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What is the difference between hubs, switch, bridges and routers? How are they implemented?

Hubs

A hub connects individual devices on an Ethernet network so that they can communicate with one another. The hub operates by gathering the signals from individual network devices, optionally amplifying the signals, and then sending them onto all other connected devices. You should use a hub or a switch on your Ethernet network if the network includes more than two clients, servers, or peripherals.

Switches

Like a hub, a switch is a device that connects individual devices on an Ethernet network so that they can communicate with one another. But a switch also has an additional capability; it momentarily connects the sending and receiving devices so that they can use the entire bandwidth of the network without interference. If you use switches properly, they can improve the performance of your network by reducing network interference.

Switches have two benefits:

1. they provide each pair of communicating devices with a fast connection; and
2. they segregate the communication so that it does not enter other portions of the network.

(Hubs, in contrast, broadcast all data on the network to every other device on the
network.)

Bridges

A bridge connects two or more networks, or segments of the same network. These networks may use different physical and data link protocols. For example, you can install a bridge to connect a small lab of Macintosh computers using LocalTalk to the school's main Ethernet network. Bridges filter network traffic. They examine each set of data, transmitting only appropriate data to each connected segment. (Hubs, by contrast, broadcast all information to each connected computer, whether or not that computer is the intended recipient.) In this manner, bridges help reduce overall network traffic.

Bridges are relatively simple and efficient traffic regulators. However, in most networks they have been replaced by their less expensive or more powerful cousins—hubs, switches, and routers.

Most bridges operate by examining incoming or outgoing signals for information at OSI level 2, the data link level.

(OSI Level 2:At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking. Source: Webopedia.com)


Routers

Like bridges, routers connect two or more networks. However, routers are much more powerful than bridges. Routers can filter traffic so that only authorized personnel can enter restricted areas. They can permit or deny network communications with a particular Web site. They can recommend the best route for information to travel. As network traffic changes during the day, routers can redirect information to take less congested routes.

If your school is connected to the Internet, then you will most likely use a router to make that connection. Routers ensure that your local area network traffic remains local, while passing onto the Internet all your electronic mail, Web surfing connections, and other requests for Internet resources.

Routers are generally expensive to purchase and difficult to configure and maintain. Be sure that your staff have the resources necessary to manage them well.
Routers quickly become critical components of your network. If they fail, your network services will be significantly impaired. As part of your network plan, you should consider how you might deal with the failure of key routers on your network. Many sites include redundant connections— additional routers and network cable connections—configured to take over if one router or connection fails.

Most routers operate by examining incoming or outgoing signals for information at OSI level 3, the network addressing level.

(OSI Level 3: This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing. Source: Webopedia.com)

Source: EDC.org

What are the different multiplexing techniques?

Multiplexing (also muxing or MUXing)- In telecommunications, is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). The reverse of this is known as inverse multiplexing, demultiplexing, or demuxing. George O. Squier (1863–1934) invented the principle in 1910 using a carrier frequency to combine multiple telephone signals on one telephone line.

In electrical communications, the two basic forms of multiplexing are time-division multiplexing(TDM) and frequency-division multiplexing (FDM). In optical communications, FDM is referred to as wavelength division multiplexing (WDM).

• Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e., bit stream, by interleaving pulses representing bits from different channels.
  
  In some TDM systems, successive pulses represent bits from successive channels, e.g., voice channels in a T1 system. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called "time slot").
  
  What distinguishes coarse time-division multiplexing from packet switching is that the time-slots are pre-allocated to the channels, rather than arbitrated on a per-time slot basis.
  
  Uses of time-division multiplexing:
  
  * The PDH and SDH network transmission standards
  * The GSM telephone system
  
  (Source:Wikipedia.org)
  


• Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
  
  Historically, telephone networks used FDM to carry several voice channels on a single physical circuit. In this, 12 voice channels would be modulated onto carriers spaced 4 kHz apart. The composite signal, occupying the frequency range 60 – 108 kHz, was known as a group. In turn, five groups could themselves be multiplexed by a similar method into a supergroup, containing 60 voice channels. There were even higher levels of multiplexing, and it became possible to send thousands of voice channels down a single circuit.
  
  Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.
  
  FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.
  
  Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA).
  
  FDMA is the traditional way of separating radio signals from different transmitters.
  
  The analog of frequency division multiplexing in the optical domain is known as wavelength division multiplexing.
  
  (Source:Wikipedia.org)
  

(Source:Wikipedia.org)