For us to utilize “integrated services” including voice and non-voice communications and the use of some new media, such as facsimile in offices and home, the following features must be provided for user-network interfaces :
- Different services for each call
- A switching mode (packet switched/circuit switched function) can be selected.
- The data transmission speed can be selected.
- A plural number of terminals can be concurrently connected.
- The portability of terminals can be ensured.
2) Basic Structure of User-Network Interface.
The basic conditions for structuring the user-network interface that satisfy the preceding requirements can be summarized into the following three points :
- Multi services
- Common use of various services telephone/non-telephone and existing/new services. As shown in Fig.12, ISDN termianls, personal computers, FAX machines, etc. are connected to S/T points to offer various services.
- Multi points
- Up to eight (8) terminals can be connected to one (1) NT as well as point to point connection.
- Fig 14 shows the multi points connection.
- Terminals can be carried from place to place and connected to different sockets for use, just as home electrical appliances can be carried around and plugged into AC outlets.
3) Channel Classification
Various channels can be used to transmit information between a terminal and the switching system. These include B, D and H channels. Each channel has a different bit rate and information carrying attributes.
- The B-channel carries user information such as voice and packet data at a rate of 64 kbps. However, the B-channel does not carry signaling information.
- The D-channel interface carries mainly signaling information such as originating or terminating subscriber number, call origination and disconnect signals for circuit switching and packet switched user data at 16 kbps or 64 kbps.
- The D-channel also permits multiple logical channels to be established for use in packet communications.
- The H-channel carries high-speed user information such as high-speed facsimile, video, high-speed data, etc. H channels do not carry signaling information for circuit switching by the ISDN.
- Table 1 outlines these three channel types and characteristics.
Table 1: Channel Types and Characteristics
|Channel Type||Bit Rate||Function|
|B||64 kbps||· To carry user information
· Circuit switchingmode and packet switching mode
|· To carry signaling information for circuit switching|
|H||H0: 384 kbps
H11: 1536 kbps
H12: 1920 kbps
|· To carry high-speed packet data such as facsimile and video
· An H channel does not carry signaling information for circuit switching by the ISDN
|Note :||· H0 : 64K X 6 = 384 kbps
· H11 : 64K X 24 = 1536 kbps
· H12 : 64K X 30 = 1920 kbps
3) Typical Interface Structures
- Basic Interface
- This interface is primarily for home use.
- The basic interface is set at a transmission speed of 144 kbps. This provides two (2) 64 kbps B-channels for user information exchange and a 16 kbps D-channel for signaling and control. The interface is thus referred to as 2B+D.
Fig.15 shows the basic interface structure.
- Primary Group Interface
- These interfaces are primarily for business use. The primary group interface for the ATT system consists of a 1.544 Mbps line. This line can thus provide up to 23 B-channels at 64 kbps and a single D-channel at 64 kbps.
- In Europe and other countries using CEPT system standards, the primary group is 2.048 Mbps and the interface is 30B-channels and single 64 kbps D-channel. This line is used for PABX etc.
- Fig 16 shows the primary group interface structure.
- Table 2 shows the typical user network interface structure.