Category Archives: Legacy telecom

Practices , Network , Architectures and terminology jargon used in Legacy networks

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 .

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.

Services part 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.

Evolution of voice Communication

The telecom landscape has evolved, as far as infrastructure, services and contents are concerned. Industry  is  witnessing a  migration from  Legacy to  NGN.  Next Generation  Network  (NGN)  is  being implemented globally as a means to change the cost base, agility and service capabilities of telecoms providers. The evolved architecture for the transition is one that provides flexibility to service providers by enabling them to deploy new services on IP based technologies, while leveraging existing services and infrastructure as long as it makes sense.

This post describes the evolution of voice communication in access ,transport and  session layers respectively.

ip transformation in access layer

ip transformation in access layer

ip transformation in transport layer

ip transformation in transport layer

ip transformation in session layer

ip transformation in session layer

2nd and 3rd generation of telecommunication

Although the history of telecom evolution begins with PSTN and switches we shall oit them as they are truly legacy now .  We have seen the evolution of second to third generation of telecom most recently .  Where 2 G is referred to as the GSM era  , 2.5 G as the GPRS with GSM era . The following two diagram denote the service operators architecture nodes in both these times .

Note that in pure 2G there was only circuit switched communication services .


The advent 2.5 G bought packet switching for data access along with existing circuit switching for voice network .


Note that the processes such as billing etc had begun merging for both the circuit switched and packet switched networks .

However as the mobile became smarted and hungry for faster internet , it bbecame necessary to bring in faster speed and hence was born 3G. . Now 3G was further succeeded by 3.5G ( HSPA – High Speed Downlink Packet Access ) eventually 4G ( LTE Long Term Evolution ) as we can see now but that is another story .

WebRTC web service to PSTN – diagram

webrtc to pstn callflow

webrtc to pstn callflow

Public Switched Telephone Network

What is a Circuit Switched Telephone call ?

circuit switched – During the set up portion of a telephone call, a circuit is created, which is then torn down when the call is completed.

Originally, the entire telephone network was analog, but it is now mostly digital. The digital parts include the switches and backbone lines between them (called trunk lines).The connection between the customer premises equipment and the first telephone switch, called the local loop, is still analog.