When computer networks are working well, which we hope is most of the time, the inner workings of the system modules are transparent to the average user. The most common components on a computer network, not counting cables, are "routers", "hubs", and "switches". Many of these modules can be similar cosmetically. Each assembly typically has Ethernet connectors (RJ45, which looks like an oversized telephone plug) and LED indicators. However, these modules function differently, and it is important to understand the differences.
Modern network hardware operates on the "Open System Interconnection" (OSI) standard. This standard defines how communications on a network should be implemented. By conforming to this standard, modules from different manufacturers can coexist on the same network. Wireless networks (WiFi) and the 802.11X standard are an additional subset of network systems.
When a message is sent between computers, it is broken into parts. At the base level, the message is reduced to "1" and "0" bits. The next level is a group of bits called a "frame". A frame contains its control information, including target address and error detection. The next level is a group of frames called a "packet". The terms frame and packet are sometimes used interchangeably. If a message is sent on a complex network like the Internet, some of the packets may take a different path, and be recombined at the destination.
Error detection is used with both frames and packets. The most common routine is called "Cyclic Redundancy Check" (CRC). CRC sums of all the "1"s in the frame or packet. This number is stored as a hexadecimal value at the end of the frame/packet. At the receiving end, the process is repeated. If the two hexadecimal values agree, the test passes. If not, the receiving device requests that the sending device resend. Most of this functionality is performed by integrated circuits (chips) inside the network modules.
The network hub operates on the first layer of the OSI standard, called the "physical layer". The hub is the most simple of the three modules. A hub is not aware of the contents of the message that is processed; it handles the message as bits. It simply records the signal, and rebroadcast it to all, including back to the port that sent the message. An "active" hub will clean the electrical signal of noise and amplify the signal before rebroadcast. A "passive" hub does not amplify the received signal; it merely receives a signal, and rebroadcast the signal as received to each port. Hubs are sometimes used to link multiple computers with a printer.
The Network Switch
The network switch operates on the second layer of the OSI standard, called the "data link layer". The network switch, as indicated by its name, switches signal paths, so that a message frame goes to a specific destination. A switch will improve a networks performance, especially on networks with many computers. A switch has enough on-board intelligence to remember the path to each destination. The network switch handles a message in frames.
When you connect a computer to a network switch, the switch will record the Media Access Control or “MAC” address of the computer's network interface card (NIC). This is called address protocol, or "ARP". When a frame is received intended for a specific computer, the switch sends the frame only to that computer. By preventing paths of the network from being utilized by every frame, network resources are conserved. Computer A can send a frame to computer B, while simultaneously, computer C is sending a frame to computer D.
The Network Router
The network router operates on the third layer of the OSI standard, called the "network layer". A router's name is also indicative of its role. Routers have some of the same capabilities as switches, but routers are most often used to connect two or more networks. For example, a router could be used to connect a wireless network with a conventional local area network (LAN). Another common use is to connect a LAN with the Internet (a "wide-area network", or "WAN"). In this role, the router uses "Network Address Translation" (NAT) so that all of the computers attached to the LAN can share a single IP address. A network router handles the message in packets. A router uses the IP addresses in the packets to route them between multiple networks.
A personal computer can be configured to handle the function of a router if it is equipped with router software and two or more network interface cards (NIC). A separate NIC is needed for each network.
A router is capable of advanced functions, including serving as DHCP (domain host control protocol) server and Firewall. A Firewall protects computers from potential hazards from the other computers outside the network. Linking multiple networks often requires the conversion of protocols.
A router is an extremely diverse classification. Routers may provide connectivity inside offices, between different locations, and between businesses and the Internet. The largest routers connect Internet service providers, are used in very large business networks, or connect a business with a satellite link to a distant corporate locale. Advanced routers are powerful computers, complete with microprocessors. Very sophisticated routers are used by the Internet to manage the network traffic most efficiently.
A router maintains a table called “routing information base” (RIB) that tracks information about the available routes. The RIB can be static (manually determined by a network administrator) or dynamic (continually updated based on changing conditions). A RIB is simple on a small LAN, but can be extremely complex in the very large routers used on the Internet.
There are more types of network modules than the three discussed here, and there can be significant overlap in roles. For example, an "intelligent hub" can have many of the characteristics of a network switch. Wireless networks (WAN) have much in common with their conventional LAN cousins, but additional protocol is added for the special security and interference concerns specific to wireless networks. Multiple roles may be combined into a single assembly. Network routers sometimes have subassemblies that function as network switches.
Brian Bradshaw is a Certified Technical Specialist (InfoComm CTS) and CompTIA A+ Computer Technologist. Areas of expertise include Video, Audio, Computer Networks, WiFi, [parked domain bradshaw-vacuum-technology.com] HDTV, Satellite Systems and Communications.
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