ISDN; Frame Relay; ATM

In WAN as in most areas of business the desire to maximise the impact of any investment in technology is a core issue. Careful analysis of data traffic as well as business objectives will help to successfully match business applications with the networking services.

Some Business Considerations for choosing systems:

(The greater the bandwidth indicates faster or greater data transfer capability)

 

 

ISDN (Integrated Services Digital Network)

 

 

 

ISDN is deliverable on two service levels:

Basic Rate Interface (BRI) – also called 2B + D

Primary Rate Interface (PRI) – also called 23B +D

The PRI can have 23 x 64 Kbps B Channels and a 64 Kbps D Channel. These are the minimum rates but they can be greater. This gives a bandwidth of 1.544 Mbps. With additional overheads businesses could consider mapping this onto the T-1 circuit because PRI has the required bandwidth to make use of T-1 transmission rates.

[The T-1 circuit is a point-to-point transmission technology with high-speed digital lines that use two wire pairs (one to send and one to receive). Other countries use a similar service called E-1 (European Digital Signal 1) – signalling rate of 2.048Mbps.]

Note: One of the above rates does not have to be used exactly. You can increment your ISDN capability rather than jumping straight up to PRI from basic rate interface. Multi-rate ISDN uses the inverse multiplexing technique which is a collection of 64 Kbps B-Channels dialled up together into a single logical channel to meet the application needs e.g. videoconferencing can be used to attain variable amounts of bandwidth.

 

ISDN has been criticised for only having isolated deployment of the infrastructure in many regions but the coverage situation has improved over the past 3 to 4 years with more its widespread use.

Considerations and Hindrances:

Distance: Signalling requirements can limit ISDN installations, as they must be within 3.4 miles of an ISDN switch.

External AC Power Supply: Unlike today’s analog phone lines premises with ISDN require an External AC Power Supply because it is digital. There is some concern of losing the ISDN-based phone service during power failures whereas today’s analog phone lines would not be affected.

Charges:

Incompatibility: ISDN switches have had slightly different specifications but elimination of compatibilities is taking place with interoperability specifications developed through ‘National ISDN-1’. This defines a national standard for ISDN switches and inter-switch communication.

ISDN Ordering and Implementation: To properly link end user’s equipment such as remote access servers must be programmed with service profile identifier numbers to properly identify the carrier’s equipment must link (must interface).

Uses of ISDN

  1. Single user-to-office
  2. Single user Internet connectivity
  3. Office-to-office connectivity

Applications for Single User

Applications Office-to-Office

ISDN Terminal Adaptor

It is the equivalent of an ISDN modem and puts voice or data onto the ISDN. It allows analog devices such as phones and fax machines to interface to all digital ISDN. Software and drivers must be compatible with installed network operating systems as well as with the purchased ISDN terminal adapter.

Network Terminator Unit –1 is required to physically connect the ISDN line to the user’s ISDN equipment.

Broadband ISDN (B-ISDN):

B-ISDN delivers voice, data and image from the joint network architecture of ATM and SONET. SONET is the optical transmission interface and mechanism that will deliver B-ISDN services. B-ISDN should be the service

Frame Relay

Technology

Advantages

Multiple PVCs can exist within frame relay clouds and provide the advantage over circuit switched options that the multiple PVCs can be supported from only one access line

It was introduced to take advantage of the decreased bit error rate on today’s transmission systems. By making this assumption point-to-point error correction and flow control have been eliminated from within the network itself. Instead the end nodes look at any problems encountered along the way. Thus data is transferred faster.

Errors are still detected but the frame relay switches do not try to correct the bad frames but instead discard them. The correction and request for retransmission of bad frames is left to the end node devices of modems and computers.

Disadvantages

There is no way for end nodes to either monitor or manage flow control within the frame relay network itself. Only end nodes can manage flow control between themselves and whatever frame relay network access device they are linked to.

 

Business

Advantage

Unlike the uniform cells of ATM, Frame Relays transmit frames or packets of variable length with a maximum of 8,000 characters. The potentially large frames also create high throughput and low delay.

Disadvantage

As there is no defined length to a frame there is no guarantee as to how quickly a frame can be forwarded. This is only an issue with time-sensitive information such as digitised voice and video. For this reason if a business makes frequent use of either of these media, ATM is recommended instead. As a result of this drawback of variable length packets, frame relay is often described as a data-only service. Data frames need a lot slower end to end connectivity.

 

ATM (Asynchronous Transfer Mode)

Technology

Business