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Updated:December 13, 2018
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This document introduces the topic of voice dial peers and call legs. It explains the call setup process through a packet network that uses Cisco
IOS® software voice-enabled gateways/routers.
For other topics that discuss dial peers, see the Related Information section of this document.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Types of Dial-Peers
Cisco IOS uses two types of dial-peers. They are defined as:
Plain old telephone systems (POTS) dial peer - These
define the characteristics of a traditional Telephony network connection. The
POTS dial peer maps a dial string to a specific voice port on the local
router/gateway. Normally, the voice port connects the router/gateway to the
local public switched telephone network (PSTN), private automatic branch
exchange (PBX), or telephone.
Voice-Network dial peers - These define the attributes
of a packet voice network connection. Voice-Network dial peers map a dial
string to a remote network device. Some examples of these remote network
devices are listed here:
Session initiation protocol (SIP) server (for Voice over IP
Open Settlement Protocol (OSP) server (for Voice over IP that uses
Mail Transfer Agent (MTA) Server (for Multimedia Mail over IP
The specific type of Voice-Network dial peer depends on the packet
network technology used. Different technologies used by dial peers are
Voice over IP (VoIP) - The dial peer is mapped to the IP address,
Domain Name System (DNS) name, or server-type of the destination VoIP device
that terminates the call. This applies to all VoIP protocols such as H.323,
SIP, and Media Gateway Control Protocol (MGCP).
Voice over Frame Relay (VoFR) - The dial peer is mapped to the
data-link connection identifier (DLCI) of the interface from which the call
exits the router.
Voice over ATM (VoATM) - The dial peer is mapped to the ATM virtual
circuit for the interface from which the call exits the
Multimedia Mail over IP (MMoIP) - The dial peer is mapped to the
e-mail address of the Simple Mail Transfer Protocol (SMTP) server. This type of
dial peer is used for Store and Forward Fax (on-ramp and off-ramp
The Cisco IOS command to enter into the dial peer configuration mode
maui-nas-07(config)#dial-peer voice number ?
voatm Voice over ATM
vofr Voice over Frame Relay
voip Voice over IP
Relationship Between Dial Peers and Call Legs
A voice call over a packet network is segmented into discrete call
legs. These are associated with dial-peers (a dial-peer is associated with each
call leg). A call leg is a logical connection between two router/gateways or
between a router/gateway and an IP Telephony device (for example Cisco
CallManager, SIP Server, and so forth). To illustrate this concept, see Figure
1 and Figure 2 here:
Figure 1. Voice Dial Peers / Call Legs Toll-bypass
In Figure 1 (toll-bypass), a voice call comprises four call legs, two
from the perspective of the originating router/gateway and
two from the perspective of the terminating
Figure 2. Voice Dial Peers / Call Legs: Call Manager System
with IOS Gateway Scenario
In the Figure 2 (CallManager system with IOS Gateway), a voice call
compromises two call legs.
Note: The terms originating router/gateway and terminating router/gateway
are dependent on the source to destination direction of the call.
Note: Hair-Pinning is the name given to calls that originate and terminate
on the same router/gateway. On POTS-to-POTS Hair-Pinning calls, the
router/gateway matches an inbound POTS dial-peer and an outbound POTS dial-peer
to terminate the call. This is supported on POTS interfaces. However,
VoIP-to-VoIP Hair-Pinning is not supported on Cisco IOS voice-enabled platforms
except in CallManager Express with certain IOS releases.
Call Setup Process
A call is segmented into call legs with a dial peer associated to each
call leg. The process for this is listed here:
The POTS call arrives at the originating router/gateway. An
inbound POTS dial-peer is matched. (See Note 3 later in
After it associates the incoming call to an
inbound POTS dial-peer, the originating router/gateway
creates an inbound POTS call leg and assigns it a Call ID
(Call Leg 1 in Figure 1).
The originating router/gateway uses the dialed string to match an
outbound Voice-Network dial-peer.
After it associates the dialed string to an outbound Voice-Network
dial-peer, the originating router/gateway creates an outbound Voice-Network
call leg and assigns it a Call ID (Call Leg 2 in Figure
The Voice-Network call requests arrive at the terminating
router/gateway. An inbound Voice-Network dial-peer is
After the terminating router/gateway associates the incoming call
to an inbound Voice-Network dial peer, the terminating router/gateway creates
the inbound Voice-Network call leg and assigns it a Call
ID. (Call Leg 3 in in Figure 1)
The terminating router/gateway uses the dialed string to match an
outbound POTS dial-peer.
After it associates the incoming call setup to an
outbound POTS dial peer, the terminating gateway/router
creates an outbound POTS call leg. It assigns it a Call
ID, and terminates the call. (Call Leg 4 in Figure 1)
In scenarios where a Cisco CallManager is present with a Cisco IOS
router/gateway assume these :
For outbound calls from the CallManager system
through an IOS router/gateway, the IOS router/gateway behaves as a terminating
device.( See steps 5 through 8)
For inbound calls to the CallManager system
through an IOS router/gateway, the IOS router/gateway behaves as an originating
device. ( See steps 1 through 4)
Note: At this stage, if configured on the inbound
POTS dial-peer, non-default inbound POTS services and/or Toolkit Command
Language (TCL) applications are used. When you use such services or
applications, it is important to be certain that the correct
inbound POTS dial-peer is matched. Some examples of
services / applications include:
DID (direct inward dial)
TCL Based Applications such as IVR ( interactive voice response),
VoIP SIP Transfer, On-Ramp Faxing (in the context of Store and Forward
Note: At this point, both routers/gateways negotiate Voice-Network
capabilities and applications (if required). Default capabilities are not
displayed on the router/gateway IOS configuration output. Use the command
show dial-peer voice number
to view the configured capabilities, services, and applications on POTS and
Default capabilities include codec
g729r8, vad enable, dtmf-relay disable, fax-relay
disable, req-qos best-effort,
acc-qos best-effort, and session
protocol cisco (for H.323).
Examples of TCL applications include Remote IP Authentication and
Note: When non-default capabilities or applications are requested by the
originating router/gateway, the terminating router/gateways needs to match an
inbound Voice-Network dial-peer that is configured for
such capabilities or applications.