Computer Networking

Transport Layer

Communication Devices

Communication devices are hardware’s which are used in both ends (sender and receiver) to transmit data.

  • The Communication device at sending end, sends the data through communication medium.
  •  The Communication device at receiving end, receives the data through communication medium.

Communication devices may convert the analog data to digital data or digital data to analog data according to requirement.

Types of Communication devices are explain below

1. Dial up modem

  • Modem is a communication device that sends and receives data from one computer to another on the internet through telephone lines.
  • Connection is establish by dialing a number from phone.
  • Sending and receiving computers both must have dialup modes.
  • Computer sends the digital data, Modem converts it into analog signals to transmit it over the telephone lines, its called modulation.
  • Modem at receiving ends converts the analog signals back into digital signals to transmit it to receiving computer which is known as demodulation.
  • Dial-up connection must be re-established each time the modem is use.
  • Data transfer speed is much slower up to 56 Kbps. Dial up modem is not use now days.
  • It is replace with Digital modems.

Computer 1 -> modem-> antenna -> antenna -> To ISP provider -> internet -> from ISP -> antenna -> antenna -> modem -> Computer 2

2. Digital modem

Digital modem also called broadband modem. Types of digital modems are

  • ISDN modem
  • DSL modem
  • Cable modem

3. Wireless Modem

  • Wireless modem transfers data through air instead of cables.
  • Wireless modem also known as radio frequency modem.
  • It is use in notebook, desktop computers, smart phones, handled computers and other devices to connect to internet.

4. Network interface card (NIC)

  • NIC is use to connect a computer to internet.
  • It is circuit board install on the motherboard.
  • Now days, most computer system have built in NIC on motherboard.
  • Each computer in the network must have at least one NIC.
  • NIC supports wired and wireless networks

Note: TP-link is alternative to NIC.

5. Wireless Access Point

6. Repeaters and Amplifiers

  • Repeater is a device which is use to regenerate the original signals to transmit it over the large distance.
  • Repeaters are available both for wired and wireless networks.
  • Digital and analog both repeaters are available.
  • Antenna can also use to increase the range of Wi-Fi network. The strength of antenna is measure in decibels (dB)
  • Repeaters works on physical layers that why it cannot recognize MAC and IP address.
  • Note: if any device is use in data link layer then it will use MAC address and those devices which are use in network layer will use IP address.


  • Repeaters have two ports.
  • It forward data and can’t send it back.
  • Repeater cannot filter the data. It means it sends data as received.
  • Collision is maximum. It up to n number of devices.

Repeater vs Amplifier

Repeater regenerates the signal so that the noise can be reduced or eliminated. Amplifier increases both the amplitude and the noise of the signal.

Let explain with example,

If there is signal X which is down to half (X/2) after some distance then amplifier may increase it up to 2X, 3X.. up to NX. But the repeater is use to regenerate the original signal as X.

7. HUB

  • Hub is a central device which contains multiple ports to connect multiple computers to a network.
  • HUBs are non-intelligent because when sender send data to hub then hub automatically passed it onto every device/computer on the network through ports.
  • It is use in physical layer like repeaters.
  • One end of cable is connected with HUB and other is connected with computer.
  • HUB can handle only one way information traffic at a time.


  • Hub has many ports to connect different devices. That’s why it is called multiport repeater.
  • It forward data and can’t send it back.
  • Repeater cannot filter the data. It means it sends data as received.
  • Collision is maximum. It contains up to n number of devices.

8. Switch

  • Switch is more intelligent device than HUB.
  • It receives the message from any device connected to the network and transmits this message to only the device, for which the message was send by sender.
  • Switch works at data link layer and use the mac address (physical address) of each device connected to port of switch.
  • It can also handle multiple connections channels at the same time
  • “we cannot access internet by using only hub or switch”

 9. Router

A router is communication device which connects multiple networks using similar or different protocols. It works on network layer

  • Hub and switch are used to share the data within the network. Both are not able to access the data outside the network.
  • To access outside the network, a device needs to be able to read I.P addresses.
  • Every network has an IP address and Routers reads the IP address of destination device and send data to that device.
  • It works at three layers physical, data link layer and network layer.
  • It checks the MAC address at data link layer and IP address at network layer.
  • IP address is use to access the internet.


  • If the Router can’t understand that where the message is to transfer then it broadcast this message to every device connected in the network.
  • It can filter the message.

It use the routing table and some protocols (RIP protocol, distance vector routing protocol) to transmit the message.

Router uses its own buffer so collision is minimum

For example there are three networks A, B and C. when a sends data to C then Router reads the message-data along with destination IP-address and then transmit data to C network.

 10. Gateway

  • Gateway is a device that connects two or more networks with their different types of protocols. . i.e. one network use Token Ring protocol and other network use Ethernet protocol.
  • It is intelligent device
  • It converts data according to protocols
  • Connection of personal computer to mainframe computer is example of gateway

11. Bridge

Bridges are used in data link layer but cannot used in network layer.

A network bridge can be used in computer networks to interconnect two LANS and interconnect separated network segments.

Bridge cannot send the message if the sender and receiver both are present in the same segment. 

12. Multiplexers and concentrators


Multiplexing is a technique which is used to combine different inputs and send the multiple data streams over a single medium. It is done by hardware called MUX.

DEMUX is a device at receiving ends which separates a signal into its component signals (one input and n outputs)


Concentrator combines the multiple messages and then sends them through a single transmission medium. It sends in such way that all the individual messages are active simultaneously instead of sending as a single combined message.

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