Router? Bridge? Switch? Hub? What's the difference? their principle?

Reference: LAN and WAN Subnetworks Under IP - Lan Interconnection, by Thomas A. Maufer


Hub
A hub is a repeater, which is a OSI model device, the simplest possible. Hubs are a common connection point for devices in a network and are commonly used to connect segments of a LAN. A hub takes the incoming data packet that comes into a port and copies it out to all the other ports in the hub. It doesn't perform any filtering or redirection of data. Although it's actually a little more complicated, a good analogy might be that of an Internet Chat room. Everything that everyone types in the chat room is seen by everyone else. If there are too many people trying to chat things get bogged down.

A passive hub serves simply as a conduit for the data, enabling it to go from one device (or segment) to another. So-called intelligent hubs include additional features that enables an administrator to monitor the traffic passing through the hub and to configure each port in the hub. Intelligent hubs are also called manageable hubs. A third type of hub, called a switching hub, actually reads the destination address of each packet and then forwards the packet to the correct port.



Bridge
Bridges (sometimes called "Transparent bridges") work at OSI model Layer 2. This means they don't know anything about protocols, but just forward data depending on the destination address in the data packet. This address is not the IP address, but the MAC (Media Access Control) address that is unique to each network adapter card. The bridge is the device which is used to connect two local-area networks (LANs), or two segments of the same LAN that use the same protocol.

With a Bridge, all your computers are in the same network subnet, so you don't have to worry about not being able to communicate between computers or share an Internet connection. DHCP servers will work fine across Bridges, or if you assign your own IP addresses, you'll use the same first 3 "octets" of the IP address (Example: 192.168.0.X)

However, the only data that is allowed to cross the bridge is data that is being sent to a valid address on the other side of the bridge. No valid address, no data across the bridge. Bridges don't require programming. They learn the addresses of the computers connected to them by listening to the data flowing through them.

Bridges are very useful for joining networks made of different media types together into larger networks, and keeping network segments free of data that doesn't belong in a particular segment.

Switches
Switches are the same thing as Bridges, but usually have multiple ports with the same "flavor" connection (Example: 10/100BaseT).

Switches can be used in heavily loaded networks to isolate data flow and improve performance. In a switch, data between two lightly used computers will be isolated from data intended for a heavily used server, for example. Or in the opposite case, in "auto sensing" switches that allow mixing of 10 and 100Mbps connections, the slower 10Mbps transfer won't slow down the faster 100Mbps flow.

Although switch prices are dropping so that there is very little difference from hub prices, most home users get very little, if any, advantage from switches, even when sharing broadband Internet connections. Broadband connections for most users are in the 1-2Mbps range, far below even 10Mbps speeds. Since you share that bandwidth, you can see that your speedy 100BaseT connection isn't even breaking a sweat when you're using the Internet.

Router
Routers forward data packets from one place to another, too! However routers are OSI model Layer 3 devices, and forward data depending on the Network address, not the Hardware (MAC) address. For TCP/IP networks, this means the IP address of the network interface.

Routers isolate each LAN into a separate subnet, so each network adapter's IP address will have a different third "octet" (Example: 192.168.1.1 and 192.168.2.1 are in different subnets). They are necessary in large networks because the TCP/IP addressing scheme allows only 254 addresses per (Class C) network segment.

Routers, like bridges, provide bandwidth control by keeping data out of subnets where it doesn't belong. However, routers need to be set up before they can get going, although once set up, they can communicate with other routers and learn the way to parts of a network that are added after a router is initially configured.

Routers are also the only one of these four devices that will allow you to share a single IP address among multiple network clients.

Networking Device

Difference between router, switch, bridges, and hubs including their pinciples






router



A device that connects any number of LANs.
Routers use headers and a forwarding table to determine where packets go, and they use ICMP to communicate with each other and configure the best route between any two hosts.
Very little filtering of data is done through routers. Routers do not care about the type of data they handle.




bridge

A device that connects two local-area networks (LANs), or two segments of the same LAN that use the same protocol, such as Ethernet or Token-Ring.











switch

In networks, a device that filters and forwards packets between LAN segments. Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Reference Model and therefore support any packet protocol. LANs that use switches to join segments are called switched LANs or, in the case of Ethernet networks, switched Ethernet LANs.








hub

A hub is a "unintelligent" broadcast device -- any packet entering any port is broadcast out on every port. Hubs do not manage any of the traffic that comes through their ports. Since every packet is constantly being sent out through every port, you end up with packet collisions, which greatly impedes the smooth flow of traffic on your LAN.


References:
http://www.experts-exchange.com/Hardware/Routers/Q_20381112.html

Multiplexing Techniques

Multiplexing is the process where multiple channels are combined for transmission over a common transmission path.

There are two predominant ways to multiplex:







Frequency Division Multiplexing
Time Division Multiplexing



Frequency Division Multiplexing (FDM)

In FDM, multiple channels are combined onto a single aggregate signal for transmission. The channels are separated in the aggregate by their FREQUENCY.
There are always some unused frequency spaces between channels, known as "guard bands". These guard bands reduce the effects of "bleedover" between adjacent channels, a condition more commonly referred to as "crosstalk".
FDM was the first multiplexing scheme to enjoy widescale network deployment, and such systems are still in use today. However, Time Division Multiplexing is the preferred approach today, due to its ability to support native data I/O (Input/Output) channels.

FDM Data Channel Applications

Data channel FDM multiplexing is usually accomplished by "modem stacking". In this case, a data channel's modem is set to a specific operating frequency. Different modems with different frequencies could be combined over a single voice line. As the number of these "bridged" modems on a specific line changes, the individual modem outputs need adjustment ("tweaking") so that the proper composite level is maintained. This VF level is known as the "Composite Data Transmission Level" and is almost universally -13 dBm0.
Although such units supported up to 1200 BPS data modem rates, the most popular implementation was a low-speed FDM multiplexer known as the Voice Frequency Carrier Terminal (VFCT).

FDM Voice Channel Applications

Amplitude Modulation (AM), using Single Sideband-Suppressed Carrier (SSB-SC) techniques, is used for voice channel multiplexing. Basically, a 4 KHz signal is multiplexed ("heterodyned") using AM techniques. Filtering removes the upper sideband and the carrier signal. Other channels are multiplexed as well, but use different carrier frequencies.
Advances in radio technology, particulary the developments of the Reflex Klystron and integrated modulators, resulted in huge FDM networks. One of the most predominate FDM schemes was known as "L-Carrier", suitable for transmission over coaxial cable and wideband radio systems.








Time Division Multiplexing

Timeplex is probably the best in the business (IMHO) at Time Division Multiplexing, as it has 25+ years or experience. When Timeplex was started by a couple of ex-Western Union guys in 1969 it was among the first commercial TDM companies in the United States. In fact, "Timeplex" was derived from TIME division multiPLEXing!
In Time Division Multiplexing, channels "share" the common aggregate based upon time! There are a variety of TDM schemes, discussed in the following sections:
Conventional Time Division Multiplexing
Statistical Time Division Multiplexing
Cell-Relay/ATM Multiplexing
Conventional Time Division Multiplexing (TDM)
Conventional TDM systems usually employ either Bit-Interleaved or Byte-Interleaved multiplexing schemes as discussed in the subsections below.
Clocking (Bit timing) is critical in Conventional TDM. All sources of I/O and aggregate clock frequencies should be derived from a central, "traceable" source for the greatest efficiency.

Bit-Interleaved Multiplexing

In Bit-Interleaved TDM, a single data bit from an I/O port is output to the aggregate channel. This is followed by a data bit from another I/O port (channel), and so on, and so on, with the process repeating itself.
A "time slice" is reserved on the aggregate channel for each individual I/O port. Since these "time slices" for each I/O port are known to both the transmitter and receiver, the only requirement is for the transmitter and receiver to be in-step; that is to say, being at the right place (I/O port) at the right time. This is accomplished through the use of a synchronization channel between the two multiplexers. The synchronization channel transports a fixed pattern that the receiver uses to acquire synchronization.
Total I/O bandwidth (expressed in Bits Per Second - BPS) cannot exceed that of the aggregate (minus the bandwidth requirements for the synchronization channel).
Bit-Interleaved TDM is simple and efficient and requires little or no buffering of I/O data. A single data bit from each I/O channel is sampled, then interleaved and output in a high speed data stream.
Unfortunately, Bit-Interleaved TDM does not fit in well with today's microprocessor-driven, byte-based environment!

Byte-Interleaved Multiplexing

In Byte-Interleaved multiplexing, complete words (bytes) from the I/O channels are placed sequentially, one after another, onto the high speed aggregate channel. Again, a synchronization channel is used to synchronize the multiplexers at each end of the communications facility.
For an I/O payload that consists of synchronous channels only, the total I/O bandwidth cannot exceed that of the aggregate (minus the synchronization channel bandwidth). But for asynchronous I/O channels, the aggregate bandwidth CAN BE EXCEEDED if the aggregate byte size is LESS than the total asynchronous I/O character size (Start + Data + Stop bits). (This has to do with the actual CHARACTER transmission rate of the asynchronous data being LESS THAN the synchronous CHARACTER rate serviced by the TDM).
Byte-Interleaved TDMs were heavily deployed from the from the late 1970s to around 1985. These units could support up to 256 KBPS aggregates but were usually found in 4.8 KBPS to 56 KBPS DDS and VF-modem environments. In those days, 56 KBPS DDS pipes were very high speed circuits. Imagine!
In 1984, with the divestiture of AT&T and the launch of of T1 facilities and services, many companies jumped into the private networking market; pioneering a generation of intelligent TDM networks.