A Set Of Simple Network Components Definitions

Many folks, when they start connecting their computers, get lost in the terminology. Justifiably so, I would think. I can hear the pain in their voice.

• I want to connect my computers. One guy in the store showed me a hub. Somebody else told me to buy a switch. And in the forum, I was told that a router was the only way to go. Help!
  
• I want to connect my computers, but avoid using cables everywhere. One guy told me to buy a wireless router. Somebody else recommended an access point. And I hear about bridges, and repeaters.
  

• Introduction to Networking
  
• Wired Devices - Bridges, Hubs, Routers, and Switches.
  
• Wireless Devices - Access Points, Bridges, Repeaters, Routers, and Switches.
  
• Summary and References
  

Introduction to Networking
To learn about network components, and what each one does, you first need to learn the concept of the OSI network model. All network components are defined in term of the network layer which they work in. Components in any one layer connect to other components in that layer, or to components in the layer above or below.

A network cable would be an example of a component in the Physical, or bottom, layer. Ethernet, which is one of the most common network standards in use today, incorporates both the Physical Layer, and the Data Link Layer. An Ethernet cable, then, connects thru both the Physical (Layer 1) and Data Link (Layer 2) layers, and can connect to a Network device, such as a router.

Routers, which operate at Layer 3, connect networks that use Internet Protocol (IP). For intensive instruction in IP networking, see Microsoft TCP/IP Fundamentals for Microsoft Windows.

WiFi, which is not a totally physical medium, is similar to Ethernet, excepting that it uses a radio channel, instead of cable.

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Wired Devices - Bridges, Hubs, Routers, and Switches

Hubs, routers, and switches are devices used to connect computers, that are physically attached (using cables), or logically attached (using WiFi).

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A hub is one of the most basic network components; like a cable, it is a Physical (Layer 1) device. It is not addressable, it connects passively to a group of Ethernet (or other media) cables.

A hub effectively connects a group of computers in one big conversation, much like an old fashioned telephone party line. With all computers in a network connected by hubs, only one computer will be able to transmit, to another computer, at any time. Computers connected in this way must use a communications technique called Carrier Sense Multiple Access/Collision Detection (CSMA/CD).

CSMA/CD is a pretty inefficient protocol. If you are chatting with a friend, maybe over the telephone, do you ever notice that sometimes one of you wants to speak when the other is still talking? How about if both of you start talking simultaneously? What if a group of you, and many friends, try to carry on a conversation that way? Sometimes, you have to spend as long deciding who's going to speak next, as actually speaking.

In effect, with a hub connecting your computers, the more computers that get connected, the less productive network work will get done. Hubs are just not scalable - that is, you can't keep adding computers to a hub, and get any decent production out of a network.

Since the purpose of networking computers is to transmit massive amounts of data between those computers, the switching hub was developed.

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A switch is a Data Link (Layer 2) device. A switch, which was originally called a switching hub, connects specific computers to each other selectively, much like a telephone switch, for individual conversations. Individual computers are addressed (selected), by a switch, using their MAC addresses.

With a switch, individual pairs of computers can carry on simultaneous conversations. Essentially, a switch is to private line telephone (which is the telephone service we all take for granted) as a hub is to party line telephone (if any of you are old enough to remember that). A switch operates in full duplex mode (each computer can send and receive simultaneously), where a hub operates in half duplex mode.

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A bridge is a type of switch. Where a switch, in general, connects two or more networks that use identical media (such as Ethernet), a bridge may connect networks that use different media. In Internet connectivity, a modem will act as a bridge, and connect:

• The Public Switched Telephone Network (PSTN) to a serial cable, leading to a computer or router (or connect as an internal component in your computer). A properly selected NAT router (though not all NAT routers) can connect to a properly selected external modem.
  
• A cable broadband network to Ethernet, or USB cable.
  
• A DSL broadband network to Ethernet, or USB cable.
  

A WiFi Bridge connects (bridges) Ethernet to WiFi.

There is one other difference between a bridge and a switch. A switch, by definition, connects multiple computers, and has 4 or more ports. A bridge generally connects only 2 different networks, and has 2 ports. A bridge with more than 2 ports generally has a hub or switch attached to one of the bridge ports.

Since the MAC address is factory assigned, and intentionally unique in all circumstances, it would be practically useless to designate groups of computers by MAC address. Switches are more effective than hubs for connecting large groups of computers, but the groups need to be local to each other for efficiency.

To associate groups of computers, where not all groups are local to each other, you need the ability to associate computers in location based groupings. This is where Internet Protocol addressing comes in to use - the IP address is assigned by physical grouping of computers.

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A router is a Network (Layer 3) device, that connects networks that use Internet Protocol. A router connects specific computers to each other selectively, like a switch. Unlike a switch, which addresses individual computers by their MAC addresses, a router addresses computers by their IP addresses.

Since a router addresses computers by their IP addresses, a router only transports Internet Protocol traffic. IPX/SPX and NetBEUI, which are alternate transports, do not produce routable traffic. Networks which use either alternate transport must be connected by hubs or switches, they won't work with routers.

Since Ethernet connects thru both the Physical (Layer 1) and Data Link (Layer 2) layers, an Ethernet cable can connect either a hub (Layer 1), a switch (Layer 2), or a router (Layer 3). A group of computers, connected by Ethernet or WiFi, thru a collection of hubs, switches, and routers, makes up a Local Area Network (LAN), or a Wide Area Network (WAN). Since the IP address is assigned to each group of computers based upon their physical location, all computers in one physical location can be easily identified by IP address grouping, or subnet.

A router is essentially a big switch, with multiple connections, each connection leading to one or more subnets. A subnet can be locally attached (by Ethernet), or distantly attached (by a long distance communications line). By knowing what subnet is accessible (immediately, or distantly), from any connection, a router can decide which connection should be used for a packet destined for any given IP address or subnet.

Now if you are buying, or just bought, a router for your home or small office, you probably are looking at a NAT router. A NAT router has the functionality of a regular router, and more. For a description of a NAT router, please see my article What Is A NAT Router?.

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Wireless Devices - Access Points, Bridges, Repeaters, Routers, and Switches

A WiFi channel is similar to a hub, in that all computers using a single WiFi channel have to share it with each other. They can choose not to listen to the conversations of their neighbors (properly designed software won't participate in conversations which don't apply to the network that it connects to), but you should not assume this to be true in all cases. You absolutely must practice WiFi security.

And whether or not a WiFi device listens to a conversation on another network, it won't be able to use the channel, while the other network is using the channel. The WiFi channel can only be used by one conversation at any time. All WiFi devices, within range of each other (able to detect radio from each other) have to share the channel, and only one device can transmit at any time. This is why we say that WiFi is a half duplex medium.

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A WiFi router is similar to a wired router, but with one extra component - a radio connected to the LAN switch. The computers that connect by WiFi become peers to the computers connected to the Ethernet LAN ports.

All computers connected directly to the Ethernet switch have the capability of multiple simultaneous, full duplex, communications, with all other computers connected directly to the Ethernet switch. All computers that connect by WiFi, though, have to share the channel with all other nearby WiFi devices.

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A WiFi bridge is similar to a wired router, but with one extra component - a radio connected to the WAN port. Like the wired router, all client computers connect to the bridge by Ethernet.

All computers connected directly to the Ethernet switch have the capability of multiple simultaneous, full duplex, communications, with all other computers connected directly to the Ethernet switch. Connection to the rest of the network, thru the WiFi WAN port, will have to share the WiFi channel with all other nearby WiFi devices.

You can buy WiFi bridges made for that purpose, and some WiFi NAT routers can be converted to bridge configuration. The Linksys WRT54G, with third party firmware, can be configured as a bridge.

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A WiFi access point is a wired switch, with a radio. As with a WiFi router, the computers that connect by WiFi become peers to the computers connected to the Ethernet LAN ports on the switch.

The computers connected to the Access Point - both wired and wireless - will have the same capabilities and restrictions as those connected to a WiFi router.

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A WiFi repeater is, simply, a radio that alternately receives and sends. Placed at a distance from a WiFi router (at a midpoint between the router and the clients), a repeater can extend the range of the router. A repeater that operates on one channel, though, will be very slow. It has to:

• Receive a packet from one WiFi computer, that's intended for another.
  
• Retransmit (repeat) the same packet.
  
• Wait for a reply from the computer that the packet was intended for.
  
• Pass that reply back to the sending computer.
  
• Receive another packet from the sending computer.
  

Summary and References
All equipment, excepting the NAT Router component of any device, operates at Layer 2 of the OSI network model. Excepting the NAT Router component, all equipment will transport IPX/SPX and NetBEUI network traffic, in addition to IP traffic.

Any configuration of equipment, done thru your browser, will typically require Internet Protocol though. Most network components, when designed to be managed thru the network, is addressed (managed) by IP address. Both WiFi routers, and Wired Routers, will only transport IP traffic.

For additional discussion about wired components, see Hard Forum Networking FAQ Q1.

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