (UDP) packets to send
  and receive Voice Over IP samples.
• Each RTP stream is called a logical Channel.
• A Logical Channel is a Unidirectional RTP stream,
• To Have a two-way conversation, you must have two
  logical channels opened one from the calling
  device to the called device and one from the
  called device to the calling device.

18
Audio Stream Setup

• In the following section you can see how CM asks
  the IP phone to open a connection to receive RTP
  streams.
• You can also see how that CM asks the IP phone
  for specific parameters, including the codec and
  packet size.
• StationD 000000004 OpenReceiveChannel
  conferenceID0 passThruPartyID1000091
  millisecondPacketSize20 compressionType4(Media_P
  ayload_G711Ulaw64k) qualifierIn?. myIP
  8001a8c0 (192.168.1.128)ltCLIDStandAloneClustergt
  ltNID192.168.1.140gtltCT1,100

19
Audio Stream Setup

• Upon receiving an OpenReceiveChannel message, the
  I phone selects the UDP port number it wants to
  use to receive RTP packets and reports this
  information back to the CM in an
  OpenReceiveChannelAck message.
• StationInit 000000004 OpenReceiveChannelAck
  Status0, IpAddr0x8001a8c0, Port17198

20
Audio Stream Setup

• The same handshake of OpenReceiveChannel and
  OpenReceiveChannelAck is done at the called
  phone.
• When CM receives the OpenReceiveChannelAck from
  the CIPC phone, containing the UDP port number,
  it forwards this information to the calling phone
  in a StartMediaTransmission message
• StationInit 000000005 OpenReceiveChannelAck
  Status0, IpAddr0xf001a8c0, Port24600
• StationD 000000004 StartMediaTransmission
  conferenceID0 passThruPartyID1000091
  remoteIpAddressf001a8c0(192.168.1.240)
  remotePortNumber24600 milliSecondPacketSize20
  compressType4(Media_Payload_G711Ulaw64k)

21
Audio Stream Setup

• At this point the two phones are sending RTP
  messages to each other.
• Notice that for the duration of this call, the
  two phones never have sent nor received any
  Skinny signaling to or from each other.
• This is because all the signaling goes through CM
• The only time IP phones send packets to each
  other is for the actual Voice Stream.

22
Labs

• CCM Traces
• Challenge Lab Capture The Call Setup and Media
  Transport Processes using Ethereal