1. Implementing Voice Gateways
Lecture 3

2. 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

3. 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 – x

4. 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

5. 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

6. 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)

7. 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

8. 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

9. (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
port 1/0/0
dial-peer voice 2 pots
destination-pattern
port 1/0/1

10. (2) Two Analogue phones – 2 Gateways
Voice Gateway Router 1 Configuration File
voice-port 1/0/0 !
dial-peer voice 1 pots
destination-pattern
port 1/0/0
dial-peer voice 10 voip
destination-pattern
session target ipv4:
codec g711ulaw
Voice Gateway Router 2 Configuration File
voice-port 1/0/0 !
dial-peer voice 2 pots
destination-pattern
port 1/0/0
dial-peer voice 20 voip
destination-pattern
session target ipv4:
codec g711ulaw

11. (3) Each Connected to Their Own Voice Gateway Routers Using the G
(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
port 1/0/0
dial-peer voice 2 voip
destination-pattern
voice-class codec 1
session target ipv4:
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
port 1/0/0
dial-peer voice 2 voip
destination-pattern
voice-class codec 1
session target ipv4:

12. (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
port 1/0/0
dial-peer voice 2 voip
destination-pattern
session target ipv4:
Voice Gateway Router 2 Configuration File
voice-port 1/0/0 !
dial-peer voice 1 pots
destination-pattern
port 1/0/0
dial-peer voice 2 voip
destination-pattern
session target ipv4:

13. (5) Two IP Phones Connected over a Frame Relay Network
Voice Gateway Router 1 Configuration File
interface Serial0/0
ip address
encapsulation frame-relay
Voice Gateway Router 2 Configuration File
interface Serial0/0
ip address
encapsulation frame-relay
clock rate
dial-peer voice 1 pots
destination-pattern
port 1/0/0
dial-peer voice 1000 voip
preference 1
destination-pattern
session target ipv4:
dial-peer voice 1 pots
destination-pattern
port 1/0/0
dial-peer voice 2000 voip
preference 1
destination-pattern
session target ipv4:

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

15. Configuration for Router 1
translation-rule 21
Rule 1 ^2 5552
translation-rule 11
Rule 1 ^5551 1
interface FastEthernet0/0
ip address
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:
dial-peer voice 20 pots
preference 2
translate-outgoing called 21
port 1/0/0
forward-digits all

16. Configuration for Router 2
translation-rule 11
Rule 1 ^1 5551
translation-rule 21
Rule 1 ^5552 2
interface Ethernet0/0
ip address
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:
dial-peer voice 10 pots
preference 2
translate-outgoing called 11
port 1/1/0
forward-digits all

17. Digit stripping We want to dial 5554329 #dial-peer voice 3 pots
#destination-pattern
#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

18. Digit stripping ctd.. We want to send all digits to 5554329
#dial-peer voice 3 pots
#destination-pattern
#no digit-strip
#port 2/0/0

19. Prefixes
The prefix dial-peer command adds digits to the beginning of the dial string
#dial-peer voice 3 pots
#destination-pattern
#prefix 8
#port 1/0/1
The peer at other end might require a 5 digit number so the number is forwarded

20. Number Expansion
People find it easier to just remember the 3 digit extension number e.g.363
Ext Dest-Pattern Num Expand Command
num-exp