Networks are not just made up of cables connected to such devices as computers, terminals and printers.
Several special-purpose hardware devices are needed to successfully construct a network.
Network interface card
Each computer connected to the network requires a special network interface card.
A network interface card (NIC, pronounced ‘nick’) is an expansion card that fits into an expansion slot of a computer or other device, so that the device can be connected to a network.
Most NICs require a network cable connection and have connectors on the card for different types of cables.
The type of NIC depends upon the type of network.
NICs package data according to the rules of the network operating system and transmit data along the connecting network cable.
Network cards have three main types of connectors. Below is an example of what a network card may look like.
4.6.1 BNC connector:As illustrated in the above picture the BNC connector is a round connector, which is used for thin net or 10Base-2 Local Area Network.
4.6.2 DB9 (RJ45 JACK): The DB9 connector not to be confused with the Serial Port or sometimes referred to as the RJ45 JACK not to be confused with the RJ45 connection is used with Token Ring networks.
4.6.3 DB15 Connector: The DB15 connector is used for a Thick net or 10Base-5 Local area network.
4.6.4 RJ45 connector: Today one of the most popular types of connections used with computer networks. RJ45 looks similar to a phone connector or RJ11 connector however is slightly larger.
LED - The LED's as shown in the above illustration indicates if it detects a network generally by a green light which may flash as it communicates and then a red light which indicates collisions which will generally flash or not flash at all.
A server is a computer that provides services to other computers on the network.
Individual computers log on to the server, which gives them access to files, applications or peripheral devices.
There are different types of servers, such as file servers, print servers, mail servers and Web servers.
file server is a controlling computer in a network that stores the programs and data shared by users.The files stored on this server can be retrieved by any node provided it has access rights.
print server is a computer in a network that controls one or more printers and stores data to be printed. A print server can be used with or without a file server.
mail server is a computer in a network that provides email facilities. It stores incoming mail for distribution to users and forwards outgoing mail to appropriate devices.
A Web server is a computer in a network that provides a connection to the Internet. All the Internet traffic is directed through this server.
Routers and switches
Data often travels between networks. This requires networks to be linked to other
networks, paths to be established between the networks and signal strength to be
boosted.
Devices used to determine the path between networks include:
Router—a device that determines where to send a data packet between at least two networks.
Its decision is based on its current understanding of the networks.
A router maintains a table of the available routes and their conditions.
It uses this table together with distance and cost algorithms to determine the best route for a given data packet.
Data packets often travel through a number of networks and routers before arriving at their destination.
Switch—a device that directs data packets along a path.
It may include the function of a router.
In general, a switch is a simpler and faster mechanism than a router as it does not maintain knowledge of the networks.
A switch is not always required in a network.
Many LANs are organised so that the nodes inspect each data packet.
A switch can be thought of as an intelligent hub or a multi-port bridge.
Switches determine the MAC address of the sender and intended receiver that precedes each message.
The receiver’s address is used to identify the destination node and forward the message to that node only.
In essence, a switch sets up a direct connection between the sender and the receiver;
therefore each node exists on its own segment, the switch being the only other device on the segment.
As no other nodes exist on each segment each node is free to transmit messages at any time without the need to detect or avoid collisions.
Switches are able to simultaneously receive and forward messages from and to multiple pairs of nodes.
As long as both the sender and the receiver of each message do not conflict with other simultaneous messages then the switch will direct the message correctly.
Most switches allow nodes to communicate in full duplex.
Bridges and gateways
The actual interconnection between networks is achieved using devices such as
bridges and gateways.
Bridge
a combination of hardware and software to link two similar networks.
It often connects LANs that use the same protocol, such as Ethernet.
A bridge to a connected LAN.
A bridge separates a network into different segments at the data link layer. Bridges were once used extensively to segment Ethernet logical bus networks – today switches perform this function.
Bridges determine the destination MAC address of each frame.
If the destination node with that MAC address is on the other side of the bridge then the frame is repeated onto that segment, otherwise the frame is dropped.
Essentially a bridge splits a logical bus network into two collision domains.
Bridges are faster than routers because they connect networks that are using the same protocol.
Gateway
a combination of hardware and software to link two different types of networks.
This usually involves converting different protocols.
For example, a gateway could be used to convert a TCP/IP packet to a NetWare IPX packet.
A Gateway is a router.
A default gateway to a computer on a network is nothing more that the default router.
In other words, if the computer needs to communicate to a device that isn’t on its own network then it sends that to the default gateway which uses the routing protocols it has to find a route to that destination IP address.
Repeater.
A signal may be corrupted due to long distances or interference from electromagnetic fields.
A repeater is used to rebuild a fading signal to its original strength and shape before transmitting it onwards.
It ensures that the data is received as it was sent.
A network repeater is used in a LAN to connect network segments.
Repeaters are less intelligent than a bridge or gateway.
Hubs
A hub is a central connecting device in a network.
Data arrives at the hub from one or more devices and is forwarded out using just one cable (see Figure 3.13).
When a hub receives a packet of data it simply amplifies and retransmits the packet to all attached nodes.
As a consequence hubs are also known as multi-port repeaters.
Hubs are dumb devices that operate at the physical layer of the OSI model.
They make no attempt to identify the destination node for each message.
For example, four cables from three computers and a printer are connected to a hub and then a single cable connects the hub to a server.
A hub can also include a router.
Most hubs were originally passive.
The data simply passed through the hub without any change.
Intelligent hubs are more frequently used in today’s networks.
They often contain a CPU and network operating system.
This allows them to perform some of the functions of a server.
