Legacy Telecom Networks


I use the term legacy telecom system many a times , but have not really described what a legacy system actually is . In my conferences too I am asked to just exactly define a legacy system . Often my clients are surprised to hear what they have in current operation is actually fitted in our own version of definition of ” Legacy system “. This write up is an attempt to describe the legacy landscape . It also describes its characteristics, elements and transformation.

Source : Unknonw. Figure describes an IP based communication system catering to various endpoints and networks

Characteristics of Legacy Systems

1. Analog Signals

1G , introduced in 1980s , used analog signals as compared to digital in 2G onward. In 1G voice was modulated to higher frequency and then converted to digital while communication with radio towers .

2.Legacy system have ATM / Frame Relay transmission .

This is basically Hardware Specific and results in High Expenses.

3. Legacy systems have POTS / PSTN / ISDN as their access layer technology .

Access layer is the first layer of telecom architecture which is responsible for interacting directly with the end use / subscriber . Legacy system technologies are again Hardware  Specific , bear High Expenses and offer Low stability.

Physical transmission media include :

  • Twisted wire (modems)
  • Coaxial cable
  • Fiber optics and optical networks – Dense wavelength division multiplexing (DWDM)

4. Legacy system use Traditional Switches / ISDN in their Core Layer

Core layer is the main control hub of the entire telecom architecture . Using old fashioned switches render high CAPEX ( capital Expenditure ) and OPEX ( Operational Expenses ) .

5. In the service delivery front legacy system employ Traditional IN switches

These are very Hardware Centric.

PSTN (Public Switched Network)

MAN (Metropolitan area networks) connected ~30 miles of LAN (local networks) with a BN (backbone network). WAN(Wide Area Network) connected these MANs for longer distances ~100 miles. The telco were carriers with these larger internetworks and connections such as At&T or BellSouth. This led to development of PSTN.

PSTN were Circuit – Switching telephone network which were designed to carry analog data. Connnecting over PSTN required a modem convertor which converted analog to digital at 28/56 Kbps. During the set up portion of a telephone call, a special circuit is created, which is then torn down when the call is completed.

TDM (Time Division Multiplexing)

TDM was designed to provide continuous, compact and orderly data transfer for voice and video. It does so using a constant bandwidth over a secure connection.

ISDN (Integrated Service Digital Network)

ISDN can provide end-to-end digital communications for Fax, Data, audio, image and video transmission. It is a switched digital telecommunication line that can be delivered over regular copper wires.
Speed for a single ISDN line is 128 Kbps. Multiple ISDN lines can be combined together to achieve higher communication speeds. It was mostly used to provide remote access to LANs.

Digital lines with ISDN

  • Basic Rate Interface or BRI are used for home or SOHO connection
    • 2B + D channel
  • Primary Rate Interface or PRI is used for large businesses
    • 23 B channels and 1 D channel

B channels are known as bearer channels and is used to carry information
D channel is known as the Delta channel used for signalling purposes

Services of Legacy Telecom Networks

a) Virtual Private Network (VPN)

An Intelligent Network (IN) service, which offers the functions of a private telephone network. The basic idea behind this service is that business customers are offered the benefits of a (physical) private network, but spared from owning and maintaining it

b) Access Screening(ASC)

 An IN service, which gives the operators the possibility to screen (allow/barring) the incoming traffic and decide the call routing, especially when the subscribers choose an alternate route/carrier/access network (also called Equal Access) for long distance calls on a call by call basis or pre-selected.

c) Number Portability(NP)

An IN service allows subscribers to retain their subscriber number while changing their service provider, location, equipment or type of subscribed telephony service. Both geographic numbers and non-geographic numbers are supported by the NP service.


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