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NETWORKING

Networking is connecting two or more devices to allow communication between them with the purpose of sharing information and resources. Examples of these devices are computers, printers, routers, hubs, modems, and PDAs. The information and resources being shared can be anything from MS Office documents and e-mail to printers and fax devices. Internetworking is connecting multiple networks with the purpose of creating one large network. The Internet is the most common example of an internetwork.


Client/server vs Peer-to-peer

Most of today's networks use the client/server model. In this model at least one computer acts as a server. Servers hold resources that are accessed over the network by clients. Examples of resources are shared files, e-mail messages and even applications. Another common server is the print server that allows access to network printers.
In a peer-to-peer network model every computer can act as a client and a server at the same time. An example is a network with 4 Windows XP Professional computers in a workgroup using file and print sharing.


LAN/WAN

The terms LAN and WAN mainly refer to the geographical area of the network. LAN is short for Local Area Network and is a high-speed network typically within a building. WAN is short for Wide Area Network and refers to low-speed networks that cover a large distance, for example a network that spans several cities or the entire globe even. The Internet can be considered the largest WAN, but actually consists of many different WANs, which, in turn, include LANs. The connection between LANs in an internetwork is also referred to as a WAN connection, although a network diagram of a WAN often includes the LANs in it.


Private vs Public Networks

Two other terms used to categorize networks are private networks and public networks. A private network is typically within the premises of a corporation and can be accessed only by users working for, or related to, that corporation. A public network Internet can be accessed by multiple individuals and/or corporations, the best example of a public network is again, the Internet.


Media

The physical connection used to transport electrical signals (bits; 1s & 0s) between the network devices is called the media. Examples of network media are copper cabling, fiber optic cabling and infra-red. The most common types of media are outlined later in this TechNote.


Protocols

To be able to communicate with each other, network devices need a common language. The language network devices use is called a protocol. There are many different types of protocols available, and most protocols are actually a suite of several protocols, each with a different function. For example, one protocol allows data transfer between hosts and another can be used to retrieve email from a mail server. Today's most common protocol, TCP/IP, and several older, less common protocols, are described later in this TechNote.


Addressing

If you want to contact somebody by snail-mail or by telephone you need some sort of address. In a telephone network you need to enter a telephone number to reach your intended communication partner. Similar, devices in a network need an address. There are two types of addresses, the first type is configured in software by a network administrator and uses protocols to define the addressing scheme and format, this type is known as network or layer 3 addressing. The other type of address that devices in a network use, is most commonly referred to as MAC address; this address is burned into the chip of the physical network interface.


Network Topologies

A physical topology depicts how network devices are connected physically, the cabling.
A logical topology depicts the route a the signal takes on the network.

	Bus - Devices are connected to a central cable, in this type of network both cable ends are terminated.
	Star - Devices are connected through a central hub. The hub forms a single-point-of-failure.
	Ring - Every device is connected to two other devices, forming a ring.
	Mesh - In a full mesh every device in the network is connected to every other device. In reality a partial mesh is often used in backbone environments.

Collisions

A collision indicates that two or more network nodes have tried to access the network simultaneously, and both put a signal on the network cable at the same time which collide with each other. Occasional occurrences are normal and will not affect network performance.


Access Methods

The access method defines how signals are put on the network media.

NETWORK INTERFACE CARDS (NICs)

A Network Interface Card (NIC), typically an expansion card in a computer, is used to connect to the physical network media. Some mainboards and most portable computers are equipped with a built-in (onboard) NIC. NICs are available for different types of network media, the most common today being Ethernet NICs with a RJ-45 socket for UTP/STP cabling. To install a network interface card you need a free ISA or PCI expansion slot and an appropriate driver that the computer's operating system will use to communicate with the NIC. Some older ISA NICs can be manually configured to use a particular IRQ. This is done by setting jumpers or dip switches. Some other NICs allow the IRQ to be configured through the use of configuration software.


An image of a Fast Ethernet network interface card.

Many of today's NICs are equipped with status indicators in the form of leds. These leds can be used to troubleshoot network problems. Typically one green led indicates the NIC is physically connected to the network and flashes when activity occurs, i.e., the port is transmitting or receiving data, this is also known as a heartbeat. When the NIC supports multiple speeds, for example 10 and 100 Mbps, there can be a green led for each speed, of which one is lit indicating the current speed, possibly auto-negotiated with a hub or switch. Some NICs, as well as other network devices such as hubs, include and orange or red led which flashes when collisions occur. If the collision LED flashes repeatedly or continuously, the NIC maybe be configured incorrectly or may be malfunctioning, or there may be other devices utilizing the network heavily.

As described earlier, networks interfaces are physically configured with an address known as the MAC address (MAC is short for Media Access Layer), layer 2 address, Burned In Address (BIA), or physical address. Here's an example of a MAC address: 00-10-E3-42-A8-BC. The first 6 hexadecimal digits specify the vendor/manufacturer of the NIC, the other 6 define the host. MAC addresses are supposedly unique across the planet.

Half duplex vs. Full-duplex communication

Half-duplex means that only one host can communicate at a given time, two hosts communicating with each other will take turns transmitting. In full-duplex communication both hosts can transmit at the same time, in theory allowing twice as much data to be transmitted over the same connection.
The connections able to run at full-duplex are cross-cable connections and connections to a port on a switch, where collisions cannot occur because each end has it's own wire pair (separate segment).


MEDIA

The list below shows some important considerations to make when choosing the proper media for a particular network solution:

RS-232

The RS-232 standard specifies serial cabling using DB-9 or DB-25 connectors. The maximum length for a RS-232 cable is 50 feet (15.25 meters) at a baudrate of 19200. The RS-232 standard is designed to supports data transfer rates up to 20 Kbps.


Example of an RS-232 serial cable with a female DB-9 and a male DB-25 connector.

TWISTED PAIR

The most common type of twisted pair cabling is Unshielded Twisted Pair (UTP) cabling. This type of cabling is typically made up of 4 twisted pairs of copper wires as depicted in the image below. Each wire has its own cover, and so does the complete bundle.

UTP cabling is categorized using a number. The required category depends on the network technology and the desired transmission speed. Following are the UTP categories:

Another, more expensive type of twisted pair cabling is Shielded Twisted Pair (STP). STP cabling includes a metal cover shielding the bundle of wires, reducing electrical interference and cross-talk.

In a cross-over cable wire 1 & 3, and 2 & 6 are crossed, these cables are typically used to connect a pc to pc, or switch to switch for example.

UTP cabling in networks use RJ-45 connector as depicted below:



10BaseT Ethernet, 100BaseTX Fast Ethernet, 1000BaseT and Token Ring are the most common networks that use twisted pair cabling and are described below.

10BaseT

The 10BaseT specification uses Cat 3, 4 and 5 UTP cabling in a star/hierarchical topology. Devices on the network are connected through a central hub.

10BaseT specifications:
- Maximum segment length is 100 meters
- Maximum data transfer speed is 10Mb/s
- Cat 3, 4 and 5 Unshielded Twisted Pair (UTP)

100BaseTX (Fast Ethernet, 802.3u)

Is similar to 10BaseT, except it requires at least Category 5 UTP or Category 1 STP cabling. Only uses 4 of the 8 wires like just like 10BaseT. The maximum data transfer rate is 100 Mb/s.

802.5 (Token Ring)

Token Ring uses the token passing method described earlier in this TechNote. While the logical topology of a Token Ring network is a ring, the physical topology is star/hierarchical as illustrated in the diagram below. Stations connect to MultiStation Access Units (look a bit like hubs) using UTP cabling which in turn are connected in a physical ring.

Token Ring specifications:
- Data transfer rate is 4 or 16 Mb/s
- Uses Twisted Pair cabling (Cat 3 for 4 MB/s, Cat 5 for 16 Mb/s)
- Logical topology ring, physical topology is star

Token Ring is originally created by IBM, and was later standardized by IEEE under the 802.5 specification. The original IBM Token Ring specification uses IBM Class 1 STP cabling with IBM proprietary connectors. This connector is called the IBM-type Data Connector (IDC) or Universal Data Connector (UDC), and is male nor female.


COAXIAL

Coaxial cabling is used primarily in 10Base2 (Thinnet) and 10Base5 (Thicknet) Ethernet networks. Coaxial cable uses a copper core with a protective shield, to reduce interference. The shield is covered with the outside cover made from PVC or plenum. The most common types are listed in the following table.

10Base2

Commonly referred to as Thinnet, uses a bus topology. Stations are attached using BNC T-connectors represented in the picture below. Both cable ends are terminated using a 50 ohm terminator.


BNC (British Naval Connector) T-connector.

10Base2 specifications:
- Maximum segment length is 185 meters
- Maximum data transfer speed is 10Mb/s
- 0.2 inch, 50 ohm RG-58 coaxial cable (Thinnet)

10Base5

Commonly referred to as Thicknet, commonly uses a bus topology. Stations are attached to the cable using MAUs, a transceiver that is attached to the cable using vampire taps that pierce the cable. A cable with AUI connectors is used to connect the transceiver to the network interface on for example a computer, hub or repeater. Both cable ends are terminated using a 50 ohm terminator.

AUI connectors	MAU transceiver

SC connectors	ST connectors	MIC connectors