ISDN Tutorials

1.       Overview

ISDN is a digital communication networks providing transmission rates in multiples of 64 kbits/sec. Typically described as 2B+D, Basic Rate Interface(BRI) logically used 2 64kbit/s data channels (B channels) and one 16kbit/s signaling channel (D channel). ISDN has yet to become a single global standard. Individual countries however, have their own flavors of ISDN all of which are based to a varying degree around the ITUs ISDN standard.


2.       The D Channel

An ISDN conversation is controlled via the D channel, which supports a three layer protocol. The three layers are as follows:
Layer 1             Physical  Clock/Synchronization
Layer 2             Basic protocol setup, Handshaking
Layer 2             Signaling       Initiation, Progress (all call handling)
Basically the D Channel controls the call. It is possible to analyze the D channel with an ISDN monitor which will provide a decoded display of the three layers. It is also possible to send X.25 packet data via the D channel

2.1. Layer 1

Layer 1 is a physical layer and ensures that a connection exists between the network and the ISDN terminal (TE). Obviously as only 2 wires exists over which  to transfer data the channel can not be physically separate. Instead the channels are separated by time. Layer 1 uses frames to transport layer 2 and 3 information. A typical layer 1 frame might look like:

The Framing, Echo and Balancing information are used to help control the data on the line and help in the control of Framing, Collision Detection and to avoid DC levels on the line, respectively.

2.2.         Layer 2

Layer 2 is used to control the handshaking between the network and the TE. Establishing a layer 2 connection requires that a TE has what is known as a tei (terminal equipment identifier). This is assigned by the network and will be unique to that TE and is used by the network to identify the TE on layer 2 at all times.

Whilst the network is trying to assign a tei, communication with a TE still needs to be uniquely identify in some way. This is achieved by using a Ri or reference indicator. Once a tei is assigned the Ri is no longer used.

Once a tei has established the TE sends out a SABME (set asynchronous balanced mode extended) this requests that the network be set for full duplex communication. The network response is a UA (user acknowledge) indicating it accepts theSABME.

From this point forward the layer 2 information is generally just an I or a RR. I is indicating that the TE or NT is sending an information frame (it is this frame that is the layer 3 information). RR (receiver ready) indicates that the NT or TE is ready to receive information.

The TE and Network continue to handshake during an ISDN call and while the line is idle to ensure both parties are present and available.

A disconnection is issued at layer 2 as well in the form of DISC to which the receiving responds to UA. If no layer 3 information is received after 10 seconds then the network will automatically issue a DISC.

2.3.         Layer 3

Layer 3 consists of:
1.       Messages.
2.       Information elements.

An information element must always be tagged to a message. Layer 3 message are used to control the content and progress of a call.
A typical Layer 3 conversation may look like that illustrated in figure 1 overleaf.

3.       The B Channel

The transmission of data over ISDN is achieved via multiples of 64kbit/s B channels. The protocol used to achieved successful digital communication within the B channels (inband signaling) is up to the ISDN end used. TANDBERG used the ITU-T, H.320 standard. Whatever the inband signaling, the bearer capability requested by a system when setting up a video call will be Unrestricted Digital Information (UDI). Requesting UDI indicates to network that it should provide a full 64kbit/s bandwidth per channel and must not apply any compression/decompression algorithm to the transmitted information.

Speech is typically compressed before sending long distance, multiplexed with other speech channels and de-multiplexed and decompressed at the far end. To the human ear the effect is inaudible but for a network provider, this technique allows far more data to be transmitted using the same bandwidth. Such a technique is not suitable for the transmission of data, hence the request for UDI.

4.        The Network & Connectivity

Diagrammatically the network can be illustrated as Figure 2. The ISDN network is represented by cloud from which individual lines (2 wire copper), via interface hardware, are run.
The 2 wire from the network are connected at the user site to a network terminating device known as an NT1. The connection between the network and the NT1 is known as the U Interface. Physically the U Interface is 2 wire copper and is known as ‘local loop’. Essentially it is the same as that used for analogue phones. The maximum length of a line for a U Interface is approximately 11km.
The NT1 links to the user’s equipment (TE) via 8 wires, 2 Tx and 2 Rx as a balanced pair, 2 power supplies (for low current devices) and 2 Grounds. Up to 8 TEs may be connected to a single NT1 i.e. each NT1 can support up to 8 separate MSNs with a TE on each. The connection between the NT1 and the TEs is known as the S/T Interface. The cable between a TE and the NT1 is a flat cable i.e. there is no crossover of Rx/Tx pairs.
It is also possible to obtain a 2MB link to the ISDN network. In this case the interface at the user site is usually made via an Inverse Multiplexer (IMUX) or a PBX. The connection between the ISDN network and the IMUX is known as a Primary Rate Interface.
Although referred to as 30B+D, a 2MB stream supports 32, 64kbit/s channels comprised of:
          30 B channels
          1 D channel
          1 Sync. Channel
Each connected to the network at the user end will have a unique MSN. This is a number allocated by the network provider to the user, which may be used to identify at least one TE.
In addition to an MSN it is possible to set a SUB. This is a sub-address and is sent as a suffix to an MSN. A SUB can be up to 4 digits long and enables separate TEs Sharing the same MSN to be addressed individually.
Although all the 64kbits/s channels required may be provided by the same network for a single conversation there is no guarantee that they will travel simultaneously. This can result in significant delays between the separate channels. Usually these delays will never exceed 1 second.

5.       ISDN Cabling

Systems connect to the S/T interface using an 8 core straight through cable. Such a cable has the following properties:
·    Each end is terminated with an RJ-45 jack.
·    Pins 1,2,7 & 8 are used by ISDN to provide a power source for low power consumption ISDN TEs such as ISDN telephones
·    Pins 3,4,5 & 6 must be connected as they carry the Tx, Rx balanced pairs required for data communication.
The ISDN standard determines that a 100Ω resistive load be used to terminate both ends of S/T interface, this is done to prevent reflections from the ends of the cable. Such a load is usually present in an NT1 and is often found in ISDN wall sockets.

5.1.         How to connect the video system to the ISDN

Here are some examples on how the videoconference system can be connected to the ISDN line through the NT1.

·    NT1 and videoconference system in the same room. If the video system is the only equipment that is connected to the NT1 and the distance between the NT1 and the video system is less than 10 meters then the ISDN cable from the video system can be connected directly into NT1. The NT1 then needs to be terminated with a 50Ω resistive load.

·   ISDN S0-bus use. If there is need for more than one ISDN wall socket on the particular ISDN line or if the distance between the video system and the NT1 is up to 100 meters then the NT1 must have a S0-bus. The 100-meter distance on the S0-bus is dependent on the quality on the cable used. It is possible to have up to 8 ISDN wall sockets connected to that particular NT1. The S0-bus must be terminated in both ends with a 100Ω resistive load.

Please note that each Tx pair and Rx pair must be terminated with a resistive load according to the above.



No comments:

Post a Comment