The Physical Layer

Note on 16 Jul 2002
Topologies - Bus & Ring

• Bus is a network topology in which all nodes connect to the network via a central cable called the bus. The bus acts as the shared communication medium that the devices are attached to. Any device that wants to communicate with other device on the network will send its data over the bus which will be send to all attached devices but the intended recipient will only process that packet. Thus bus topology is good and easy to setup for only a small number of devices, as devices and network utilization increases the performance issues and problems arise. If the bus is damaged then the whole network fails making bus topology a less preferred option.
• In a Ring topology every device/node is connected to exactly two other nodes one on either side of it in closed loop fashion. All messages travel though the ring either in a clockwise direction or anti-clockwise direction. Ring topology is very rarely used today because they are expensive, difficult to install and manage. A failure in any single connection disrupts the ring topology thus also making ring topology a rare choice for network topologies.

Bus

Ring

Topologies - Fully & N-connected

• A fully connected network is a communication network in which each of the nodes is connected to each other. In graph theory it known as a complete graph. A fully connected network doesn't need to use switching nor broadcasting. However, its major disadvantage is that the number of connections grows quadratically with the number of nodes, per the formula c=n(n-1)/2, and so it is extremely impractical for large networks. A two-node network is technically a fully connected network.
• Responsibilities
  Defines what a bit is and isn't
  Sends streams of bits from sender to receiver, possibly via other machines

Fully & N-connected

Topologies - Star & Hubs

• Star topology is the most common topology and is the widely implemented. In a Star Topology every device is connected to a central device such as a switch. Star topology requires more cable as compared to other topologies but it mode robust as a failure in one cable will only disconnect the specific connected computer via that cable to the central device. The messages between systems will always flow via the central device and so if the central device fails the entire network will fail. Star topology is very easy install, manage and troubleshoot making it the most common topology in home and office networks

Star

Star & Hub

Topologies - Channel bonded

• Simple way of increasing the bandwidth for any of the previous topologies - stick in extra network card/TA/Modem
• Useful when the next fastest technology is expensive
• Requires rewriting of Data layer

Switching

• In higher connected topologies there is a need to determine routes for signal
• Two main methods of doing this
  - Packet Switching
  - Circuit switching

Circuit Switching

• Old fashioned telephone exchange
• Dedicated "copper" path
• Fixed bandwidth
• Wasted bandwidth
• Standard route
• Time required to setup call
• Congestion can occur at setup time - not during transmission
• Charging is per time interval

Packets Switching

• Modern digital exchange
• No dedicated path
• Bandwidth assigned dynamically
• Little or no bandwidth is wasted
• information must be routed - no standard path
• Connection time is virtually nil
• Congestion can occur at any time
• Charging is per quantity of information sent

Transmission Media

• Sneakernet
• Open wire
• Multi-wire
• Twisted pair
• Coaxial
• Fibre
• Wireless

Factors affecting transmission media

• Distance
• Rate
• Interference
• Cost

sneakernet

• Physical transport of magnetic/optical media
• Extremely high bandwidth
• Extremely poor latency
• Cheap

Open wire

• Standard serial link - simplest form of communication
• 2 wires connected to each device
  
  • Carries signal
  • Carries ground/reference
• Modest but rate < 19.2kb/s
• Care needed to avoid crosstalk caused by
  
  • capacitive coupling
  • electromagnetic interference

Multi-wire

• Standard parallel connection
• N*8 data wires + common ground
• Faster bit rate than serial, but expensive
• More caare must be taken to avoid crosstalk
• Common variant is to for each data wire to have own ground (e.g. UltraDMAxxx)

Twisted pair

• Similar to open wire, but wires are twisted into helix
• commonly used in POTS and ether
• 100m to 1km, 1-100mb/s

Coaxial

• Two common types
  • baseband (digital)
  • broadband (analogue)

Baseband Coax

• Used to be very common (thick and thin ether), but largely replaced by fibre and UTP
• Mainly used now as LAN backbone
• Up to 2Gb/s for up to 1km

Broadband Coax

• Very common (large existing network from cable TV) - use analog transmission on standard cable TV cabling, braodband > 4kHz
• Capable of transmitting up to 1Gb/s for up to 100km without repeaters
• Two main types
  • dual cable - 2 indentical cables running in parallel. Cable 1 transmit data with head-end at the root of cable. Then head-end send back the signal to cable 2.
  • single cable - low frequency band used for communication from computer to head-end and high frequency band to re-broadcast
• Diiferent between baseband and broadband: Broadband need amplifier to strengthen the signal and only transmit in one direction

Fibre

• also called as optical fiber cable, is a type of Ethernet cable which consists of one or more optic fibers that are used to transmit data. Fiber optic cable transmits data as pulses of light go through tiny tubes of glass. The transmission capacity of optical fiber cable is 26,000 times higher than that of twisted pair cable.
• Extremely high bandwidth (in the order of 100's of GB/s) for 10's km
• Small size means that many fibres (100's) can be bunched together
• Needs light source (preferably coherent) & light detector
• For long stretchs also needs repeaters

Copper vs Fibre

	Copper	Fibre
Security	Easy to tap	Extremely difficult to tap
Interference	Susceptible	not susceptible
Weight	Heavy	Light
Bandwidth	Medium	High
Wire cos	Reasonable	Reasonable
Connection cost	Cheap	Expensive
Direction	Bi-directional	Unidirectional

Wireless

• Extremely useful in mobile situations - no need to lay cable
• Can also pass through material obstacles

Radio

• Limited bandwidth
• Congested
• Capable of travelling long distances (certain frequencies can be bounced off ionosphere)
• Restricted usage

Microwave

• Higher bandwidth than radio
• Extremely directional
• Susceptible to weather
• Potentially dangerous
• Relatively cheap
• Congested & restricted

IR

• Reasonable bandwidth
• Extremely poor penetration ability (limited reflection ability)
• Unregulated
• Cheap
• Not suitable for outdoors

Visible light

• Can be used similar manner to microwave
• Affected by weather
• Potentially dangerous
• Uncongested & (partially) unregulated
• Fibre is better (where possible)

UV, X-Ray & Gamma

• UV- possible to use in fibre, but gains often not worth risks
• Gamma & X-Ray
  • Too dangerous
  • Difficult to send and receive signals
  • Difficult to focus

Science Fiction?

• Recent experiments have shown that entanglement can be used to transmit information
• Transmission is probably instantious
• So far to complex to be used in networks, but in the future...

RS232C

• Standard PC Serial cable
• Used to connect modems & computers together
• Maximum transmission rated is stated at 9.6kb/s for 15m (often ignored when connecting peripherals to computers - why?)
• Signal levels: > +3V = 0, < -3V = 1

Telephone

• PSTNs are good example of existing networks
• PSTNS consist of 3 main components
  • Local loops
  • Trunks
  • Switching centres
• PSTNS use a mixture of analogue and digital technology

Modem

• Digital signals are unsuited to sending down analogue lines
• Signal has to be modulated before entering line and demodulated at the other end
• Main types of modulation are
  • amplitude - 2 different voltage levels used to represent O & 1
  • frequency - 2 different tones used
  • phase - Systematically shifted 45, 135, 225/315 degree at uniformly spaced intervals
• These can be combined to give greater bandwidth

ISDN

• Data requirements have meant demand for digital to digital links
• These have previously been met in the form of ISDN (Integrated Services Digital Network)
• 2 main ISDN channels
  • B - 64kb/s
  • D - 16kb/s
• 2 main type of ISDN
  • Basic rate (2*B, 1*D)
  • Primary rate (30*B, 1*D)

xDSL

• New technology with many variants
• 2 main variants are
  • (R)ADSL
  • SDSL
• Allows high(ish) bandwidth on POTS (provided not too far from exchange)
• Also provides benefit of "always on"

Mobile phones

• Becoming increasing common
• Operates on radio/microwave frequencies
• Uses cell structure to maximise bandwidth and minimise interface
• Suitable for voice or limited data transfer
• New technologies (3G) said to improve speed, but display is still and issue
• One major problem is different system in Europe & in USA

ATM

• High speed point to point packet switched network
• Commonly used to link sites requiring high bandwidth links (e.g. different campuses)
• Normally uses fibre, but may use UTP & coax
• Capable of transfer rates of 155Mbps or 622Mbps

Cable

• Normally used for video, but increasingly being used for data transfer
• Capable of high speed (often approaching LAN) communication
• Restricted availability
• May by unidirectional or restricted unidirectional

Satellite

• Increasingly common for high bandwidth
• Normally only incoming data is sent via satellite - outgoing data sent via modem
• Extremely useful for multicast data (eg. video, usenet)