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 1/0/0no comfort-noise ! needs ‘no vad’ on VoIP dial-peercptone GBtimeouts interdigit 3 ! timeout when gathering dialled digitsdescription 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 Filevoice-port 1/0/0!voice-port 1/0/1!!dial-peer voice 1 potsdestination-pattern 5551234port 1/0/0!dial-peer voice 2 potsdestination-pattern 5555678port 1/0/1

(2) Two Analogue phones – 2 Gateways

Voice Gateway Router 1 Configuration Filevoice-port 1/0/0!dial-peer voice 1 potsdestination-pattern 5551234port 1/0/0!dial-peer voice 10 voipdestination-pattern 5555678session target ipv4:10.5.6.7codec g711ulaw

Voice Gateway Router 2 Configuration Filevoice-port 1/0/0!dial-peer voice 2 potsdestination-pattern 5555678port 1/0/0!dial-peer voice 20 voipdestination-pattern 5551234session target ipv4:10.2.3.4codec g711ulaw

(3) Each Connected to Their Own Voice Gateway Routers Using the G.729 Codec as 1st choice

Voice Gateway Router 1 Configuration Filevoice class codec 1codec preference 1 g729r8codec preference 2 g711ulaw!voice-port 1/0/0!voice-port 1/0/1!!dial-peer voice 1 potsdestination-pattern 5551234port 1/0/0!dial-peer voice 2 voipdestination-pattern 5555678voice-class codec 1session target ipv4:10.5.6.7

Voice Gateway Router 2 Configuration Filevoice class codec 1codec preference 1 g729r8codec preference 2 g711ulaw!voice-port 1/0/0!voice-port 1/0/1!!dial-peer voice 1 potsdestination-pattern 5555678port 1/0/0!dial-peer voice 2 voipdestination-pattern 5551234voice-class codec 1session target ipv4:10.2.3.4

(4) Analogue Phone and an IP Phone Connected over an IP Network

Voice Gateway Router 1 Configuration Filevoice-port 1/0/0!dial-peer voice 1 potsdestination-pattern 5551234port 1/0/0!dial-peer voice 2 voipdestination-pattern 5555678session target ipv4:10.5.6.7

Voice Gateway Router 2 Configuration Filevoice-port 1/0/0!dial-peer voice 1 potsdestination-pattern 5555678port 1/0/0!dial-peer voice 2 voipdestination-pattern 5551234session target ipv4:10.2.3.4

(5) Two IP Phones Connected over a Frame Relay Network

Voice Gateway Router 1 Configuration Fileinterface Serial0/0ip address 10.5.6.6 255.0.0.0encapsulation frame-relaydial-peer voice 1 potsdestination-pattern 5551234port 1/0/0dial-peer voice 1000 voippreference 1destination-pattern 5555678session target ipv4:10.5.6.7

Voice Gateway Router 2 Configuration Fileinterface Serial0/0ip address 10.5.6.7 255.0.0.0encapsulation frame-relayclock rate 2000000dial-peer voice 1 potsdestination-pattern 5555678port 1/0/0dial-peer voice 2000 voippreference 1destination-pattern 5551234session target ipv4:10.5.6.6

(6)Using the PSTN in the event of an IP network failure

Configuration for Router 1translation-rule 21Rule 1 ^2 5552translation-rule 11Rule 1 ^5551 1

interface FastEthernet0/0ip address 10.1.1.2 255.255.255.0

voice-port 1/0/0translate called 11

dial-peer voice 1 potsdestination-pattern 1000port 1/1/0

dial-peer voice 2000 voippreference 1destination-pattern 2000session target ipv4:10.1.1.3

dial-peer voice 20 potspreference 2destination-pattern 2000translate-outgoing called 21port 1/0/0forward-digits all

Configuration for Router 2translation-rule 11Rule 1 ^1 5551translation-rule 21Rule 1 ^5552 2

interface Ethernet0/0ip address 10.1.1.3 255.255.255.0

voice-port 1/1/0translate called 21!dial-peer voice 1 potsdestination-pattern 2000port 1/0/0!dial-peer voice 1000 voippreference 1destination-pattern 1000session target ipv4:10.1.1.2!dial-peer voice 10 potspreference 2destination-pattern 1000translate-outgoing called 11port 1/1/0forward-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 dial-peer 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 Command363 719155363 num-exp 00353 719155363