ISDN: Integrated Services Digital Network

Earlier telephone lines were analog (POTS) but with the invention of digital technology, came the existence of ISDN. This is a circuit –switched technology where voice and data can be transmitted over the existing phone lines.

ISDN was a new technology and had better speed than the dial-up connections, thus it started to be used as backup line for the dedicated leased lines. ISDN standards have been defined by the ITU-T,

ISDN is the outcome of the earlier IDN technology created by the telephone companies. ISDN is not just one protocol but a set of protocols.

Using ISDN voice and Data can be transmitted simultaneously.

-         Digital Signal
-         Uses minimum speed for 128Kbps
-         The Call Establishment is faster
-         It gets terminated to the ISDN Switch at the exchange.
-         Multiple services can be used at a time.

ISDN is charged by ISPs based on the call duration and the number of calls, thus ISDN is only used occasionally when any problems with the other connectivity (Leased lines).

The Data and Voice are carried by the B channel (Bearer channel) which uses the bandwidth of 64kbps (bits per second)

For Data the channel is the D channel for which the signaling is handled at 16kbps or 64kbps

Types of ISDN

-         BRI (Basic Rate Interface):  This contains 2 B + 1 D Channels. Each B channel is of 64 kbps and the D channel is of 16 kbps. The B channel is used for data transmission whereas the D channel is used for signaling.

Considering 2B +1D channel, (2*64) + (1* 16) = 144kbps

B Channel:  Bearer
D Channel: Delta

-         PRI (Primary Rate Interface):  PRI is intended for higher usage users. Here the B and D channels are both of 64kbps. Under the PRI we have two standards.

a)     European Standard ( E1) : consists of 30B and 1 D channel, which makes it a total of : (30* 64) + (1*64) = 2.048 Mbps

b)    US Standard (T1): consists of 23 B and 1 D channel, which makes it a total of : (23*64) + ( 1*64) = 1.544 Mbps

ISDN Components

  • Terminal Equipment (TE)
  • Terminal Adapter (TA)
  • Network Termination 1 (NT1)
  • Network Termination 2 (NT2)
  • Exchange Termination (ET) 

Terminal Equipment (TE): This is any device or equipment that is as per ISDN standards and is used for communicating. Some examples could be: ISDN Data terminals, digital phones, computers, Fax machines etc.

Generally or in most cases the TE would provide a full BRI access i.e. 2B + 1D but it may also use only 1 B or only D channel at times.

TE1: Is Type 1 equipment, which would designate a device with native ISDN interface.

TE2:  Is a Type 2 equipment or device that would require a terminal Adapter (TA) for ISDN signals as it does not have a native ISDN interface.

Terminal Adapter (TA): It’s an interface conversion device which is used for communications between devices that are not as per ISDN standards and devices that conforms to ISDN standards. The TE2 device would be using this co convert EIA/TIA-232, V.35 and other signals to be converted to BRI signals.

Network Termination (NT1 and NT2): These devices form the logical and physical boundary between the provider network and customer premises.

NT1: This device converts BRI signals to a form so it can be used by the ISDN Digital line. The logical interface functions of switching and local signaling is performed by NT1.  It acts as a boundary between the carrier’s ISDN network and the Customer premises.

NT2:  this device is used to perform the physical interface conversion between different customer and network sides of the interface. It would aggregate and then switch all the ISDN lines at customer location.  This device would usually be placed at the corporate PBX (Private Branched Exchange).

Exchange Termination (ET): This is the part which forms a boundary (logical and physical) between the carrier’s switching office and the local loop. It is similar to the NT but performs its functions at the end office.

The ET also will be used for separating the B channels and have them placed on proper interface trunks to their ultimate destinations. It also would terminate the signaling path of the D channel at the customer end. The required end-to-end signals from the ISDN D channel signaling protocol are converted to switch-to- switch trunk signaling protocol of the carrier.

ISDN Interfaces – Reference Points

In the customer’s connection to the network ISDN specifies four interfaces which are: R, S, T and U.

The interfaces are not ‘real’ physical interfaces but are standard reference points and physical interfaces are necessary. The physical interfaces would use the name of these reference points.

The R Interface:  is the point between non ISDN devices i.e. TE 2 (Terminal Equipment 2) and a Terminal Adapter (TA) that provides to and from translation for such device.  This can be either physical or logical.

The S Interface:  is the point between the ISDN device TE1 or the TA and the NT2 (Network Termination Type 2) device. This can be either physical or logical.

The T Interface: is the point between NT1 and NT2 (NT: Network Termination). As in most implementations, NT1 and NT2 would exist in the same physical device, thus there is no real T interface.

The U Interface: is the point between the NT and local transmission loop of ISDN carrier. This also acts as the legal demarcation between the carrier’s loop and customer premises.

ISDN functions on the three lower layers of the OSI model i.e. Physical, Data-Link and Network Layers. For ISDN connectivity to function properly all the 3 layers need to be operations.

Layer 1: Being the physical layer, deals with the physical connectivity between the router and the ISDN circuit.

Layer 2:  At the data link layer the protocol Q.921 is used on the D channel and the B channel encapsulation (HDLC and PPP). The signaling between the router ISDN interface and the ISDN switch is handled by Q.921.

Layer 3: This is the network layer where protocols where protocols Q.931 protocol is included over the D channel and over the B Channel it uses the network layer protocols (IP, IPX, etc). The call setup between calling and called parties is handled by the Q.931 protocol.

ISDN Protocols

The exchange of data on the D channels is governed by using ISDN signalling protocols.

The protocols that make up the layered stack are:

-         Link Access Protocol for D Channel, LAPD which is also known as Q.921 which provides services to the Layer 2 i.e. Data-link. LAPD is similar to the HDLC and SDLC which is simple and bit-oriented data-link protocol.

-         Q.931 protocols that is used for service used on the higher layers. The Q.931 signaling protocol is feature rich communications protocol which is considered to be one of the most complex one.

LAPD (Q.921)

The LAPD protocol operates over the D channels and between TE and NT.  The unit of transmission is FRAME. As in other protocols the frames are demarcated

The LAPD frame will contain the below fields:

  • INFORMATION (only in frames carrying higher-layer data)


The ISDN D-Channel Signalling Protocol. The goal of integrated services over other services is to make Q.931. Between the end user‘s TE and ISDN’s career office Q.931main job is to carry signalling information about the type of ISDN service required for calls or the data sessions.

Configuring ISDN: BRI

We must first configure the switch-type for ISDN

Router_A(config)# isdn switch-type basic-ni 

Now configure the BRI interface for encapsulation ppp. Also assign the IP address to the interface and then configure the isdn spids based on the B channels to be configured, this is assigned by ISP provider..

Router_A(config)#int bri0/0
Router_A (config-if)#encapsulation ppp
Router_A (config-if)#ip address
Router_A (config-if)# isdn spid1 077721300100 7773123
Router_A (config-if)# isdn spid2 012312230100 7773124
Router_A (config-if)#dialer string 4441111
Router_A (config-if)# no shut

The phone number of the remote router is specified using the command dialer string.  While activating the ISDN connection this number is dialed.

Testing ISDN 

Use the below command to initiate a test call on an ISDN BRI interface:

Router_A# isdn test call interface bri0/0 4441111
And when a call needs to be disconnected:

Router_A# isdn test disconnect interface bri0/0 all
Troubleshooting ISDN

show isdn active : checks if call is active and number it dialed to show isdn status: would output the information of status between the router and ISDN switch. This command will be very useful to check connectivity issues.

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