- Services.
- Digital voice, high-speed data (circuit or packet switched), fax,
slow-scan video.
- Videotex, teletex, terminals, telemetry (energy management, alarm and
emergency services).
- There are three ISDN channels.
- The bearer (or B) channel is a 64 kb/s pipe for user data.
- B channels can be circuit switched, packet switched, or
semipermanent.
- The data (or D) channel is a 16 or 64 kb/s pipe for user and network
data.
- The high-speed (or H) channel, is a 384, 1536, or 1920 kb/s pipe for
user data.
- All channels are full duplex.
- There are two main ISDN rates based on channel allocations.
- Basic access consists of 2 B channels and a D channel (2B + D) for
2*64 + 16 = 144 kb/s (bit tricks can up this to 192 kb/s).
- There can also be a B + D basic service.
- Primary access consists of 23 B channels and a D channel (23B + D) for
23*64 + 16 = 1.4 mb/s.
- More generally, primary access can be nB + D for n < 30.
-
- ISDN exploits digital communications in the PSTN, but has problems with
the analog local loop.
- 4 kHz bandwidth can be modulated to a 8-16 kb/s data rate.
- Compression multiplexing: data are sent in bursts in alternate
directions.
- Simultaneous transmission with echo cancellation.
- User access.
- User access is divided into reference points and functional groupings.
- Reference points are interfaces between functional groupings.
- A functional group delimits a set of related functions.
- Network termination 1 is the physical and electrical termination at
the user location.
- Network termination 2 is the distribution mechanism (PBX, or LAN).
- Terminal equipment type 1 are for ISDN-based equipment.
- Terminal equipment type 2 are for non-ISDN equipment.
- Terminal adaptors bridge non-ISDN equipment to the ISDN connection.
- The reference points are T for network termination, S for the system
interconnect, and R for rate adaptation.
- ISDN protocols.
- There are a huge number of protocols (and standards) associated with
ISDN.
- Physical layer: I.430, I.431.
- Data-link layer: LAP-D (D channel, I.441) and LAP-B (B channel X.25)
- Network layer: Call control (D channel, I.451), packet switching (D
and C channel, X.25);
- There is some friction between ISDN and the OSI Framework.
- D and B channel linked protocols.
- Multi-media calls with multiple linked information flows.
- Multi-point connections.
- The D channel.
- Uses the new LAP-D link-layer protocol.
- Supports control signaling, packet switching, and telemetry.
- Control signaling (I.451) does connection management.
- The B channel.
- B channels are used for circuit switching, semipermanent circuits,
and packet switching.
- Circuit switching and semipermanent circuits are just end-to-end bit
pipes.
- Packet switching is X.25.
- ISDN Connections.
- B channels support both packet and circuit switching.
- The D channel supports packet switching.
- Circuit switching involves both the B and D channels.
- The D channel does connection management; the B channels do data
transport.
- B channels provide layer 1 functions only; other layers are the
user's responsibility.
- The D channel is a three level protocol.
- Packet switching can be carried on either D channel or B channels.
- LAP-D D channel protocol.
- Provides acknowledged and unacknowledged information transfer.
- Unacknowledged information transfer is like UDP.
- Acknowledged information transfer is like X.25.
- Unacknowledged service uses unsequenced frames with no error or flow
control.
- Acknowledged service uses sequenced frames with error and flow
control.
- LAP-D frames have flags at both ends, a Service access point
identifier (6 bits), a command-response bit, a 7-bit terminal
identifier, control or information payloads, and a 2-byte FSC.
- LAP-D frames multiplex twice: between devices on the physical
interface and again within a device.
- There need to be two addresses: the service access point
identifier and the terminal identifier.
- Physical layer functions include
- Full-duplex transmission on either channel, as well as timing
information.
- Physical circuit activation and deactivation.
- Power and terminal management (identification and fault isolation).
- Rate adaptation.
- D-channel contention access (for multi-terminal endpoints).
- Signaling system 7 (ss7) provides internal control and network
intelligence to digital networks and ISDN.
- A complex set of specs (38 recommendations).
- SS7 architecture uses a common-channel signaling.
- Essentially a packet-switched overlay on a circuit switched network.
- However, it doesn't have to be implemented that way.
- SS7 has four layers:
- Signaling data link (physical layer).
- Signaling link (data-link layer); reliable sequenced delivery.
- Signaling network routes across the network.
- End-point determination.
- Further distribution (based on control-point functions, for
example).
- Out-link selection (routing).
- Also link-state management (congestion control).
- Signaling connection control part
- Augments the basic services with sequencing, connections, and flow
control.
- The first three layers are known as the message transfer part.
- It provides reliable datagrams.
- All four layers are known as the network service part.
- Control messages.
- Forward set-up messages specifies end-points and call quality
parameters.
- General set-up messages provide communication during call set-up.
- Backward set-up messages provide accounting control during call
set-up.
- Call supervision messages provide status and further control during
call set-up.
- Circuit supervision messages manage established circuits:
terminate, suspend, or block circuits.
- Circuit group supervision messages more of the same.
- In-call modification messages manage an active call (end-points,
parameters).
- End-to-end messages provide application-level signaling between
endpoints.
This page last modified on 14 November 2004.