» Call Signaling

By | September 24, 2012

Here you will find answers to CVoice – Call Signaling Questions

Question 1

Which option is true concerning the MGCP call agent?

A. acts only as a recorder of call details

B. provides only call signaling and call setup

C. manages all aspects of the call and voice stream

D. monitors the quality of each call after setup

Answer: B


MGCP Call Agent is a central control component to remotely control various devices. When the MGCP call agent exists in the network, calls are routed via route patterns on the Call Agent (Cisco Unified Communications Manager), not by dial peers on the gateway.


The messages sent between the voice gateway and the MGCP Call Agent are just used for call signaling and call setup only. In summary, the Call Agent will instruct the gateways what to do in each stage: receive dialed digits, find the destination gateway, send connection request… Finally, the Call Agent will allow gateways to establish RTP Streams with each other. Notice that the voice streams only flow between the two voice gateways, not to the Call Agent.

At the conversation finishs (one of the endpoints goes on-hook), that gateway notifies the Call Agent and the Call Agent sends Delete Connection (DLCX) Requests for both gateways.

Question 2

At what point does the MGCP call agent release the setup of the call path to the residential gateways?

A. after the call agent has been notified that an event occurred at the source residential gateway

B. after the call agent has been notified of an event and has instructed the source residential gateway to create a connection

C. does not release call path setup

D. after the call agent has sent a connection request to both the source and destination and has relayed a modify-connection request to the source so that the source and destination can set up the call path

E. after the call agent has forwarded session description protocol information to the destination from the source and has sent a modify connection to the destination and a create-connection request to the source

Answer: D


Below is the call flow between two voice gateway through a MGCP Call Agent


The MGCP call agent releases the setup of the call path to the residential gateways when the conversation begins. After sending the Modify Connection (MDCX), the two gateways have enough information to start the conversation so the duty of the Call Agent finishs.

Question 3

Which three services are supported by CUBE when supporting H323-to-SIP calls? (Choose three)

A. SIP cause codes

B. media flow-around

C. media flow-through

D. codec transparent support

E. Transport Layer Security

F. H.261, H.263, and H.264 video codecs

Answer: C D E


Media flow through and media flow around mode is supported on the Cisco Unified Border Element (CUBE). The CUBE is always involved in the call setup (signaling) portion of the call, but the media (RTP bearer stream) may flow through the CUBE or be routed around the platform. Media flow through must be used to support many of the features available like IP address translation and IP address hiding. Media flow around allows the CUBE greater scalability in the number of calls that can be processed by one CUBE router.

For “Media flow through” option, the media packets are passed through the CUBE, they will get terminated and re-originates with CUBE’s IP address and port number, so here we cannot find the original gateway’s ip address. This is one of the security feature in the CUBE. The default option is “media flow-through”.

Use the “codec transparent” command to configure codec pass-through. Use this command to enable endpoint-to-endpoint codec negotiation

without a Cisco UBE router -> D is correct.

Transport Layer Security (TLS) is a security protocol that enables encrypted network communications. TLS on CUBE can be configured on a per-leg basis in order to allow a TLS to non-TLS SIP call. For example, CUBE uses IPSec in order to secure signaling and support calls from H.323 to SIP with the H.323 leg, while the SIP leg uses TLS -> E is correct

The questions only let us choose 3 answers but I think B – media flow-around can be used for H323-to-SIP calls. We just can’t use media flow-around for SIP-to-SIP calls.

For your information, H.323-to-SIP interworking is configured using the allow-connection h323 to SIP command. Then issue the allow-connections sip to h323 command to enable SIP to H.323 calls.

Question 4

Which two are attributes of SCCP? (Choose two)

A. It is Cisco proprietary.

B. It is a supervisory signaling protocol.

C. It is classified as client/server architecture.

D. SCCP devices are considered intelligent endpoints.

Answer: A C


SCCP is the only Cisco-proprietary VoIP protocol currently in use. The purpose of SCCP protocol is to provide a signaling protocol between the Cisco Unified Communications Manager and Cisco IP phones. Similar to MGCP, SCCP is a client/server protocol -> A & C are correct.

Supervisory signals involves the detection of changes to the status of a circuit (on-hook, off-hook, ringing). Any event causes a message to be sent to a Cisco UCM -> We can say SCCP is more than a supervisory signaling protocol because it tells the phone exactly what to do. From the on-hook, off-hook, buttons pressed, lamp on/off, through the prompt, key settings, and even the dialtone -> B is not correct.

The beauty of SCCP is that it makes the endpoints very cheap comparing to the H.323 devices. The end stations (telephones) that use SCCP are

called Skinny clients, which consume less processing overhead and they do not contain call control intelligence -> D is not correct.


SCCP devices (in this case the Cisco IP Phones) become “dump” devices when using this protocol because they have to ask the Unified Communications Manager for every action they need to do -> D is not correct.

Question 5

Refer to the exhibit. All IP phones are SCCP phones. Phone D makes an internal call to phone G. Which call setup signaling statement is true?


A. Phone D signals phone G directly. Call setup is handled by the phones.

B. Phone D signals gateway A, which processes the call and signals phone G.

C. Phone D signals gateway B, which processes the call and signals phone G.

D. Phone D signals gatekeeper. The gatekeeper processes the call and signals phone G.

E. Phone D signals the call agent. The call agent processes the call and signals phone G.

Answer: E


This is a …weird and wrong question. Maybe the phone they want to ask here is Phone A, B or C because only these phones can use SCCP to communicate with the Call Agent. Phones D and E can’t use SCCP to talk with a H.323 Gateway.

Phone A, B or C are SCCP Phones so they hand over the call control intelligence to the Call Agent and the Call Agent need to process the call and signals phone G before these phones can talk with each other.

Question 6

Which statement is true about MGCP?

A. Call completion is always shared, with some intelligence on the endpoint, some on the call agent.

B. Endpoints always take all actions to complete calls.

C. Endpoints may act alone or cooperate with call agent to complete calls.

D. Call agents order and direct each step of call completion for the endpoints.

Answer: D