Networking Fundamentals.

Local area Networks (LAN's) these were developed in the early 1970's to connect an ever increasing range of computing equipment and ancillary hardware. Typically data rates lie between 1 and 100 Mb/s The geographical span seldom exceeds 10Km and is more likely to be around 1Km. Most LAN's connect equipment in a single building or a group of buildings in close proximity. Also most LAN's are privately owned.

Graphically distributed sites are connected by a Wide area network (WAN) through the use of telecom lines, multiplexers and modems. Such networks can also be connected to a LAN by means of a gateway.

There are three basic types of LAN configuration

Star

Bus / Tree

Ring.

Ethernet Bus Topology (Coax Cable) Top Drawing

Ethernet Star Topology (Cat 5) Bottom drawing.

(IEE 802.3)

Ring Topology.

Ring topology implemented using a hub (Token ring)

(IEE 802.5)

There are four basic LAN types

Baseband

Broadband

Hybrid

PBX

Baseband.

In a baseband system direct encoding is used to convey digital signal information over the transmission medium (Co-Ax, Twisted Pair, Fibre Optic, Ribbon) Digital signals are transmitted without modulation. The entire frequency spectrum of the medium is available and if there is more than one session at a time then Time Division Multiplexing techniques are employed. Since no modulation is involved (there is signal encoding) the interface equipment is simpler. The baseband LAN's support data transmission up to 10Mb/s.

Broadband

In broadband systems the digital signal from the attached devices is modulated by a Radio Frequency (RF) carrier using radio frequency modems. By operating the modems at different frequencies (FDM) a single medium can support a number of simultaneous information channels without interference and resultant disruption. Devices are connected to the network via frequency agile modems which transmit and receive signals at different frequencies under external control. A commercial example is the Local Net 20 broadband system. In this system the in-channel group occupies 36Mhz from 70 to 106 Mhz and the out channel group occupies 36Mhz from 226 to 262 Mhz. 120 channels occupy each channel group giving 300kHz bandwidth to each channel.

Consider device a which wishes to communicate with device b . although the two devices are located physically adjacent to one another the connections are made via the head end translator a device which converts incoming signals at one frequency to outgoing signals at another frequency. Device a transmits on to the channel at f1 hertz and the signal travels to the head end translator where it is shifted in frequency to f2 Hz and transmitted on the out channel to be received by device b.

Comparison of baseband and broadband systems

The two types of network address quite distinct needs. The baseband system is more suited to data applications in small and medium networks. Broadband is used over longer distances where a variety of applications are required.

Baseband

Advantages

Simple to install and maintain

Supports any technology.

Supports high data rates.

Disadvantages

Limited transmission distances

Broadband

Advantages

Long distance transmission 10's of KM

High Capacity

Supports different traffic Voice, Video Data

Disadvantages

Difficult to install and maintain.

Experienced RF staff required to service equip

Propogation times longer (reduced efficiency)

Network topologies

Bus/Tree Topology

This consists of a single main transmission path which is known as the bus. For one device to communicate with another it must gain exclusive access to the bus. The difficulty is that a medium or chanel sharing mechanism must be employed.

The bus needs to be terminated at each end to prevent signals being reflected down the line. The bus structure is often extended to the tree structure to make it more flexible. Each interconnecting bus is called a segment.

Further problems arise if the bus is damaged the entire system fails. One terminal on the bus can become faulty and continually occupy the bus preventing other terminals from gaining access.

Ring / Star Topology

The basic ring structure is closed and the signals put onto it must be removed by the initiating device after the circuit is completed allowing other devices to gain access. The ring can be considered a series of point to point links but devices must be granted the authority to use it. Devices attached to the ring regenerate and repeat the data as it passes only copying the data if it is addressed to them.

Problems with Ring Topology include. If there is a break I the medium there is no return path and the system will fail. This problem has been overcome by providing a distribution panel. A fault in a network card can also cause a system failure.

Medium Access Techniques

How the LAN's access the medium is important to the overall LAN performance. There are two basic schemes Contention in which all devices contend with each other to use the medium and non-contention where a device is given permission to use the medium. I will consider the two main strategies from which most of the other variants are derived.

CSMA/CD A contention protocol for LAN's

This is Carrier sense multiple access collision detect and is the most popular contention protocol.

The following are the features

  • Nodes listen for a carrier
  • Nodes transmit only when no energy is detected
  • Nodes transmit and continue to listen for a collision
  • If a collision is detected the node sends out a jammer signal to reinforce the collision thus ensuring all nodes detect it and abort transmission sequences.
  • Node waits a random time (listening at the same time) before attempting a re-transmission.
  • If further collisions occur the random wait time of a node is increased and the node tries again. There is a maximum numner of times a node is allowed to attempt retransmission.

CSMA/CD is used in the widely used and extremely popular ETHERNET LAN

The Ethernet lan is specified by IEEE 802.3

The specification includes the following

  • Based upon a single length of 50 Ohm Co-ax
  • Maximum length of a cable segment is 500m
  • Overall the maximum separation between two nodes is 2.5km. this is possible by the use of repeaters with no more than two repeaters between any two nodes
  • Up to 1024 nodes are permitted with no more than 100 on any one segment.
  • Point to point link between receivers cannot be more than 1km
  • The minimum separation between taps on a segment is 2.5 meter
  • Nodes are to be connected to the cable by transceiver cables which are to be no more than 50m
  • Access method onto the lan is by contention and is refered to a CSMA/CD
  • The data encoding is split phase or manchester encoding..

TAP and TRANSCEIVER

This performs the physical connection between the devices and the medium. In tihch Ethernet it consists of electronic components and a metal pin which penetrates the cable to make contact with the inner conductor.

The functions of the TAP and TRANCIEVER are:-

  • Transmission and reception of data.
  • Sensing traffic (carrier) on the medium
  • Monitoring the medium for collisions
  • Provision of an electrical interface and protection against malfunctioning nodes.

NETWORK INTERFACE UNIT

Provides interface between bus and device wishing to use it:

It's functions are

Encode and Decode

Purpose to encode the binary data to produce a binary signal with zero d.c. component. It also produces the 64 or 32 bit pattern which precedes the frame providing synchronising information.

Link Management and Data formatting

  • Provides the (MAC) medium access control
  • Provides error detection using CRC but takes no remedial action. Errors are passed to upper levels.
  • The frame (Ethernet packet) is constructed here.
  • The minimum length of the packet is 72 octets.

Interfacing.

Provides the electrical interface between the bus and the communications device.

Token Ring Lan(A non-contention protocol for a ring lan)

There is a special frame passing around the lan containing a token. This frame circulates around the network and a station receives permission to transmit when it is in possession of the token. Once the transaction is completed the token is released for use by another station. This is usually used in ring lans. Connection to the lan is usually by means of a distribution panel. The ring forms a point to point connection between adjacent stations on the network. The stations are linked by screened twisted pair cables and can operate at either 4Mb/s or 16Mb/s.

Token ring passing is covered by IEEE 802.5 and encompasses layer 1 and 2 of the ISO 7 layer model.

A station can be in one of two states. It can be either transmit or repeat. In the transmmit mode it sends out it's own message when permitted to do so. While in the repeat state it outputs a received frame bit by bit onto the ring.

When a station requires to send information the MAC unit (medium access unit) encapsulates the data and awaits a token. This lan works on a priority basis so the station assigns a priority to the message. Only when a token of less than or equal priority is received is the encapsulated frame transmitted.

When a token arrives at the station the MAC unit reads it's priority and if higher than the frame awaiting transmission then it replaces the priority with that of the waiting frame.

When the frame circulated the ring the sending MAC unit removes the transmitted frame from the ring.

It is possible to send more frames provided they satisify the above requirements and the maximum holding time for a token is not exceeded. Finally a new token frame is generated and forwarded on the ring to allow other stations to transmit.

There are three types of frame

  • Token
  • Data or Message
  • Control

Ring Network.

Star Network