Hi ,
Just as a reminder when you sit in the lab, try to not forget to put the correct Calling number , type and plan and this for all situation so it is true as well for SRST.
So a good vision of the call routing is important
Sometimes , when you defined your mgcp config and before to activate it with the CUCM config. You can experience issue where you can not send or receive calls on this gateway only because your link is not up.
This is caused because the binding between the MGCP gateway and the CUCM is not properly set up. So in order to fix this issue under your serial interface , remove the command isdn bind-l3 ccm-manager and put it again just after. The result is direct and your PSTN will go up .
Here is a basic stuff to implement the mgcp config
!
ccm-manager fallback-mgcp
ccm-manager redundant-host 172.17.123.201
ccm-manager mgcp
ccm-manager music-on-hold
ccm-manager config server 172.17.123.200
!
mgcp
mgcp call-agent 172.17.123.200 service-type mgcp version 0.1
mgcp bind control source-interface GigabitEthernet0/0.102
mgcp bind media source-interface GigabitEthernet0/0.102
!
mgcp profile default
!
Real-time fax over IP operates in a similar way of a regular fax transmission. The fax machines involved in the transmission synch up and then the fax data is sent between them over the intervening IP Network.
There are 2 methods of transporting fax in real time across the network:
Fax-relay
Fax-passthrough
When using fax-relay, the T30 fax signal from a connected fax machine is demodulated by the sending fax gateway and sent over the IP Network to a remote fax gateway. The remote fax gateway then recontructs the T30 fax signal and send it to the fax.
There are 2 types of fax-relay mechanisms:
Cisco fax-relay
T38 fax-relay
Cisco fax-relay is an older method. So a fax gateway terminates T30 fax tones from a local fax machine and then sends the fax data across an IP network by breaking the tones into HDLC frames and then transmitting them using RTP.
T38 fax-relay is the ITU standard T30 fax signal, it is demodulated at the local gateway and encapsulated into IP packets for transport over a network to a remote fax gateway which will then reconstruct the signal and play it to the fax. T38 includes also a mechanism by which a fax gateway can inform the remote gateway of its desire to change the media type from voice to data. T38 can also use TCP or UDP connections but will use more UDP.
For fax pass-through, modulated fax data is sent in-band across the IP network by a fax gateway using a voice codec (like G711 without any VAD or echo-cancellation). Also with fax pass-through, T30 fax calls are not distinguished from regular voice calls, they are simply sent in-band over the IP Network. With the fax detection tones, the gateway must be able then to switch to high-bandwidth codec. Fax pass-through is relatively bandwidth hungry and is sensitive to delay,jitter and packet loss
There is two types of security that you can enable with Call Manager
- Mixed mode : In this mode, depending the security configured on each phone, you can have secure calls when both devices are security-enabled and when one of the phones is missing security, your call will be nonsecure.
- Nonsecure mode : As all phones are not set up with security (default configuration), all calls are nonsecure.
When you device to put security on phones , they can support the three following levels:
- Nonsecure mode : secure calls are not supported
- Authenticated mode : the phone will be able to authenticate calls
- Encrypted mode : the phone will be able to support encrypted calls
If you enable the authentication and the encrytion on your network , you are then able to secure the media traffic as well the voice signaling.
If you want to have security on the media flow, it is then mandatory to secure also the signaling as the keys which are used to secure the media traffic are exchanged during the signaling phase.
SCCP messages sent by IP Phones and Call Manager can be secured using TLS, it is the signaling part. Then for the protection of the media traffic so the RTP packets , you will use the Secure RTP which is providing a framework for encryption and authentications of your stream.
SRTP will be also use between your MGCP gateway and your IP Phone but you need to know that your SRTP keys are exchanged in cleartext session between the MGCP gateway and the Call Manager.
If we want to have SRST on MGCP gateways, ensure that you have the two following commands:
!
service alternate
call-manager fallback-mgcp
!
This will ensure that the MGCP gateway can provide Call Processing with SRST. Don’t forget also to have an H323 config to take over in SRST as MGCP hasn’t any Call Control.
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