ISDN Background

ISDN Interfaces• ISDN interfaces are digital TDM (Time Division

Multiplexing) trunks that support multiple voicechannels (also known as Bearer channels) over asingle circuit.

• There are two common types of ISDN interfaces.They are BRI and PRI.

Bearer Channel (Media)• The bearer capability for BRI and PRI is voice/speech

at 3.1 kHz audio voice channel of 8 bits sampled at8 kHz with G.711 encoding, and data at unrestricted64 kbps, restricted 64 kbps, 56 kbps.

• Each one of these channels is known as a DS0

Data Channel (Signaling)• The signaling channel (D) uses Q.931 for signaling with the

other side of the link.Q.931• Q.931 is used to transmit and receive call signaling

messages according to the H.225 protocol for digitaltelephone services. The messages in Q.931 include setup (asignal indicating the establishment of a connection), call-proceeding (a signal indicating that the call is beingprocessed by the destination terminal), ring-alert (a signalthat tells the calling party that the destination set is ringing),connect (a signal sent back to the source indicating that theintended destination phone set has received the call), andrelease/complete (a signal sent by either the source or thedestination indicating that the call is to be terminated).

Basic Rate Interface

The entry level interface to ISDN is the Basic(s) Rate Interface (BRI), a 128 kbps service delivered over a pair of standard telephonecopper wires. The 144 kbps payload rate is broken down into two 64 kbps Bearer channels ('B' channels) and one 16 kbps signalingchannel ('D' channel or delta channel). This is sometimes referred to as 2B+D.

The interface specifies the following network interfaces:• The U interface is a two-wire interface between the exchange and a

network terminating unit, which is usually the demarcation point innon-North American networks.

• The T interface is a serial interface between a computing device anda terminal adapter, which is the digital equivalent of a modem.

• The S interface is a four-wire bus that ISDN consumer devices pluginto; the S & T reference points are commonly implemented as asingle interface labeled 'S/T' on an Network termination 1 (NT1).

• The R interface defines the point between a non-ISDN device and aterminal adapter (TA) which provides translation to and from such adevice.

• BRI-ISDN is very popular in Europe but is muchless common in North America. It is alsocommon in Japan - where it is known as INS64.

• ScopTEL supports many SIP to BRI gateways(Mediatrix, Patton, Audiocodes, Quintum,Vegastream)

• ScopTEL only supports Woomera and Wanpipe(Sangoma ) BRI PSTN interfaces (using the CardDetection Wizard).

Primary Rate Interface

• The other ISDN access available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbps) in most parts ofthe world. An E1 is 30 'B' channels of 64 kbps, one 'D' channel of 64 kbps and a timing and alarm channel of 64 kbps.

• In North America PRI service is delivered on one or more T1 carriers (often referred to as 23B+D) of 1544 kbps (24 channels).A PRI has 23 'B' channels and 1 'D' channel for signaling (Japan uses a circuit called a J1, which is similar to a T1).

• Inter-changeably but incorrectly, a PRI is referred to as T1 because it uses the T1 carrier format. A true T1 (commonly called"Analog T1" to avoid confusion) uses 24 channels of 64 kbps of in-band signaling. Each channel reserves 56 kb for data orvoice and 8 kbps for signaling and messaging. Analog T1 circuits do not support CLID.

• PRI uses out of band signaling which provides the 23 B channels with clear 64 kb for voice and data and one 64 kb 'D' channelfor signaling and messaging. PRI Circuits supply CLID before the user’s phone starts ringing.

• In North America, NFAS (Non-Facility Associated Signaling) allows two or more PRIs to be controlled by a single D channel, andis sometimes called "23B+D + n*24B". D-channel backup allows for a second D channel in case the primary fails.

• On a single span Channel 24 is normally reserved for the D Channel on North American T1 PRI circuits.• On a single span Channel 16 is normally reserved for the D Channel on E1 PRI Circuits.• PRI Circuits use DNIS to route calls to the CPE (Customer Provided Equipment) and most PRI circuits have many DNIS numbers

associated with the circuit number (usually sold in blocks of 10 or 30).

DNIS (Dialed Number Information Service)

• DNIS is the routing number the PRI CPE (Customer Provided Equipment) circuit receives from the carrier (also known as Received Digits)• The received digits length (also known as Digits to Out Pulse by some Telco’s) can vary in length typically from 3 to 10 digits in length.• DNIS digits, as received from the carrier, are used to route calls via Incoming Lines objects in the ScopTEL IP PBX

Example 1:

• The customer’s BTN is 555-555-1234

• The Received Digit length set by the Telco is 4

• The resulting DNIS or received digits is equal to 1234

Toll Free Services:

• The customer has a published toll free number 1-800-555-2234

• Toll free numbers must be associated with a local phone number which is referred to as the conversion number

• The conversion number can be any DNIS number associated with the PRI circuit

• In this example the conversion number for 1-800-555-2234 is the BTN number 555-555-1234

• Therefore the DNIS number for 1-800-555-2234 is 1234

ISDN Interfaces – Clocking

Systems with digital interfaces need to synchronize to the network in order to function. Synchronization is performed in a hierarchical way, where eachdevice/switch obtains the network clock from the device/or switch above it in the synchronization hierarchy and passes the network clock to the device/switchbelow it in the synchronization hierarchy. The synchronization levels are referred to as strata. Sangoma and Digium ISDN interfaces both include an internalCSU/DSU as do Mediatrix T1/E1 gateways.

• NT (Network Terminator): The interface does not obtain timing from the network, but transmits the systems timing to equipment connected to it. This isthe mode selected when connecting to the telco or when setting up a private back to back ISDN connection (ISDN tie trunk).

• TE (terminal Equipment): The interface derives clocking from the NT interface (usually the Telco but can also be a primary interface in a private ISDN tietrunk.

• Primary reference: The interface obtains the timing reference from the network, to which the system synchronizes (usually the Telco).• Secondary reference (Secondary): The interface acts as a standby reference. If there are excessive errors on the primary reference T1 link, or the interface

designated as primary reference fails, this interface will obtain the timing reference from the network, which the system synchronizes to.• Thirdary: The interface acts as a standby reference. If there are excessive errors on the secondary reference T1 link, or the interface designated as

secondary reference fails, this interface will obtain the timing reference from the network, which the system synchronizes to.• Quaternary: The interface acts as a standby reference. If there are excessive errors on the thirdary reference T1 link, or the interface designated as

thirdary reference fails, this interface will obtain the timing reference from the network, which the system synchronizes to.

Background and Pre-requisites

ScopTEL’s:

• SIP VoIP Accounts must be created on ScopTEL server for ISDN registration.

• Incoming Lines must be created on ScopTELserver to route incoming calls.

• Outgoing Lines must be created on ScopTELserver to route outgoing calls.

• Class of Service must be edited to allow Outgoing Lines to access Mediatrix ISDN.

Mediatrix’s:

The Mediatrix ISDN Gateways have two network interfaces:• Lan1 eth2 (default management interface IP address

192.168.0.10 255.255.255.0)• Uplink eth1 (DHCP client)

The Uplink eth1 port is the default SIP signaling and media interface (sip-default is bound to this interface).This document focuses on T1E1 interfaces due to their high density of channels and flexible routing options.

Mediatrix Configuration

Log into the webui http://<IP of Mediatrix Uplink eth1>/ .

The default user is ‘admin’ and password is ‘administrator’.

Mediatrix Interop Settings

• Since SIP OPTIONS messages will besent by ScopTEL to the MediatrixGateway the Mediatrix Gatewaymust be configured to reply to theseSIP OPTIONS messages.

• SIP>Interop>OPTIONS MethodSupport: Always 200 OK.

SIP Registrar and Proxy Settings

• The Mediatrix Gateway needs to know which ScopTEL server to register to by configuring SIP>Servers>Default Servers.

• Configure the Registrar Host and Proxy Host with the IP address or Fully Qualified Domain Name of the ScopTEL server.

Mediatrix Configuration

• Click on the SIP Menu tab and add a Trunk named isdngateway• Type = Trunk

• Signalling Network must be set to the interface on the same subnet as the ScopTEL server (in this example it is Lan1)

• Click Apply

VoIP Accounts

• Use the ScopTEL Interfaces Manager to build the SIP VoIP Account for the Mediatrix ISDN Gateway using secure passwords.

• Only one VoIP Account to process PSTN calls to and from the Mediatrix Gateway is required

VoIP Accounts, cont’d

• The Account name and Username must match.• Your password must be secure.• Hostmode is dynamic so the Mediatrix gateway

can register with the ScopTEL server.

VoIP Accounts, cont’d

• Insecure should include both Port and Invite.• Qualify should be enabled.• Keepalive Interval should be set to a value short

enough to address UDP timeouts in some firewalls.

VoIP Accounts, cont’d

• DTMF Mode RFC 2833 is recommended.• P-Asserted-Identity is recommended.• Only select CODEC’s which are compatible

with your PSTN interface and Region.• Careful CODEC selection is required to avoid

transcoding.• The G.729 CODEC cannot support FAX

transmissions.

Mediatrix Gateway

• The Mediatrix Gateway needs a configured Unit Gateway Registration to register with the ScopTEL VoIP Interface and process PSTN calls to and from the ScopTEL server

• The User Name must match the ScopTELVoIP Account Username created for the Gateway.

Mediatrix Gateway, cont’d

• The Mediatrix Gateway needs to authenticate to the ScopTEL VoIP Account.• SIP>Authentication Click the edit button to configure authentication.

Mediatrix Gateway

• Criteria must be Endpoint from the drop list selection.• Validate Realm must be Disable.• Endpoint must match the Slot where the ISDN module is installed.• The Username must equal the Username defined in the ScopTEL VoIP ‘isdngateway’ Interface.• The Password must equal the Password defined in the ScopTEL VoIP ‘isdngateway’ Interface.• Apply & Refresh Registration for the Mediatrix Gateway to register with the ScopTEL VoIP Interface.

Mediatrix Call Router

Overview of Call Router configuration:• The final configuration must resemble this screenshot.• Route Index 1 shows that all incoming calls to isdn-Slot1/E1T1 will be re-directed to sip-isdngateway ‘as is’.

These calls are configured as Incoming Lines in ScopTEL.• Route Index 2 shows that all incoming calls from sip-isdngateway will be re-directed to isdn-Slot1/E1T1 ‘as is’.

These calls are configured as Outgoing Lines in ScopTEL.

Incoming Lines

• Incoming Lines on the ScopTEL server will use Called Number/DNIS supplied by the Mediatrix Gateway over the configured VoIP Interface.

• For each configured DNIS a unique ScopTEL Destination can be applied as in this example:

Outgoing Lines

• Outgoing Lines use pattern matching to steer the dialed number to the isdngateway as in this example where the digit 9 is configured as the PSTN access code.

• Digits prefixed with 2-5 followed by any two digits from 0-9 are an example of using the isdngateway as a private ISDN tie trunk to connect a legacy PBX to ScopTEL.

Outgoing Lines, cont’d

• The leading digit 9 will be detected in the dial plan and route any outgoing call to the isdngateway based on pattern priority configured in the ScopTEL Class of Service applied to the extension.

• In this case the 9 will be stripped before the call is passed to the isdngateway and all remaining digits as dialed will be sent to the isdngateway T1E1 interface and routed to the PSTN.

NOTE: if you don’t want to strip the 9 setthe ‘Number of digit to strip’ to 0. This canbe useful when ISDN tie trunking theScopTEL server to a Legacy PBX. The 9 andall following digits will be steered by theLegacy PBX dial plan to the Legacy PBX’sPSTN interface. This is known as tandemdialing.

Tie Lines

• The leading digit 2-5 will be detected in the dial plan and route any outgoing call to the isdngateway based on pattern priority configured in the ScopTEL Class of Service applied to the extension.

• This can be useful when ISDN tie trunking the ScopTEL server to a Legacy PBX. Any 3 digit pattern starting with 2-5 will be steered by the ScopTEL Class of Service priority applied to the extension to the isdngatewayinterface. The Legacy PBX can be configured with incoming routes to steer these calls to local PBX extensions.

Application Notes

• ISDN Gateways are very practical for adding ISDN interfaces to the ScopTEL server when SIP trunks are not used.

• Additional features can be added to Legacy PBX’s using ISDN Gateways and leveraging ScopTEL to:• Add or replace existing voicemail servers• Add conference bridging • Add ACD (Automatic Call Distribution/Contact Center)• Add CDR (Call Detailed Reports) via ScopSTATS• Add Contact Center Reporting via ScopSTATS• Add Call Recording via ScopSTATS• Add VoIP phones and other mobility features