ISDN connection uses existing copper wires in the local loop and on-site wiring. Depending on the configuration, you will need several different devices to configure an ISDN connection, especially if you will be connecting non-ISDN devices (such as an analog telephone).
General information about ISDN network has been discussed previously. The following diagram shows ISDN components you need to be familiar with in order to understand ISDN connection.
NT1 (network terminator) in ISDN connection is the connection point between the local loop and ISDN network. A Cisco router might be classified as an NT1.
An NT2 (network switching equipment) connects with an NT1. It lets you connect multiple devices, and or split the signal into data and voice transmissions. NT2 in ISDN connection is optional.
ISDN Connection diagram
TA (terminal adapter) is any device that generates traffic on an ISDN connection line. More specifically, the term is generally used to describe a device that converts non-ISDN device signals to ISDN signals. The TA in ISDN connection is often called an ISDN modem, although it does not convert digital signals to analog signals. Rather, it converts ISDN signals to non-ISDN signals.
TE2 (Terminal endpoint device type 2) in ISDN connection is a non-ISDN compatible device such as a computer without an ISDN adapter.
TE1 (Terminal endpoint device type 1) in ISDN connection is an ISDN-compatible device such as a router, computer or ISDN telephone.
ISDN connection reference points
The ISDN standard defines several reference points, where different protocols and devices connect with each other. It identifies proper connections with the following interface designations.
1. R: Interface between a TA and non-ISDN equipment (TE2)
2. S: Interface between the NT2 and an ISDN device (TE1 or TA)
3. T: Interface between an NT2 and the NT1
4. U: Interface between the NT1 and the local loop (ISDN wall connection)
Because they are electrically the same, the S and T interfaces in ISDN connection are often identified as an S/T interface. When connecting devices on an ISDN connection, pay attention to the interface type (R,S,T, or U) of each device. Be sure to connect the correct interface.
Note: S and T interfaces are similar, so they are often lumped together and identified as an S/T interface.
For example, to make an ISDN connection from a router, you might have the following options (depending on the configuration of the router)
1. Connect a router with an S/T interface to either an NT1 or an NT2 (but not directly to the local loops)
2. Connect a router with a U interface directly to the local loop. Do not connect U interface to an NT1 or an NT2
3. Connect a router with a serial interface (i.e. an R reference point) to a TA. The TA can then connect to the NT1 or NT2
When making an ISDN connection – by connecting the ISDN equipments, pay attention to the reference point (interface type) of each device. Connecting devices to the wrong type of interface will disable communication and could damage the equipment.
ISDN is a network layer protocol that operates over a specific hardware interface configuration. For this reason, ISDN has it’s own network and data link addressing. ISDN uses the following addresses:
1. Service Protocol Identifier (SPID)
SPID in ISDN connection is a network layer address used to identify ISDN devices. Depending on the specific ISDN implementation, each device can have one or more SPIDs. The following are common SPID assignments:
- One SPID is assigned to the entire device
- Each B channel has its own SPID
- Each B channel can have more than one assigned SPID
ISDN Connection SPID
Your WAN service provider will give you the SPIDs to use to configure the router.
2. Terminal Endpoint Identifier (TEI)
The terminal endpoint identifier (TEI) in ISDN connection is a data link layer address used with ISDN. TEIs are assigned to the router by the ISDN switch when the connection is made. A TEI is similar to an Ethernet MAC address except that they are dynamically assigned when the router connects to the ISDN switch. Each ISDN device is assigned one TEI.
ISDN Connection - TEI
The following is a summary how ISDN connection make a communication process by initializing the router.
1. The router uses the D channel to perform Data Link (layer 2) initialization. TEIs are dynamically assigned to identify the router.
2. The router uses the D channel to perform Network (layer 3) initialization. It uses its preconfigured SPIDs (if required) to set up the B channels
When another ISDN connection would like to communicate with the router, the following process is performed.
1. The sending device requests a connection through the D channel
2. The receiving device answer and the link is established.
3. The B channel is used to transmit the data. The D channel is used for session maintenance
4. After the transmission is completed, the D channel is used to drop the link.
After having familiar with the ISDN connection technically, the ISDN network is ready for configuration.
See also:
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