Switcher and Hubs

Switches

A network switch is a device that forwards and filters OSI layer 2 datagrams between ports based on the MAC addresses in the packets. A switch breaks the collision domain but represents itself as a broadcast domain. Switches make forwarding decisions of frames on the basis of MAC addresses. A switch normally has numerous ports, facilitating a star topology for devices, and cascading additional switches.

HUB
In computer networking, a hub is a small, simple, inexpensive device that joins multiple computers together. Many network hubs available today support the Ethernetstandard. Other types including USB hubs also exist, but Ethernet is the type traditionally used in home networking.
Working With Ethernet Hubs To network a group of computers using an Ethernet hub, first connect an Ethernet cable into the unit, then connect the other end of the cable to each computer's network interface card (NIC). All Ethernet hubs accept the RJ-45 connectors of standard Ethernet cables.

Bridges and Routers

BRIDGES

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges broadcast to all ports except the port on which the broadcast was received. Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port.

ROUTERS

A router is an internetworking device that forwards packets between networks by processing information found in the datagram or packet (Internet protocol information from Layer 3 of the OSI Model). Is processed in conjunction with the routing table. Routers use routing tables to determine what interface to forward packets.

Multiplexing

In telecommunications and computer networks, multiplexing (also known as muxing) is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy, and is now widely applied in communications.
A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX).
Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

Transmisssion modes

A given transmission on a communications channel between two machines can occur in several different ways. The transmission is characterised by:

• the direction of the exchanges
• the transmission mode: the number of bits sent simultaneously
• synchronisation between the transmitter and receiver

Simplex, half-duplex and full-duplex connections

There are 3 different transmission modes characterised according to the direction of the exchanges:

•A simplex connection is a connection in which the data flows in only one direction, from the transmitter to the receiver. This type of connection is useful if the data do not need to flow in both directions (for example, from your computer to the printer or from the mouse to your computer...).

•A half-duplex connection (sometimes called an alternating connection or semi-duplex) is a connection in which the data flows in one direction or the other, but not both at the same time. With this type of connection, each end of the connection transmits in turn. This type of connection makes it possible to have bidirectional communications using the full capacity of the line.

•A full-duplex connection is a connection in which the data flow in both directions simultaneously. Each end of the line can thus transmit and receive at the same time, which means that the bandwidth is divided in two for each direction of data transmission if the same transmission medium is used for both directions of transmission.

types of signal

Analog Signals

The pictures we saw above are examples of analog signals:
An analog signal varies some physical property, such as voltage, in proportion to the information that we are trying to transmit.
Examples of analog technology:
1. photocopiers
2. old land-line telephones
3. audio tapes
4. old televisions (intensity and color information per scan line)
5. VCRs (same as TV)
Analog technology always suffers from degradation when copied.
When sound is transmitted or stored it may need to change form, hopefully without being destroyed.

Sound moves fast: in air, at 340 m/sec = 750 miles per hour. Its two important characteristics areFrequency (aka pitch) and Amplitude (aka loudness).

Digital Signals

With a digital signal, we are using an analog signal to transmit numbers, which we convert into bitsand then transmit the bits.
A digital signal uses some physical property, such as voltage, to transmit a single bit of information.
Suppose we want to transmit the number 6. In binary, that number is 110. We first decide that, say, "high" means a 1 and "low" means a 0. Thus, 6 might look like:

The heavy black line is the signal, which rises to the maximum to indicate a 1 and falls to the minimum to indicate a 0.

data communication

The distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single IC chip, to as much as several feet along the backplane of the main circuit board. Over such small distances, digital data may be transmitted as direct, two-level electrical signals over simple copper conductors. Except for the fastest computers, circuit designers are not very concerned about the shape of the conductor or the analog characteristics of signal transmission.
Data Communications concerns the transmission of digital messages to devices external to the message source. "External" devices are generally thought of as being independently powered circuitry that exists beyond the chassis of a computer or other digital message source. It is the aim of any communications system to provide the highest possible transmission rate at the lowest possible power and with the least possible noise

plug-in

The term plug-in may refer to any of the following.
In Automotive:

• Plug-in hybrid electric vehicle

In Computers:

• Plug-in (computing), a piece of software which enhances another software application and usually cannot be run independently
  • Browser extension, which modifies the interface and/or behavior of web browsers
  • Photoshop plugin, a piece of software that enhance the functionality of a Adobe Photoshop (a commercial software application)
  • Plug and play, is a common standard for hardware equipment installation

In Mathematics:

• Plugging in (algebra), a mathematical procedure in which substitutions are made in a formula

In Music:

• Plug-In, (band), a instrumental metal band composed of Fanalo, Frédéric Motte, Denis Cornardeau and Simon Renault.