Telegraph Network allowed transport of text messages over a long destination. It is also known as Connectionless services where no prior connection is established between source and destination addresses before the transmission of message. For message transmission, it uses Morse code to represent message as digital information and transmit it over communication lines as electrical pulses.
Telegraph system is also known as a store and forward system where first entire message which is received is decoded at every telegraph station and then forwarded to another station through encoding same. This can also be described as Message Switching.
Operation of the Network :
- Text message provided by user are initially encoded into sequence of dot and dash using ‘MORSE CODE’ by telegraph operator and then transmitted as long and short electrical pulses over transmission lines.
- Routing of message to destination station is made by operator based on destination address and this continued at all intermediate stations until destination station arrives.
- At destination station message is physically decoded by operator and forwarded to location.
- Major drawback of system was transmission lines were not utilized to its full potential, because rate of transmission of message was limited to rate at which operators typed sequence.
- Hence to improve it multiplexing was utilized, where message entered by multiple operators was multiplexed together and transmitted across single transmission line.
- Further Baudot systems were incorporated where single character was represented as 5-bit binary digits and with Baudot multiplexing with six operators, transmission rate of 120bps was achieved.
Elements of the Telephone Architecture :
- It made use of the digital transmission system such as Morse code and Baudot system for transmitting messages across network.
- Framing was used to indicating start and end of message.
- Hierarchical addressing was used for routing of message towards destination
- Multiplexing of digital information was applied to increase transmission rate.
Telephone Network :
Telephone network uses telephone devices (developed by Alexander Graham Bell in 1876, while working on telegraph) to transmit voice signals. Unlike telegraph system in telephone system real-time, two-way voice transmission was involved. It is a Connection-oriented service where initially connection is established between source and destination and then information is transmitted. It uses circuit switching for connecting users.
Operation of the Network :
In Earlier Days, telephone service for users was provided by directly connecting two end-users using separate dedicated lines between them and with an analog transmission system which converted sound into appropriate electrical signals and then transmitted across lines, this transmission involves some methods for amplifying signals to preserve quality.
As number of telephone users increased, number of transmission lines also increased, Hence telephone switches were introduced and service was redefined in following way.
After that telephone call service involves 3 phases :
- Setup Phase –
User who wants to call, sets up connection with central telephone office by picking up telephone.
- Information Transfer Phase –
After connection is set up with telephone office, user should specify destination telephone number which they desire to connect, after connection is set up by operator at office user exchanges information with other user.
- Connection Release –
When users are done, they hang up their telephones which generate a signal that call is complete. And line is made available for other users.
Traditionally connection setting up in office had human introversion where an operator used to make connections, later it got replaced by automated switches which would automatically make routing decisions.
Transmission Mechanisms Used :
During its initial days, analog systems were used for transmission of information which was replaced by digital transmission systems, where analog voice signal was converted into Binary stream & resultant digital signals were transmitted between two telephones.
Then for digital transmission ‘T1 carrier’ is used to carry voice signals which also increases efficiency, where it can transmit voice transmitted by 24 channels as one frame with 8-bit information in each channel along with multiplexing.
1 frame = 8 bits*24 channels = 192 bits 1 frame = 192 bits + 1 frame member = 193 bits 1 frame has 193 bits
Thus, one T1 carrier can transmit 8000 frames/sec
8000*193 = 1544 mb/sec is data rate achieved by standard T1 carrier. Hence in telephone networks, T1 carrier is a standard used for multiplexing and transmission.
Elements of the Telephone Network Architecture :
Telephone networks are connection-oriented, where it requires setting up of connection before transfer of information. It uses circuit switching to end to end connection and routing decision was made there itself.
- Addressing –
In Telephone number system each telephone had a unique telephone number which was provided using Hierarchical addressing.
For example the number 416-967-1111.
The first 3 digits (i.e., 416) specifies area code.
The next 3 digits specify switching facilities at central office.
The last 4 digits specify line that connects user to central office.
- User to network signaling –
For establishing connection between user and nearer telephone office was used where hearing of dial tone when user picks up telephone to make a call, signals central office and if connection is set up user hears dial tone. Else, it was assumed that telephone was dead.
- Network signaling system –
It helps in establishing a connection between the two telephones by signaling computers to set up a path.
Internet, unlike telephone and telegraph where transfer of information was in same network, provided facility for transferring of information across multiple dissimilar networks.
Earlier Semi Automatic Ground Environment (SAGE) and ARPANET made major efforts for developing network for transfer of information among ourselves and gradually due to boom computer era, internet had developed where communication between computers connected to different networks was enabled.
During initial days where computers were being used, they were costly. Hence a network named terminal oriented network allowed multiple terminals (users or input devices) to share single host computer by directly connecting through communication lines and made use of resources from host.
But terminals need to be adjacent to host computer. As modern devices for transmitting digital information were introduced through telephone network, then access to terminal located farther from host was possible. Gradually number of terminals also grew to provide individual lines to each terminal is changed(modified) by adopting methods such as.
- Medium access control –
Where all terminals shared a common communication line with host and polling mechanisms were used to get access to computer, input is directly transferred when a terminal was polled.
- Statistical multiplexers/Concentrator –
It also provided another means of sharing common communication lines by terminals with terminal connected to a multiplexer. Where input message from terminals is encapsulated into frame with header(terminal address)and all messages generated by terminals are buffered in multiplexer in an ordered queue and sent to host.
As computers became less costly, every individual had personal computer hence a computer-to-computer network was developed to enable exchange of information between computers.
Thus communication architecture evolved from a simple telegraph network to today’s advanced and most reliable computer to computer network architecture which enabled transfer of information between users in any part of world.
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