Voice Gateways: Understanding Digital Voice Ports

  • Digital Ports: T1, E1, ISDN
  • Understanding Digital Signaling
  • Configuring Digital Voice Ports

Quantization – Measurement of amplitude and frequencies and convert into binary 1s and 0s.  Voice is digitised. Nyquist Therom.

Digital Voice Interfaces

Allow for multiple calls vs single call analog

Broken into T1, E1 and ISDN standards

VWIC card can do data and or voice

2 port T1/E1 VWIC2-2MFT-T1/E1

T1 = USA/Japan/Canada etc..

E1 = Everywhere else in the world

ISDN = Used all over the world

Digital Signaling

Channel Associated Signaling (CAS) – T1/E1 Most of the time..

  • AKA Robbed Bit Signaling – RBS
  • Steals bits from user data for signaling
  • Advantage: All channels available for data
  • Disadvantage: User bandwidth reduced
  • 24 calls to the PSTN using CAS, therefore lower quality signal

Common Channel Signaling (CCS) – PRI

  • Dedicates a single channel to signaling
  • Advantages: More signaling information available, no user bandwidth used
  • Disadvantages: Loose a complete channel
  • Used by most PBXs today

More depth with CCS/PRI..

  • Two flavours of ISDN available: BRI (Europe) and PRI (T1 equivalent to ISDN)
  • Standard based Q.921 (Layer 2) and Q.931 (Layer 3) signaling protocols:

Q.921 – Error Detection/Correction
Q.931 – Call Setup/Teardown/Information

  • Non Facility Associated Signaling – NFAS Allows single D channel for multiple PRI interfaces
  • QSIG – Based on Q.931 allows PBX to PBX interconnects (Multi vendor)

CAS Configuration

show voice port summary

show controllers t1

conf t
controller t1 1/0
no shut

ds0-group (Anything to do with CAS style connection!)
ds0-group 1 timeslots 1-24 (Full T1 connection to the PSTN)
ds0-group 1 timeslots 1-24 type fxo-ground-start

show voice port summary

dial-peer voice 9 pots
destination-pattern 9……….

controller t1 1/0
no ds0-group 1 (Remove configuration)

CCS Configuration

conf t
controller t1 1/0
pri-group timeslots 1-24

isdn switch-type primary-5ess (What type of ISDN switch to connect to? Dependant on where in the world… Talk to carrier)

show ip int brief
Lots of Serial interfaces are created for individual voice channels

1 dedicated channel – 23
(If you shut this down, all channels will shut!)

All configuration applied under interface 23
isdn incoming-voice voice

If outside US, Japan and Canada use channel 15 for Europe

Signaling ports: T1 23/24 + E1 15/16 (Exam!!)