Basic LAN techniques IN common with all other computer based systems networks require both HARDWARE and SOFTWARE to function. Networks are often explained by reference to a universal ‘model’ called the OSI (Open System Interconnect) model. This model attempts to define the operation of various hardware & software components f a ‘network’

OSI Model The OSI model is HIERARCHICAL with seven layers, layer 1 being the bottom (physical) layer. Network OS Hardware A functional network must have components operating at each layer of the model.

The Network Interface Card (NIC) At this point we shall concentrate on ETHERNET version2 cards using 10/100 base T cabling systems. These cards adhere to a standard known as IEEE 802.3u, normally just called ETHERNET. NICs are available from around £6.00. Most modern motherboards have a built in NIC (LAN connector)

NIC The NIC is often referred to as a LAYER 1 (physical layer) device although it actually operates at layer 1 and 2 (data link). At layer 1 the NIC provides the means of physical (electrical & mechanical) access to the network media. At layer 2 the NIC provides Media Access and Logical Link Control by having an embedded ETHERNET chip on board.

NIC Operation This means that the Network Operating System (NOS) does not need to implement Ethernet. Layer 2 is divided into two sub layers called »LOGICAL LINK CONTROL (LLC) »MEDIA ACCESS CONTROL (MAC)

Logical Link Control The LOGICAL TOPOLOGY of the network is controlled by the LLC sub layer. (Remember, it is possible to have for example a physical star yet a logical ring topology as in token ring ieee systems)

MAC Media access control is the key to Ethernet and all other shared media type networks Ethernet systems use a media access technique called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Token passing (IBM) and CSMA/CA (Apple) are other LAN possibilities.

MAC Addressing Each node on a network is uniquely identifiable on a universal basis, much like people! This amazing feat is achieved by virtue of a two part MAC address being ‘burnt’ into the NIC at manufacture. This MAC address is a 48 bit (6 byte) binary number, written as a 12 digit hexadecimal number eg. 0067A7C11FB56

MAC address format The first 24 bits of the MAC address define the MANUFACTURER of the device and are allocated to the manufacturer. The last 24 bits are set by the manufacturer. Providing the manufacturer allocates a unique 24 bit number correctly it is impossible for any two NICs to have the same MAC address.

Simplest Possible Network The simplest possible ‘network’ consists of two PC’s fitted with NIC’s linked by a cross-over cable crossover cable Each PC is uniquely identified by its’ MAC address D0CF D0DO76

Software The NICs and cables used to construct this simple LAN exist only at the lower layers of the OSI model. We now need to add software components covering the remaining layers. These components are supplied as part of the PC operating system as is the case with MS or may be supplied by an other vendor as is the case with NOVELL.

OS / NOS The software components required to make the network functional are often referred to as ‘Network Operating System (NOS)’ components, as distinct from Operating System (OS) components. All currently supported Microsoft offerings have software components to allow at least peer to peer capability.

Login When we configure the network software we specify a ‘login’. This specifies the rules (protocol) at layer 5 (Session) and defines such things as how a communication ‘session’ is initiated, managed and terminated.

Minimum Software Needed A ‘driver’ for the NIC, supplied by the manufacturer. This allows the NIC hardware to be controlled by the PC an operates at the data link layer. A ‘protocol’ such as TCP/IP, NetBeui, Ipx/Spx etc. These are components which amongst other things control how information is ‘routed’ in networks. Protocols are sometimes described as ‘sets of rules’ for network operation. ‘Protocols’ operate at layer 3 & 4

Services These are ‘Application Layer’ services provided by the NOS. Examples of services which must be enabled are file and printer sharing Many application layer services are not explicitly installed or enabled and are essentially transparent to the user.

NIC communication All ETHERNET traffic is ultimately between NICs, so it is important to study the nature of this traffic. The actual electronic signals carried by the cables are quite complex pulses, using MANCHESTER II encoding. An exact understanding of the nature of these pulses is not essential for a understanding of networks as such.

NIC Communication These electronic signals are often shown on diagrams as strings of 1’s and 0’s, representing the streams of BINARY NUMBERS (bit streams), flowing between NICs. It is important to understand that these streams of bits are actually grouped into units called ETHERNET FRAMES (FRAMES) All ETHERNET systems use the same FRAME structure and are therefore compatible!

ETHERNET FRAME Again, a complete understanding of the FRAME is not essential, the structure is shown here for completeness and to aid understanding.

ETHERNET FRAME

PC to PC Network D0CF D0DO76

A more complex network With the addition of a HUB (a layer 1) device, a much more complex local area network may be constructed. The software requirements are the same as for the simple PC to PC network outlined earlier. CSMA/CD makes this topology possible !

Hub Based LAN

Switch Operation The switch builds up a table containing the MAC address and port number for all devices connected. if devices are removed or added then the table is updated.

Building The Switch Table Port n Name Node MAC n A324BD3B A324A1CD A24D33D2 A32763E4 A324BD3B A A324A1CD B A24D33D2 C A32763E4 X Switch Table Port MAC Address

Switch Advantages The major advantage of a switch is that it offers a dedicated full bandwidth channel between any two ports and thus any two communicating devices. In base t HUB or base 2/5BUS topologies the bandwidth of the media is shared between all communicating devices. Therefore the performance advantage of a switch compared to a hub can be very pronounced.