Understanding IP Phone Behavior

Skinny Client Control ProtocolSCCP

Prof. Yousif@

Valencia Community College

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

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???

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.

From the Trace Menu, Select Trace configuration

Collect the Traces for a Certain Period of Time

A Sample Trace for a Call Between Two IP Phones

1. IP phones use the Skinny protocol to communicate with CallManager

2. All messages to and from a Skinny device are preceded by either the word StationInit or StationD.

3. StationInit means that an inbound TCP message form a Skinny station reached CallManger.

4. Example:

StationInit: 000000004 OffHook

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 phone’s MAC Address until you find a keep alive message to that phone.

• Example: StationInit - InboundStim - KeepAliveMessage - Send KeepAlive to Device Controller. DeviceName=SEP000F8F59D304, TCPHandle=000000004, IPAddr=192.168.1.128, Port=52952, Device Controller=[1,92,1]|<CLID::StandAloneCluster><NID::192.168.1.140>

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.

Other Skinny Messages

• StationD: 000000004 StartTone tone=33(InsideDialTone) Tells the IP phone to start plying dial tone.

• CallReference=16777225: 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

Other Skinny Messages

• StationInit: 000000004 KeypadButton kpButton=4 • StationD: 000000004 StopTone. • CCM|StationD: 000000004 SelectSoftKeys instance=1

reference=16777225 softKeySetIndex=6 • StationInit: 000000004 KeypadButton kpButton=0• CCM|StationInit: 000000004 KeypadButton kpButton=0.|• StationInit: 000000004 KeypadButton kpButton=0.• StationInit: 000000004 KeypadButton kpButton=4• Note: digit * shows as e and a # shows as f• StationD: 000000004 SelectSoftKeys instance=1

reference=16777225 softKeySetIndex=8

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

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 callState=4 lineInstance=1

• StationD: 000000005 CallInfo callingPartyName='Wael Yousif' callingParty=40000 cgpnVoiceMailbox= calledPartyName='' calledParty=40004

• callReference=16777226• StationD: 000000005 SetRinger

ringMode=2(InsideRing)• StationD: 000000005 DisplayPriNotify

timeOutValue=10 pri=5 notify='€40000' content='From 40000'

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

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 callState=12 lineInstance=1

• StationD: 000000004 CallInfo callingPartyName='Wael Yousif' callingParty=40000 cgpnVoiceMailbox= calledPartyName='' calledParty=40004

• StationD: 000000004 StartTone tone=36(AlertingTone),

The CIPC Answers the Call

• StationInit: 000000005 SoftKeyEvent softKeyEvent=11(Answer) lineInstance=1

• 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

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.

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 conferenceID=0 passThruPartyID=1000091 millisecondPacketSize=20 compressionType=4(Media_Payload_G711Ulaw64k) qualifierIn=?. myIP: 8001a8c0 (192.168.1.128)|<CLID::StandAloneCluster><NID::192.168.1.140><CT::1,100

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 Status=0, IpAddr=0x8001a8c0, Port=17198

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 Status=0, IpAddr=0xf001a8c0, Port=24600

• StationD: 000000004 StartMediaTransmission conferenceID=0 passThruPartyID=1000091 remoteIpAddress=f001a8c0(192.168.1.240) remotePortNumber=24600 milliSecondPacketSize=20 compressType=4(Media_Payload_G711Ulaw64k)

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.

Labs

• CCM Traces

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