ISDN PRI (Enhanced Multipath)

bunchlearnedNetworking and Communications

Oct 30, 2013 (5 years and 4 months ago)


ISDN PRI (Enhanced Multipath)


PRI (SBC SNET product name is Enhanced Multipath) is a flexible high
speed digital
connection for voice and data between a customer’s digital PBX and SBC SNET's
switched telephone network. Each of ISDN's 23 B cha
nnels can use its full capacity (64
Kbps) for data because the 64 Kbps D channel sends all the signals required to connect
and disconnect each channel's calls. Each channel can handle Direct Inward Dial, Direct
Outward Dial, and various other call types.

The service is provided over a 1.544 Mbps channel from an SNET central office to the
customer's location and terminates at a demarcation point. The channel is multiplexed
into 24 communications paths that can be used for local service. Twenty three paths a
re B
(bearer) channels used for communications services. The twenty fourth path, or D
channel, is used for signaling. PRI allows a customer to use the same channel for multiple
call types, increasing their network’s capacity without dramatically increasing

Whether it’s voice, data, or video, PRI can help maximize network efficiency. The
service is offered on a 1, 3 or 5 year contract and is available in most central offices. To
determine availability from a particular Central Office, check with your
SBC SNET point
of contact.

With Enhanced Multipath (PRI), the customer has the additional features of Call by Call
selection and Incoming Calling Line Identification (ICLID). Both features are designed to
enhance the customer's business through operationa
l efficiencies.

ICLID enables customers to direct calls based on the calling number, or tie into a data
base offering additional information about the incoming call. Call by Call selection
enables each path to be used for incoming or outgoing services. C
onsequently, customers
are able to optimize the utilization of their trunks.

PRI provides 24 individually controlled software
defined channels for all voice and data
communications. Each channel has a 64 KBPS capacity, giving a total transmission
of up to 1.536 MBPS. With Primary Rate ISDN, the customer can predefine the
number of channels used for specific types of calls. They can easily change the number
of lines used for each application as their network traffic dictates.

Requirements for P
RI include:

Digital transport facility (DS1)

Trunks (DID, DOD, Combo, 2 way DID)

compatible equipment


With SBC SNET’s PRI a business will realize these benefits:

Clearer transmission

of voice communications

Faster connects and disconne
, which save time and money

Flexible trunk assignments

for incoming and outgoing calls.

Higher bandwidth, which enables
faster transmissions

Greater protection

from outages for the customer’s voice network

Bandwidth on Demand

Primary Rate ISDN is a di
aled service providing up to
speeds of 1.536 MBPS on demand, allowing for telecommuting, video and Internet
surfing at high speed.

Flexibility and Scalability

PRI is a switched digital service and allows 1.536 speed
on a dialed basis. Consolidated trunk
ing services, reduced call blockage, and less
cabling and interface equipment for significant cost savings.

Increased Efficiency
: Primary Rate ISDN allows customers to carry 20 to 30% more
traffic than analog trunks because of out of band signaling.

Fast C
all Setup

PRI call setup is .5 seconds compared to 3
5 seconds for analog

Incoming Call Detail

Primary Rate ISDN provides incoming call information with
Caller ID, included at no additional charge.

Disaster Recovery

Primary Rate ISDN offers a

number of optional features
including Alternate Route, Enhanced Alternate Route, and Inform 911 for
contingency planning.

Basic Features

These basic features are available with a PRI at no extra charge

Call by Call

A PRI configured as a Call
Call Tru
nk Group will allow several interworking features
to "ride" the pipe at the same time. Members of the Call
Call Trunk Group are
selected by the DPBX on a per
call basis.


The Individual Calling Line Identification (ICLID) feature is used to deliv
er the calling
party's directory number across the PRI "pipe" to the CPE on both intraswitch and
interswitched calls.

Dedicated Trunk Groups

A PRI configured as a dedicated Trunk Group will only have a single feature such as DID
or DOD. Each channel is ca
pable of handling incoming or outgoing Circuit Switched
Voice, Circuit Switched Data, or Packet Switched Data.

An entire trunk group is dedicated to a specific service. ( ie : DID,DOD or Combo
trunks )

A Dedicated Trunk Group can consist of one or more "
B" channels.

Additional Basic Features (no extra cost):


Circuit Switched Voice (CSV)

Circuit Switched Data (CSD) at 56Kbs, 64Kbs Restricted and 64Kbs CC

Data Only (56/64K CC)

Interexchange Carriers Selection

Switched Access to IEC Services (e.g
., ACCUNET, SDN, and MegaCOM 800)

Station ID (ICLID not ANI)

User Information (UUI) [User information is passed in the Initial Setup

Message on the D Channel]

Channel Availability Signaling Procedures (BCAS)


Two B
Channel Tran
sfer (TBCT)

5E13 Secured 214

Call Forward On Trunks (Alternate Routing to a Local Directory Number) Secured 273

Call (DID, DOD, SDN, 800 MegaCOM, etc.)

InterSwitch Calling Party Number/Billing Number (CPN/BN) Delivery

Electronic Tandem Ne
twork (ETN)

Feature Enhancements

These features are available on a PRI for an extra cost.

Circular Hunt

Circular Hunt is a switch feature that begins the search for an idle B channel at either the
last channel to have accepted

a call or the last channel to have attempted to place the call.
This allows all call attempts to move through the route list of available B channels within
the trunk groups in a circular fashion, better using your PRI facilities.

This plan differs from a

sequential hunt arrangement that always starts at the first B
channel in the first trunk group (i.e., an ascending sequence) and proceeds in order
towards the last B channel of the last trunk group. This current hunting algorithm does
not spread the traff
ic over available PRIs. A sequential setup always hits B channels in
the trunk groups at the beginning of the route list and may leave B Channels in the trunk
groups in the end of the list idle.

Circular Hunt dynamically points each new call attempt to the

next idle B channel
following the last channel engaged to evenly distribute traffic through all the B Channels.
This makes your traffic predictable so you can determine whether your traffic is flowing

Circular Hunt also reduces the effect of the

failure of a modem or a channel in your
hardware. A sequential hunt arrangement will continually try to complete a call on the
failed channel, and if this dead channel is is early in the search, then your traffic can be
severely affected. With Circular Hu
nt, one call would fail when it selects the dead
channel, but that specific trunk would not be selected again until all trunks in the route
list have undergone call attempts. Therefore, your overall traffic is not affected as greatly.

Calling Name Deliver

Allows ISDN PRI Calling Name Delivery with call control to send calling party name to
ISDN Class II equipment. The name provided on outgoing calls is the
main listed name

on the PRI account placing the call. This feature is available with National ISDN 2

only through Siemens and 5ESS switches.

Redirected Number

Provides the redirected number (i.e., the directory number to which the call was last
presented) to the equipment, as well as the calling number in cases where call forwarding
is invoked. If y
ou forward the call to another directory number as the call is being set up,
both the calling party number and the redirected number are delivered to the called party.
This feature is available with National ISDN 2 and only through Siemens and 5ESS

2 B Channel Transfer

Allows the ISDN PRI to connect two calls, transfer the calls together and then release
both parties from the ISDN PRI. This feature is available with National ISDN 2 and only
through Siemens and 5ESS switches


, Video and Data Applications

a PRI is suited to these applications:

Video Transmission

PBX Trunking

Demand LAN/WAN Interconnectivity

Data Connectivity or EDI

Imaging (e.g., telemedicine)

Private Line Backup/Augmentation


wide Telecommuting

nternet Backbone


The monthly cost of the PRI is dependent on:

The pricing “zone’ that the customer is located in (Check Rate Zone List for PRI)

If PRI is available from local Central Office

check PRI and Basic Multipath
Availability List) (if no
t, additional mileage will apply)

The length of the PRI contract (1,3, or 5 years) (A Request for Contract form must be
completed so that SBC SNET’s Contract Administration Group can issue a PRI

Need for feature enhancements (such as Calling Nam
e Delivery)