AT&T Telephone DS1/DMi/ISDN-PRI Uživatelský manuál stáhnout pdf (Strana 51)

INTRODUCTION

1-27

Administration software also allows services to be dedicated to specific channels by assigning a

channel to a particular trunk type. In this way, the switch always provides enough trunks for a

particular type of service. Channels administered for specific services are not available for ISDN-

dynamic uses.

With CBC Service Selection, calls requesting various types of services are routed, on a call-by-call

basis, over the same ISDN channels. To distinguish between various types of calls, the service type is

specified as a part of the message. Also included are BCC and NSF IEs.

The NSF IE identifies the feature or service provided by the network carrier (such as MEGACOM

service from AT&T). The called-party IE is used to specify routing digits such as the North

American dialing plan and the RNX.

The originating and terminating switches do channel negotiation to select a channel that is compatible

with both endpoints. Channel negotiation gives some control to the B-channel that is used for the

call. If the originating endpoint chooses a B-channel that is unacceptable to the terminating endpoint

(for example, planned use of the channel by the terminating endpoint), then the terminating endpoint

can request a change in the channel to be used for that call.

Although Q.931 recommendations allow for an asymmetrical design (that is, a user-to-network

protocol), most ISDN-PRI procedures on System 85 R2V4, Generic 1, and Generic 2 can be used

symmetrically. These communications systems can be administered as either the user side or as the

network side. When accessing a 4ESS switch, 5ESS

switch, or vendor-compatible toll-office or CO,

the customer-premises switch must have a user-side interface. For ISDN-PRI links between two

private network switches, one link must be administered for the user side and the other for the

network side.

Differences between the user side and network side are primarily related to resolving occurrences of

glare. Glare is a condition where both switches try to originate a call on the same channel

simultaneously. The network side always gains control of the channel and the user side backs down

(terminals are excused from certain protocol functions).

Calls from non-ISDN facilities (analog trunks and/or DMI-BOS trunks) may be connected to ISDN

facilities to provide end-to-end tandemed connections. The switch provides the required signal.

conversions through interworking routines. Depending on an extension's COS assignment and other

administration options, all conventional switch features and services may be used.

Depending on other administration options, message-associated user-to-user information (MAUUI)

or user-to-user information (UUI) may be transmitted from one user endpoint to the other. UUI

transfer includes the display of such things as calling number and calling party name.

Equipment manufacturers interpret the ISDN-PRI protocol in different ways. As a result, equipment

(and various equipment releases) use different approaches. Currently, one significant difference

affects codesets 6 and 7. System 85 R2V4 transfers UUI via codeset 7. System 85 R2V4 transfers

network specific information in codeset 7 according to the initial definition of ISDN. Generic 2

transfers network-specific information in codeset 6 leaving codeset 7 available for user-specific

information.

NOTE: Codesets 1 through 5 are reserved for future standards expansion.

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AT&T Telephone DS1/DMi/ISDN-PRI Uživatelský manuál Strana 51