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.
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.
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