Tuesday, June 29, 2010

CCNA Voice 06

Gateways are equipment that connect a local device to a remote network possibly of a different medium. A voice gateway can connect to a PBX, PSTN or the internet. Analog gateways are gateways that have analog ports, while digital gateways have digital ones.
Analog ports are ports that connect to analog devices. These ports are:
FXS - Foreign Exchange Station - Used to connect to an end-station device.
FXO - Foreign Exchange Office - Used to connect to the central office.
E&M - Receive and Transmit - Used to terminate tie lines (circuits between two sites)

Digital gateways typically have three port types: PRI, BRI and CAS. They are typically used to connect a VoIP network to the PSTN, but can also be used to connect to the PBX.

A Call Leg is a logical connection between two VoIP devices. A phone connected to a router connected to another phone connected to another router has four legs. This is visualized using:

Phone A -leg- Router A -leg- Cloud -leg- Router B -leg- Phone B

When a call is received on a gateway, the call must be routed to either an IP address or a voice port. This is defined using a dial-peer. A POTS call is directed at a voice port, while a VoIP call is directed at an IP address.

In the above topology, an analog phone is plugged into an FXS port port 0/0/0. To create a dial-peer for it, we'll have to type:
dial-peer voice 1 pots
destination-pattern 0001
port 0/0/0

The Call Manager will create a Voice Dial-Peer (as opposed to data) with a tag of 1 for an analog port. When it matches traffic for number 0001, it will route it to port 0/0/0.

Now suppose that we have another Call Manager connected across the WAN like this:

To enable connectivity between the two sites, we must come up with some sort of numbering plan. Let's keep it simple: PhoneA's site will have numbers starting with 1xxx, while PhoneB's site will have numbers starting with 2xxx. Now assuming both routers have full connectivity with each other, let's set up the phones starting from CCMEA:
dial-peer voice 1 pots
destination-pattern 1001
port 0/0/0
dial-peer voice 2 voip
destination-pattern 2...
session target ipv4:

From CCMAB, this is what you should have:
dial-peer voice 1 pots
destination-pattern 2001
port 0/0/0
dial-peer voice 2 voip
destination-pattern 1...
session target ipv4:

If you dial 2001 from PhoneA, you would expect PhoneB to ring. This is because when the numbers are dialed in, the 2... wildcard patern is matched, so the data is sent over the WAN to session target When CCMAB receives the call signaling, it checks that it's meant for 2001, so it routes to port port 0/0/0 which is PhoneB.

The Period is just one of the many wildcards available for use. A full list of wildcards available can be found here.

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