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Modem Switching Techniques


November 1, 2016
Published By : Pratik Kataria
Categorised in:

Switching Techniques

In large networks there might be multiple paths linking sender and receiver.
Information may be switched as it travels through various communication channels.
There are three typical switching techniques available for digital traffic.

  • Circuit Switching
  • Message Switching
  • Packet Switching

Circuit Switching

Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.
Telephone switching equipment establishes a path that connects the caller to the receiver by physical connection.
Once a connection is established, a dedicated path exists
between both ends until the connection is terminated.
Routing decisions must be made when the circuit is first established, but there are no decisions made after that time.



The communication channel (once established) is dedicated.


  • Possible long    wait    to    establish    a    connection, (10 seconds, more on long- distance or international calls) during which no data can be transmitted.
  • More expensive than any other switching techniques, because a dedicated path is required for each connection.
  • Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it.

Message Switching

With message switching there is no need to establish a dedicated path between two stations.
When a station sends a message, the destination address is appended to the message.
The message is then transmitted through the network, from node to node.
Each node receives the entire message, stores it on disk, and then transmits the message to the next node.
This type of network is called a store-and-forward network.


A message-switching node is typically a general-purpose computer.
The device needs sufficient secondary-storage capacity to store the incoming messages, which could be long.
A time delay is introduced using this type of scheme due to store- and- forward time, plus the time required to find the next node in the transmission path.


  • Channel efficiency can  be  greater  compared  to  circuit- switched  systems,  because  more  devices  are  sharing the channel.
  • Traffic congestion can be  reduced,  because  messages may be temporarily stored in route.
  • Message priorities can be established due to store-and- forward technique.
  • Message broadcasting can be achieved with the use of broadcast address appended in the message.


  • Message switching is not compatible with interactive applications.
  • Store-and-forward devices are expensive, because they must have large disks to hold potentially long messages.

Packet Switching

It tries to combine the advantages of message and circuit switching and to minimize the disadvantages of both.
There are two methods of packet switching: Datagram and virtual circuit.


In both packet switching methods, a message is broken into small parts, called packets.
Each packet is tagged with appropriate source and destination addresses.
Since packets have a strictly defined maximum length, they can be stored in main memory instead of disk, therefore access delay and cost are minimized.
Also the transmission speeds, between nodes, are optimized.
With current technology, packets are generally accepted onto the network on a first-come, first-served basis. If the network becomes overloaded, packets are delayed or discarded (“dropped”).

Analog signal from your phone is converted into a digital data stream.
That series of digital bits is then divided into relatively tiny
clusters of bits, called packets.
Each packet has at its beginning the digital address — a long number — to which it is being sent. The system blasts out all those tiny packets, as fast as it can, and they travel across the nation’s digital backbone systems to their destination: the telephone, or rather the telephone system, of the person you’re calling.
They do not necessarily travel together; they do not travel sequentially. They don’t even all travel via the same route. But eventually they arrive at the right point — that digital address added to the front of each string of digital data — and at their destination are reassembled into the correct order, then converted to analog form, so your friend can understand what you’re saying.

Packet Switching: Datagram

Datagram packet switching is similar to message switching in that each packet is a self-contained unit with complete addressing information attached.
This fact allows packets to take a variety of possible paths through the network.
So the packets, each with the same destination address, do not follow the same route, and they may arrive out of sequence at the exit point node (or the destination).
Reordering is done at the destination point based on the
sequence number of the packets.
It is possible for a packet to be destroyed if one of the nodes on its way is crashed momentarily. Thus all its queued packets may be lost.

Packet Switching: Virtual Circuit

In the virtual circuit approach, a preplanned route is established before any data packets are sent.
A logical connection is established when:

  • a sender send a call request packet to the receiver and the receiver send back an acknowledge packet call accepted packet to the sender if the receiver agrees on conversational parameters.

The conversational parameters can be maximum packet sizes, path to be taken, and other variables necessary to establish and maintain the conversation.
Virtual circuits imply acknowledgements, flow control, and error control, so virtual circuits are reliable.
That is, they have the capability to inform upper-protocol layers if a transmission problem occurs.

Packet Switching:Virtual Circuit

In virtual circuit, the route between stations does not mean that this is a dedicated path, as in circuit switching.
A packet is still buffered at each node and queued for output over a line.
The difference between virtual circuit and datagram approaches:

  • With virtual circuit, the node does not need to make a routing decision for each packet.
  • It is made only once for all packets using that virtual circuit.

VC guarantees that:

  • the packets sent; arrive in the order sent
  • with no duplicates or omissions
  • with no errors (with high probability) regardless of how they are implemented internally.

Advantages of packet switching

Packet switching is cost effective, because switching devices do not need massive amount of secondary storage.
Packet switching offers improved delay characteristics, because there are no long messages in the queue (maximum packet size is fixed).
Packet can be rerouted if there is any problem, such as, busy or disabled links.
The advantage of packet switching is that many network users can share the same channel at the same time. Packet switching can maximize link efficiency by making optimal use of link bandwidth.

Disadvantages of packet switching

Protocols for packet switching are typically more complex.
It can add some initial costs in implementation.
If packet is lost, sender needs to retransmit the data.
Another disadvantage is that packet-switched systems still can’t deliver the same quality as dedicated circuits in applications requiring very little delay – like voice conversations or moving images.

Hardware Components

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave.
An audio oscillator produces frequencies in the audio range, about 16 Hz to 20 kHz. An RF oscillator produces signals in the radio frequency (RF) range of about 100 kHz to 100 GHz.
Modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.
Amp is an electronic device that increases thepower of a signal.

Transceiver = Transmitter + Receiver



An unintelligent network device that sends one signal to all of the stations connected to it.
All computers/devices are competing for attention because it takes the data that comes into a port and sends it out all the other ports in the hub.
Traditionally, hubs are used for star topology networks, but they are often used with other configurations to make it easy to add and remove computers without bringing down the network.
Resides on Layer 1 of the OSI model


Split large networks into small segments, decreasing the number of users sharing the same network resources and bandwidth.
Understands when two devices want to talk to each other, and gives them a switched connection.
Helps prevent data collisions and reduces network congestion, increasing network performance.
Most home users get very little, if any, advantage from switches, even when sharing a broadband connection.
Resides on Layer 2 of the OSI model.


Connects two LANs and forwards or filters data packets between them.
Creates an extended network in which any two workstations on the linked LANs can share data.
Transparent to protocols and to higher level devices like routers.
Forward data depending on the Hardware (MAC) address, not the Network address (IP).
Resides on Layer 2 of the OSI model.


Used to boost the signal between two cable segments or wireless access points.
Can not connect different network architecture.
Does not simply amplify the signal, it regenerates the packets and retimes them.
Resides on Layer 1 of the OSI model.


A device that connects any number of LANs.
Uses standardized protocols to move packets efficiently to their destination.
More sophisticated than bridges, connecting networks of different types (for example, star and token ring)
Forwards data depending on the Network address (IP), not the Hardware (MAC) address.
Routers are the only one of these four devices that will allow you to share a single IP address among multiple network clients.
Resides on Layer 3 of the OSI model.

Access Points

An access point (AP) is a transceiver that connects to an Ethernet cable:

  • It bridges the wireless network with the wired network
    • Not all wireless networks connect to a wired network
  • Most companies have Wireless LANs (WLANs) that connect to their wired network topology
  • The AP is where channels are configured
  • An AP enables users to connect to a LAN using wireless technology
    • An AP is available only within a defined area

Configuring an Access Point

Configuring an AP varies depending on the hardware

  • Most devices allow access through any Web browser
  • Enter IP address on your Web browser and provide your user logon name and password

Wireless Router

A wireless router includes an access point, a router, and a switch.
A wireless router is a device that performs the functions of a router but also includes the functions of a wireless access point.
It is commonly used to provide access to the Internet or a computer network.
It does not require a wired link, as the connection is made wirelessly, via radio waves.
It can function in a wired LAN (local area network), in a wireless-only LAN (WLAN), or in a mixed wired/wireless network, depending on the manufacturer and model.