Information Technology Reference
In-Depth Information
Voice Call Legs
To accurately configure dial peers, you must first understand the concept of call legs. A
call leg represents a connection to or from a voice gateway from a POTS or VoIP source.
Figure 6-5 illustrates an example voice connection scenario.
Key
To p i c
Call Leg 1
Call Leg 2
Call Leg 3
Call Leg 4
10.1.1.2/24
FXS
T1 1/0
10.1.1.1/24
V
IP WAN
x1101
ROUTER_B
x2510
CME_A
Figure 6-5
Voice Connection Call Legs
As illustrated in Figure 6-5, the phone on the left (extension 1101) makes a call to the
phone on the right (extension 2510). For this call to pass through successfully, four call
legs must exist:
Call leg 1:
The incoming POTS call leg from x1101 on CME_A.
■
Call leg 2:
The outgoing VoIP call leg from CME_A to ROUTER_B.
■
Call leg 3:
The incoming VoIP call leg on ROUTER_B from CME_A.
■
Call leg 4:
The outgoing POTS call leg to x2510 from ROUTER_B.
■
If the call was placed in the opposite direction (from x2510 to x1101), the same number
of call legs would be needed, but in reverse. Thus, to provide a two-way calling environ-
ment that enables x1101 to call x2510 and vice versa, you need a total of eight call legs.
It is critical to understand the concept of call legs to properly configure the dial peers on
your router. Each call leg identified in Figure 6-5 represents a dial peer that must exist on
your router. These dial peers define not only the reachability information (phone numbers)
for the devices, but also the path the audio must travel. From CME_A's perspective, it re-
ceives audio from x1101 on an FXS port (call leg 1). CME_A must then pass that voice in-
formation over the IP network to 10.1.1.2 (call leg 2). From ROUTER_B's perspective, it
receives a call from x1101 on the IP WAN network (call leg 3). It must then take that call
and pass it to the PBX system out the digital T1 1/0 interface (call leg 4).
As you can see from Figure 6-5, call legs are matched on the inbound and outbound direc-
tion. In the same way, you must configure dial peers to match voice traffic in both direc-
tions. In some cases, you can use a single dial peer for bidirectional traffic. For example,
creating a single POTS dial peer for x1101 will match incoming and outgoing calls to
x1101. At other times, you must create more than one dial peer for inbound and outbound
traffic. For example, CME_A requires an outbound VoIP dial peer to send the call to
ROUTER_B (10.1.1.2). ROUTER_B needs an inbound VoIP dial peer to receive the call
from CME_A. As you see the multiple examples of dial peers in the upcoming sections,
these concepts become clearer.
