Implementing Voice Gateways
Lecture 3
Voice Gateways
• Any device with one or more TDM PSTN interfaces on
them
• TDM - Time Division Multiplexing (i.e. traditional
telephony)
• PSTN - Public Switched Telephone Network
• To be really useful, gateways also need an IP interface
on them
• Many vendors, but we will look at Cisco IOS based
voice gateways
• Both analogue and digital interfaces, we’ll look at the
more common ones
Interface Types - Digital
• ISDN primary rate circuits
• E1 (primarily used in Europe) 2 Mbit/s bearer
• 32x 64kbit/s channels. 30 for voice, 1 for
signalling (timeslot 16), 1 framing
• Common interfaces for ISP dial-in, PBX to
carrier trunks, etc.
• DSL – ADSL and HDSL
• Wireless – 802.11x
Interface Types - Digital
• Basic Rate ISDN
• 144kbit/s bearer
• 2x 64kbit/s channels + 1x 16kbit/s signalling
channel
• 2B + D
• B channels = 64kbit/s voice/data channels
• D channel(s) = signalling data channels
Interface Types - Analogue
• Only really two types:
• FXO interface - plugs into your Service
provider (Exchange or company PBX)
• FXS interface - plugs into a telephone. e.g.
ATAs
• Uses analogue signalling, limited to one DDI
per line
• Signalling is generally more ambiguous and
harder to work with than digital signalling
IOS Voice Configuration
•
•
•
•
•
For VoIP we need to configure:
voice-port - the voice ‘interface’
FXS / FXO - e.g. voice-port 1/0/0
E1/T1 signalling channel - e.g. voice-port 1/0:D
dial-peer - tells the gateway how to connect voice
ports to VoIP call legs
• For E1/T1 links we also need to configure the
physical bearer controller E1 / controller T1
• interface serial 0:15 (the signalling timeslot for an
E1)
FXS / FXO Configuration
! Some useful settings
!
voice-port 1/0/0
no comfort-noise ! needs ‘no vad’ on VoIP dial-peer
cptone GB
timeouts interdigit 3 ! timeout when gathering dialled digits
description Analog phone line
!
! Or, if you’re just having a play, the defaults will work:
!
voice-port 1/0/1
!
Dial Peers
• Basic building block on Cisco voice gateways, the
dial-peer
• All calls consists of at least two call legs:
• Originating device to originating gateway (POTS)
• Originating gateway to IP network (VoIP)
• ...and/or
• IP network to destination gateway
• Destination gateway to destination device
(1) Two Analogue Phones – 1 Gateway
Voice Gateway Router Configuration File
voice-port 1/0/0
!
voice-port 1/0/1
!
!
dial-peer voice 1 pots
destination-pattern 5551234
port 1/0/0
!
dial-peer voice 2 pots
destination-pattern 5555678
port 1/0/1
(2) Two Analogue phones – 2 Gateways
Voice Gateway Router 1 Configuration File
voice-port 1/0/0
!
dial-peer voice 1 pots
destination-pattern 5551234
port 1/0/0
!
dial-peer voice 10 voip
destination-pattern 5555678
session target ipv4:10.5.6.7
codec g711ulaw
Voice Gateway Router 2 Configuration File
voice-port 1/0/0
!
dial-peer voice 2 pots
destination-pattern 5555678
port 1/0/0
!
dial-peer voice 20 voip
destination-pattern 5551234
session target ipv4:10.2.3.4
codec g711ulaw
(3) Each Connected to Their Own Voice Gateway
Routers Using the G.729 Codec as 1st choice
Voice Gateway Router 1 Configuration File
voice class codec 1
codec preference 1 g729r8
codec preference 2 g711ulaw
!
voice-port 1/0/0
!
voice-port 1/0/1
!
!
dial-peer voice 1 pots
destination-pattern 5551234
port 1/0/0
!
dial-peer voice 2 voip
destination-pattern 5555678
voice-class codec 1
session target ipv4:10.5.6.7
Voice Gateway Router 2 Configuration File
voice class codec 1
codec preference 1 g729r8
codec preference 2 g711ulaw
!
voice-port 1/0/0
!
voice-port 1/0/1
!
!
dial-peer voice 1 pots
destination-pattern 5555678
port 1/0/0
!
dial-peer voice 2 voip
destination-pattern 5551234
voice-class codec 1
session target ipv4:10.2.3.4
(4) Analogue Phone and an IP Phone Connected
over an IP Network
Voice Gateway Router 1 Configuration File
voice-port 1/0/0
!
dial-peer voice 1 pots
destination-pattern 5551234
port 1/0/0
!
dial-peer voice 2 voip
destination-pattern 5555678
session target ipv4:10.5.6.7
Voice Gateway Router 2 Configuration File
voice-port 1/0/0
!
dial-peer voice 1 pots
destination-pattern 5555678
port 1/0/0
!
dial-peer voice 2 voip
destination-pattern 5551234
session target ipv4:10.2.3.4
(5) Two IP Phones Connected over a Frame
Relay Network
Voice Gateway Router 1 Configuration File
interface Serial0/0
ip address 10.5.6.6 255.0.0.0
encapsulation frame-relay
dial-peer voice 1 pots
destination-pattern 5551234
port 1/0/0
dial-peer voice 1000 voip
preference 1
destination-pattern 5555678
session target ipv4:10.5.6.7
Voice Gateway Router 2 Configuration File
interface Serial0/0
ip address 10.5.6.7 255.0.0.0
encapsulation frame-relay
clock rate 2000000
dial-peer voice 1 pots
destination-pattern 5555678
port 1/0/0
dial-peer voice 2000 voip
preference 1
destination-pattern 5551234
session target ipv4:10.5.6.6
(6)Using the PSTN in the event of an IP network
failure
Configuration for Router 1
translation-rule 21
Rule 1 ^2 5552
translation-rule 11
Rule 1 ^5551 1
interface FastEthernet0/0
ip address 10.1.1.2 255.255.255.0
voice-port 1/0/0
translate called 11
dial-peer voice 1 pots
destination-pattern 1000
port 1/1/0
dial-peer voice 2000 voip
preference 1
destination-pattern 2000
session target ipv4:10.1.1.3
dial-peer voice 20 pots
preference 2
destination-pattern 2000
translate-outgoing called 21
port 1/0/0
forward-digits all
Configuration for Router 2
translation-rule 11
Rule 1 ^1 5551
translation-rule 21
Rule 1 ^5552 2
interface Ethernet0/0
ip address 10.1.1.3 255.255.255.0
voice-port 1/1/0
translate called 21
!
dial-peer voice 1 pots
destination-pattern 2000
port 1/0/0
!
dial-peer voice 1000 voip
preference 1
destination-pattern 1000
session target ipv4:10.1.1.2
!
dial-peer voice 10 pots
preference 2
destination-pattern 1000
translate-outgoing called 11
port 1/1/0
forward-digits all
Digit stripping
• We want to dial 5554329
#dial-peer voice 3 pots
#destination-pattern 555....
#port 2/0/0
• The default behaviour for the router is to strip
the first three digits from the number because
these are the numbers matched in the dialpeer and send 4329 forward
Digit stripping ctd..
• We want to send all digits to 5554329
#dial-peer voice 3 pots
#destination-pattern 555....
#no digit-strip
#port 2/0/0
Prefixes
• The prefix dial-peer command adds digits to
the beginning of the dial string
#dial-peer voice 3 pots
#destination-pattern 1904678
#prefix 8
#port 1/0/1
• The peer at other end might require a 5 digit
number so the number 84678 is forwarded
Number Expansion
• People find it easier to just remember the 3
digit extension number e.g.363
Ext Dest-Pattern Num Expand Command
363 719155363 num-exp 00353 719155363