ISDN PRI Capabilities and the Asterisk Implementation …comunidad.asterisk-es.org/documentos/astricon2008/ISDN_for... · ISDN PRI Capabilities and the Asterisk Implementation Mark

© 2008 AT&T Knowledge Ventures. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Knowledge Ventures.

ISDN PRI Capabilities and the Asterisk Implementation

Mark A. Vince, P.E., AT&T Matt Fredrickson, Digium

Astricon 2008

September 25, 2008

Phoenix, AZ

© 2008 AT&T Knowledge Ventures. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Knowledge Ventures. Page 2

Why Are We Talking About ISDN?

•  PSTN has a large user community

•  ISDN PRI is an “incumbent” interface

•  Matured over 20 years

•  Standardized.... well sorta... vendor specific

•  Way to integrate VOIP and “rest of the world”

•  Learn from those who have gone before


ISDN Application Architecture

PSTN VOIP

IP ISDN PRI

Router

4E, 5E, DMS,

Siemens,

IP TDM Legacy PBX


Areas of Discussion

•  ISDN PRI overview   Layer 1 – T1   Layer 2 – Q.921   Layer 3 – Q.931

•  Asterisk PRI architecture   Zaptel (Dahdi)   Libpri

•  Connecting to Asterisk – Real examples   Avaya PRI   Cisco PRI

=>> Asterisk zaptel, zapata settings


What is ISDN PRI?

Integrated Services Digital Network   Common infrastructure for voice, data and video (circa 1980s)

  Feature rich signaling interface

  Two types: BRI and PRI

Primary Rate Interface   Provided via a T1 / E1 (1.544 Mbs / 2.048 Mbs)

  Bearer channels (B-channels, H-channels), signaling channel (D-channel)

  Two types: Facility Associated Signaling (FAS) and Non Facility Associated Signaling (NFAS)

Basic Rate Interface – BRI – 2B+D not covered today

H-channels not covered today


Primary Rate Interface – PRI – 23B+D / 30B+D Facility Associated Signaling (FAS)

B-Channel

B-Channel B-Channel B-Channel

B-Channel

D-Channel

Q.921

Q.931

T1 / E1

•  Looks like a “regular” T1

•  All signaling in the D-channel

•  B-channels are true “pipes”

ISDN Protocol


Primary Rate Interface – PRI – nB+D Non Facility Associated Signaling (NFAS)

Q.921

Q.931

T1 / E1

B-Channel

D-Channel

B-Channel

B-Channel

B-Channel

D-Channel

Primary D-Channel

Back-up D-Channel

#0

#2

#1

#n

With D-Channel Back-UP (DBU)


ISDN PRI Layer 1 – Physical Layer •  T-1, 1.544 Mbps = 24 x 64 kbps + 8 kbps (framing)

•  DS1 Signal Options   ESF – Extended Super Frame – What type of framing?   B8ZS - Bipolar 8 Zero code Substitution – How to handle all zeroes?

•  Electrical Consideration   CSU - Channel Service Unit / DSx level – How long are the wires?

•  Synchronization / Clocking   Take from telephone network, generally   Slips significantly impact modems (fax), reduce voice quality

=>> zaptel.conf dchan=24 bchan=1-23 span=1,1,0,esf,b8zs


ISDN PRI Layer 2 – Link Layer - Q.921

•  Exists Only In The D-channel

•  LAP-D – Link Access Procedure - D

•  HDLC – High level Data Link Control Protocol

•  Point-to-Point protocol only

•  Network / User concept

•  Specific Elements   SABME– Synchronous Balanced Mode Extended /

UA - User Acknowledge

  RR / RNR – Receiver Ready or Receiver Not Ready

pritimer => t200,1000 =>> zapata.conf signalling=pri_net / pri_cpe


ISDN PRI Layer 3 – Protocol Layer - Q.931

•  General Concepts   Exists only in the D-channel, on top of Q.921   Messages with Information Elements (IEs)   Variants – AT&T (4ESS, 5ESS), ETSI, NI-1, NI-2, QSIG, Nortel (DMS100)

•  Call Control / Signaling Protocol

•  Code Sets – Collections of Information Elements (IEs)

  Code Set 0 – Default set for collection of IEs   Code Set 5 – National specific set for IEs   Code Set 6 – Network specific set for IEs   Code Set 7 – User specific set for IEs

•  Locking Shift – Used to move between Code Sets

switchtype=national


ISDN PRI Layer 3 – Q.931 - Continued

•  ISDN Messages for Call Control   SETUP Message – Here is what I want....

•  Bearer Capability IE – Voice, data, encoding √ •  Channel IE – Which channel on the PRI √

•  Called Number IE – Number to be called √

•  Calling Number IE √

•  II / OLI IE (Information Indicator / Originating Line Identification) √

•  UUI IE– User-to-User Information – Data associated with the call •  Lower Layer Compatibility IE – Subrate data multiplexing scheme

•  Display Name IE – PBX user name √*CodeSet 0 only

  CONNECT Message– Here is what you got.... •  Call was accepted •  Connected Number IE – Who really answered

√ Asterisk supported


ISDN PRI Layer 3 – Q.931 - Continued

•  ISDN Messages for Call Control - Continued   Disconnect Message – I’m leaving....

•  Channel IE – Which channel on the PRI √

•  UUI IE– User-to-User Information – Data associated with the call

  Release Message – Goodbye..

•  ISDN Messages - Others   Facility Message

•  Calling Party Name IE – PSTN Calling Party Name √

•  Channel IE – Which channel on the PRI

√ Asterisk supported


ISDN PRI Layer 3 – Q.931 - Continued •  ISDN Messages for Maintenance

  RESTART Message - Set to known state (idle) √

•  Channel – Which channel on the PRI*

  RESTART-ACKNOWLEDGE Message √ •  Channel – Which channel on the PRI

  SERVICE Message - Change channel state •  Channel – Which channel on the PRI •  Change Status – Requested state change (in-service, maintenance, out-of-service)

  SERVICE-ACKNOWLEDGE Message •  Channel – Which channel on the PRI •  Change Status – Final state (in-service, maintenance, out-of-service)

* Some implementations do more than one channel at a time √ Asterisk supported

State must be

communicated to

application !

(chan-zap)

resetinterval=3600


Traditional ISDN Information Element Flow

PSTN ISDN PRI

4E, 5E, DMS,

Siemens,

Legacy PBX

Setup =>

Screen Pop

Facility =>

Calling Party Name

Restart => <= Restart Ack

Called Party Number Calling Party Number

Display Name UUI, II/OLI

T-Server CTI Link

Corp Data

Calling Party Number Display Name

UUI

Service => <= Service Ack

Channel ID Called Party Number

Bearer Capability


ISDN PRI Layer 3 – Message Associated - User to User Information

•  IE associated with setup and clearing ISDN messages

•  Typical Applications -> Call Center / Computer Telephony (CTI)   Provides a “tag” to identify supporting data regarding the call   Stays with the call   Can be modified as call is forwarded

•  Size of message depends on vendor implementation   4ESS – 198 bytes (across all Code Sets)   5ESS – 64 bytes   DMS250 – 128 bytes   DMS100 – 0 bytes   Avaya Definity – 120 bytes

•  Novel Applications   Wrong number but pass data to called party   Pass URL (alphanumeric only)


ISDN PRI Layer 3 – Temporary Signaling Connections

•  Standard D-channel messages, difference is B-channel

•  Two Types   Call Associated Temporary Signaling Connection (CA-TSC)   Non Call Associated Temporary Signaling Connection (NCA-TSC) or Call Independent Signaling Connection (CISC) (QSIG)

•  CA-TSC Application -> Distributed Communication System® (DCS)

  Pass station data that might otherwise not be passed

•  NCA-TSC Application   Carry Message Waiting Indication (MWI) information   Pass query information on call queue parameters

•  Can be used through some PSTN network switches   Allows for consolidated voicemail in the enterprise


Asterisk PRI Architecture

*


Architectural Overview




Architectural Overview – Asterisk Core

•  Asterisk Core –  Control is in the Dialplan –  Allows channels and applications to access each other in a

standard way. Regardless of underlying protocols or bearer transmission.

•  Channel Drivers –  Provide protocol independent interface for Asterisk to control –  Read, Write, Dial, Hangup

•  Applications –  Can use the Asterisk core to read and write audio data and

initiate signalling over channels.




Architectural Overview – Kernel Side

•  Zaptel/DAHDI –  Communicates with Asterisk via chan_zap/chan_dahdi –  D

oes

not actually do any of the ISDN signaling (only RBS bit signalling)‏

–  Provides services for higher layers that actually do the signalling

•  Zaptel/DAHDI Hardware Drivers –  Provide “clear channel timeslots” to which zaptel communicates –  Interrupt driven Pseudo-TDM bus


Zaptel/DAHDI Overview


Zaptel Services

•  Provides Services to Userland –  D-channel –  B-channel –  Separates channels

•  Provides Services for those Channels –  Echo cancelation –  Conferencing –  HDLC bit stuffing, unstuffing –  CRC checking/generation –  Time Slot Interchange


chan_dahdi/libpri


chan_zap/libpri

Libpri

•  Sits on top of a zaptel “dchan”

•  Handles all Q.921 (layer 2) and Q.931 (layer 3) messaging necessary for D-channel operation

•  Provides simple interface to chan_zap for making calls over D-channel

•  Q.931 Messages translate into easy to use PRI events

Chan_zap

•  Utilizes libpri to maintain

signaling services on D-channel

•  Directly interfaces with bearers through zaptel (includes

reading and writing audio data)‏

•  Provides glue interface between libpri+


Useful CLI Commands

•  pri debug span x –  Enables debugging on the given span (Q.931 call level)‏

•  pri intensive debug span x –  Enables enhanced debugging on the given span (Q.931 + Q.921)‏

•  pri show span x –  Sho

ws

some useful information about span (up, down, timer values, etc).

•  pri [set,unset] debug file [filename] –  Outputs all PRI debug enabled by the

pri

debug commands to the given filename (or disables it with unset).

•  pri show spans –  Shows information about the spans in the system (up, down)‏


Useful

Configuration Options (chan_dahdi.conf/zapata.conf) ‏•  inbanddisconnect=[yes,no]

–  Allows a caller to hear inband disconnect data that the remote end may play at disconnect, instead of releasing the B channel immediately

•  pritimer=t309,10000 –  Used in cases with periodic instability. Instead of hanging up all

calls when the D-channel drops, a guard timer (10,000 ms, or 10 secs) waits 10 seconds for the D-channel to come back up before hanging up calls.


Mistakes NOT to make

•  Don't configure a non-NFAS PRI with

the

trunkgroup and spanmap settings (those are only for NFAS)‏ –  instead use implicit method of declaration:

•  signalling=pri_net[or pri_cpe] •  channel=1-23


HOW TO – Enable Supplementary Services

•  Calling Name over Facility message –  facilityenable=yes –  You will see (in pri

debug

span x) a facility message sent containing caller name information.

•  Two B-Channel Transfer – (Works on 5ESS, NI2, DMS100 (RLT) Q.SIG)‏ –  facilityenable=yes –  transfer=yes –  Don't do anything in the Dial() application which would prohibit native bridging (any of the options that require inband

audio


What's New?

Zaptel/DAHDI

•

DAHDI will be released soon!

•  Many driver

fixes, updates, and optimizations.

•  MTP2 channel type (for SS7)‏

Libpri

•  Service message support!

•  BRI Support (1.4.4 of

libpri, 1.6.0 and trunk of Asterisk)‏ –  signallin

g=bri_net, bri_cpe, bri_cpe_ptmp‏


Configuration for /etc/zaptel.conf

span=1,1,0,esf,b8zs (Set up T1 span)

bchan=1-23 (Assign B-Channels)

dchan=24 (Assign D-Channels)


Configuration for /etc/asterisk/zapata.conf

switchtype=national (Set protocol of PRI)

resetinterval = 3600 (Set audit frequency)

signalling = pri_cpe (PRI user side)

channel => 1-23 (Designate B-Channels)


ISDN Configuration Examples

Configurations

for

Avaya and Cisco PRI

With

Asterisk


Avaya PRI Configuration for Asterisk

Three Step Process

•  Add a DS1/E1 board (TN767x or TN464x)

•  Add a signaling group

•  Add a trunk group



Add a DS1/E1 Board

Command: add ds1 xxYzzzz

•  Name: Asterisk Access

•  Line Coding: b8zs

•  Line Compensation: 1

•  Framing Mode: esf

•  Signaling Mode: isdn-pri

•  Connect: pbx

•  Interface: network

•  Country Protocol: 1

•  Protocol Version: a

•  Near-end CSU Type: other Note: Not all settings described



Add a signaling group

Command: add signaling-group xx

For FAS:

•  Associated Signaling? y

•  Primary D-Channel: 01B1124

For NFAS:

•  Associated Signaling? n

•  Primary D-Channel: 01B1124

•  Secondary D-Channel: 01A0924

•  Trunk Brd (tables) Interface ID (tables)

01B11 0

01A09 1 Note: Not all settings described



•  Group Type: isdn

•  Group Name: Asterisk PSTN Access

•  Direction: two-way

•  Outgoing Display? Y

•  Service Type: tie

•  Codeset to Send Display: 0

•  Codeset to Send National IEs: 6

•  Trunk Hunt: ascend

Add a trunk group

Command: add trunk-group xx

Note: Not all settings described


Avaya PRI Configuration for Asterisk Add a trunk group, continued

•  Maintenance Tests? Y

•  Send Name: y

•  Send Calling Number: y

•  Numbering Format: public

•  Outgoing Channel ID Encoding: preferred

•  UUI IE Treatment: service-provider

•  Replace Restricted Numbers? n

•  Replace Unavailable Numbers? n

•  Send Connected Number: y

•  Send UCID? n

•  Send Codeset 6/7 LAI IE? n

•  US NI Delayed Calling Name Update? n Note: Not all settings described


Cisco PRI Configuration for Asterisk

Three Step Process

•  Add a DS1/E1 interface

•  Add a signaling interface

•  Some additional settings



Add a DS1/E1 interface

Command: controller T1 x/y

framing esf

linecode b8zs

cablelength short 133

pri-group timeslots 1-24


Cisco PRI Configuration for Asterisk Add a signaling interface

Command: interface Serialx/0:23

description "D-Channel for Asterisk" no ip address

encapsulation hdlc

no logging event link-status

no snmp trap link-status

isdn switch-type primary-ni

isdn incoming-voice voice

isdn map address ^1 plan isdn type national

isdn map address ^011 plan isdn type international

isdn disconnect-cause busy

isdn outgoing display-ie

trunk-group 12 no cdp enable



Additional settings

Command: isdn switch-type primary-4ess

Command: trunk group xx

description "Trunk Group 12 & 23 - 4ESS PRI"

hunt-scheme sequential both down


Summary

•  High level overview of ISDN

•  Identified various Asterisk configuration settings

•  Detailed description of Asterisk ISDN internal architecture

•  Covered two real examples


References •  Digium zapata.conf Configuration, August 2006

http://www.asterisk.org/doxygen/Config_zap.html

•  AT&T PRI Specification, 1999 http://www.att.com/cpetesting/pdf/tr41459_99.pdf

•  General Information on ISDN http://www.protocols.com/pbook/isdn.htm

•  Rarig, Harry, ISDN Signal Distribution Network, IEEE Communications Magazine, Vol. 32, No. 6, June 1994

•  Stallings, William, ISDN, An Introduction, MacMillan Publishing Company, ISBN 0024154717, 1989

Documents

ISDN PRI Capabilities and the Asterisk Implementation …comunidad.asterisk-es.org/documentos/astricon2008/ISDN_for... · ISDN PRI Capabilities and the Asterisk Implementation Mark