Implementing Voice Gateways

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, well 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 802.11x

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 1/0D
• 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 015 (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 youre 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-patte
rn 5551234 port 1/0/0 ! dial-peer voice 2
pots destination-pattern 5555678 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 5551234 port
1/0/0 ! dial-peer voice 10 voip destination-patter
n 5555678 session target ipv410.5.6.7 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-patter
n 5551234 session target ipv410.2.3.4 codec
g711ulaw
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(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
ipv410.5.6.7
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
ipv410.2.3.4
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(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 ipv410.5.6.7
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 ipv410.2.3.4
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(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
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
5551234 port 1/0/0 dial-peer voice 1000
voip preference 1 destination-pattern
5555678 session target ipv410.5.6.7
dial-peer voice 1 pots destination-pattern
5555678 port 1/0/0 dial-peer voice 2000
voip preference 1 destination-pattern
5551234 session target ipv410.5.6.6
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(6)Using the PSTN in the event of an IP network
failure
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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
ipv410.1.1.3 dial-peer voice 20 pots preference
2 destination-pattern 2000 translate-outgoing
called 21 port 1/0/0 forward-digits all
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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 ipv410.1.1.2 ! dial-peer
voice 10 pots preference 2 destination-pattern
1000 translate-outgoing called 11 port
1/1/0 forward-digits all
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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

18
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

19
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

20
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