Physical Layer
Dr John C. Murray Principal Lecturer
The Physical Layer
• Last week we looked briefly at the OSI Model
– 7 Layers that detail communication – Protocols
– Standards
• Physical Layer is the basis of the OSI model
– In network communication everything goes over the physical layer
Recommended Reading
• Computer Networks
• 5th Edition
• Andrew S Tanenbaum
• David J Wetherall
• PDF Version on Blackboard
Physical Layer
• Foundation on which the network is built
• The properties of different kinds of physical channels determine the performance
– Throughput, latency, error rate, etc.
Guided transmission media
• Physical layer purpose – to transport bits from one place to another
– Node to node
• Various physical media can be used
– Each has its own niche in terms of: • Bandwidth
• Delay
• Cost
• East of installation • Maintenance
Guided transmission media
• We need to understand and appreciate differing Media types available to us
– Physical/Wireless Media is required
• To transfer “signals” from one place to another
• To impose a structure – often by the structure the media places upon the designer and installer of a network
• Data
– Although a given media is used as the transfer agent, it is the MEDIA PROTOCOL that defines the process and characteristics of the data transfer
– In other words; some media types can be used to support multiple different media protocols
Guided transmission media
• Structure
– Physical media can impose structure
– That structure is called TOPOLOGY
• Review topologies such as Star, Ring, Bus, Mesh from previous weeks
– Structure – and Media Protocol often define the limitations and expectations of a Network in terms of:
• Number of Nodes that can operate within the network • Distance over which the network will operate
• Bandwidth that the network can support
Bandwidth
• Bandwidth is a measure of the amount of data that can be sent over a network connection – it indicates the maximum transmission capacity.
• Imagine a water hose – the water flowing through the hose is the data, and the size of the hose is the bandwidth. The bigger the opening, the more water can flow through at any one time.
• Bandwidth might incorrectly be referred to as the amount of data transmitted. This is actually “data transfer,” and in the water-hose metaphor, it would be the total volume of water that went through the hose.
Broadband
• A broadband connection is one that allows data to be sent on multiple channels simultaneously. This broadens the available bandwidth.
– Measurement:
• Bandwidth is expressed in terms of how much data can be transmitted per second.
– Units
• Units of bandwidth – order of magnitude:
• Bits per second (bps)
• Kilobits per second (kbps)
• Megabits per second (mbps)
• Gigabits per second (gbps)
• Terabits per second (tbps)
– Each unit is 1,000 times larger than the one that precedes it.
Physical Media
• COPPER CABLE
– Most common form of network media
• Relatively cheap to install
• Easy to modify and manage
• Has the ability to support multiple Media Protocols
• Can support relatively high speed (high bandwidth) protocols
– 1Gbps over short distances
– Two distinct and common forms exist • Coaxial Cable
• Twisted Pair
Coaxial
Coaxial • Copper Cable
Sometimes multiple layers of “shielding” are provided particularly when very high frequencies are being used, or where extra shielding from interference is required
Coaxial
• Thin Ethernet (or sometimes called cheaper net) – http://en.wikipedia.org/wiki/10BASE2
• Thick Ethernet (a much sturdier construction, very thick cables)
• Satelliteinterconnections
• RFconnections(Radio,TVandMicrowave)
• Token Ring
– The only differences from one coaxial type to another are:
• Frequencyrangesupported(capacitanceandresistanceofcables and connectors)
• Physical diameter of construction
• Materialsusedininsulationandshieldingandconductor
– Which impose “properties” of the cable which effect the type and nature of the signals that can be transmitted down them
– Uses include:
Twisted Pair
• COPPER CABLE
• Twisted Pair
– Individual strands of copper wire, sheathed in PVC for insulation
– Paired and twisted with a second conductor
– Multiple pairs possibly included in construction
• Each pair can be individually earth shielded using braiding or “tin foil” – Which is often referred to as Shielded Twisted Pair (STP) as opposed to Unshielded Twisted Pair (UTP)
• A bundle of multiple pairs shielded using braiding or “tin foil”
• A bundle of multiple pairs without any shielding at all
– Designed for low bandwidth applications such as RS232 or Voice
– Copper cable in twisted pair format is classified using the “Category System”
• Cat I, Cat II, Cat III, Cat IV, Cat V, Cat Ve and Cat VI
• The higher the category – the greater the specification
Twisted Pair
– The category status depends on:
• The quality of the cores
• The size of the cores (according to the American Wire Guage – AWG)
• Thethicknessoftheinsulationaroundtheindividualcores
• The degree of twist (the tightness of the twist)
– The specification for categories defines:
• Howahighfrequencysignals“behave”travellingalongthecable
• The distance it will travel within a tolerance of cross-talk and
attenuation
– Cross talk is where impulses of electricity down one cable effect through “induction” another, and effectively regenerate the same (although weaker) signal in other physically close cables
• Twisted Pair
• Twisted Pair
Twisted Pair
– The higher the frequency (the greater the bandwidth) the higher the specification of cable required
– The longer the distance the higher the specification
– Simple very old telephone lines – Cat I, Cat II
– More modern telephone lines – Cat II and Cat III
– Basic data cables (serial data cables for example) – Cat IV
– Speed (100Mbps) – Cat V
– High speed (1000Mbs) – Cat Ve (enhanced)
– Very High Speed (1Gbps – 10Gbps) Cat VI
– For Cat V and VI special connectors are used to ensure high quality interconnection of nodes to cables (reducing possible signal loss and interference)
Twisted Pair
http://www.ertyu.org/steven_nikkel/ethernetcables.html
How to wire Ethernet Cables
Twisted Pair
Physical Media • Cross-over & Straight-through
NODE NODE NODE
Fibre Optic
– uses light pulses to transmit data. It operates over large distances:
• Single-mode fibre: transmits data at 100Gbps for 100km without the signal being repeated.
• Multi-mode fibre: transmits data at 100mbps for 2km without the signal being repeated.
– At one end of a Fibre optic is a transmitter which converts electric signals from copper wire (standard network), into light pulses. Immune to electrical interference and does not suffer from crosstalk.
• Fibre optic
Fibre Optic
• Each Fibre can carry many independent channels, with each using a different wavelength of light. It is more difficult to hack:
a) because there are no electrical currents running through Fibre optics they can be used in dangerous environments e.g. highly explosive fumes, without risk of ignition.
b) Fibre optic is harder to wiretap (physically hack).
Fibre Optic
Fibre Optic
– A glass fibre is composed of two parts:
• An inner core – used for transferring the light signal
• An outer sheath – used to “trap” the light signal in the inner core
– Additionally
• Individual cores are usually given a thin surface layer of coloured plastic for identification
• Cores are bundled (4, 8, 12, 16, 24 and more) into a single PVC tube. The tube is usually filled with a water resistance oil jelly for added protection (a cushion)
• Outer PVC tube is usually Blue, Green, Violet or Black depending on the installation
• Optic Fibre
Fibre Optic Cable
Fibre Optic
– Providing the light signal is within a “critical angle” – the angle of incidence to the boundary of the two glass edges, it will be “reflected”
• If it is outside the critical angle – it will escape the glass fibre and be lost
• Optic Fibre
Fibre Optic
• Fibre-optic cables usually have a minimum bend radius of 3.0 cm.
• If the cable’s bent more than this, the fibre core can develop micro-fractures, real fractures, or severely leak light.
• As it’s the light that’s carrying the network data, a loss of light means a loss of information and network errors.
Fibre Optic
• Theoretical speeds of 50,000 Gbps can be achieved using fibre
– The faster the speed of data, the smaller the pulse widths and the distance between each pulse
• Practical limits of several Gbps per second are the reality
– The light source and the receiver restrict the speed
– Imperfections in the glass or “connections” can cause losses and reflections
– Converting light to electrical signals incurs limitations
• 100Mbps over approx 2Km is practical using Multi-mode fibre
• 100Gbps over 100 Km is practical using Mono-mode fibre
• Multi-mode fibre is cheaper to manufacture than Mono-mode
• Multi-mode fibre is easier to interconnect and join than Mono-mode fibre
• Low cost laser transmitters can be used for Multi-mode
• High cost laser transmitters are used for Mono-mode (smaller aperture is required since it is a smaller fibre)
– The issue is purely one of mechanics – connectors for fibre are precision and tend to be quite expensive for mono-mode fibre
Physical Layer
• Involves the actual physical medium – Used in the transfer of messages
– Most basic network layer
– Actual signal used varies depending on types of medium
• Electrical modulation • Radio Waves
• Optical Signals
– In theory ANY medium can be used • RFC 1149
List of Services
• The Physical Layer is Responsible for:
– Bit-by-bit delivery
– Providing a standardised interface to the medium – Modulation
– Line Coding
– Flow Control
– Multiplexing
– Circuit Switching
– Forward Error Recovery
– Carrier Sense
– And many more
Bit-by-Bit Delivery
• Symbol rate or Baud Rate
– Different from bps
– Symbol is a pulse or tone that represents the data
• PHY Layer places these symbols on the medium at a fixed / known rate.
– A symbol may encode one or several bits of
data.
T S
1
fS
Modulation
• The process of modulating a signal onto a carrier
Signals
• signals can be analogue or digital;
• analogue signals vary continuously and can take any value within some given range;
• digital signals are chosen from a discrete (limited, finite) range of possibilities, e.g. {0,1} or {long, short} or {A – Z, 0 – 9}
CS15210:
Comms & 32 Telematics
Voltage
Analogue signal
Time
CS15210:
Comms & 33 Telematics
Voltage
Digital signal
Time
CS15210:
Comms & 34 Telematics
How signals get damaged attenuation
dispersion
distortion
CS15210:
Comms & 35 Telematics
Modulation
• Several Modulation Methods
– PSK – Phase Shift keying
• A finite number of phases are used
– FSK – Frequency Shift keying
• A finite number of frequencies are used
– ASK – Amplitude Shift keying
• A finite number of amplitudes are used
Frequency Shift Keying
Flow Control
• Manages the data rate between sender and receiver
– Prevents a slow receiver being overloaded by fast sender
• Stop and Wait
– Simplest flow control
– Receiver says ‘ready’ – ACK
• For each frame to be sent
• Must be received before timeout
Flow Control
1) Sender: Transmits a single frame
2) Receiver: Transmits ACK as receives frame
3) Sender receives ACK • Within timeout
4) Goto 1
• Algorithm
Multiplexing
Multiplexed
Cable
De-Multiplexed
Frequency-Division Multiplexing
Time-Division Multiplexing
Questions