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OSI Physical Layer

Network Fundamentals – Chapter 8

Dr. C. BouSaba

OSI Physical layer � OSI model layer 1

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Transport

Internet

Network Access

TCP, UDP

Ethernet,

WAN

technologies

HTTP, FTP, TFTP, SMTP

etc

Segment

Packet

Frame

Bits

Data

stream

� TCP/IP model part of Network Access layer

Physical layer topics

� Physical layer protocols and services.

� Physical layer signaling and encoding.

� How signals are used to represent bits. Characteristics of copper, fiber, and wireless media.

� Describe uses of copper, fiber, and wireless network media.

Physical layer tasks

� Takes frame from data link layer

� Sees the frame as bits – no structure

� Encodes the bits as signals to go on the medium

Physical layer tasks

Physical Layer Protocols & Services

Ways to Represent a Signal on the Medium

Physical Layer Protocols & Services

Physical layer standards define:

� Physical and electrical properties of the media

� Mechanical properties (materials, dimensions, pinouts) of the connectors and NICs

� Bit representation by the signals (encoding)

� Definition of control information signals

Physical Layer Protocols & Services

Set by engineering institutions

� The International Organization for Standardization (ISO)

� The Institute of Electrical and Electronics Engineers (IEEE)

� The American National Standards Institute (ANSI)

� The International Telecommunication Union (ITU)

� The Electronics Industry Alliance/ Telecommunications Industry Association (EIA/TIA)

Encoding and

signalling

� This can be relatively simple at very low speeds with bits being converted directly to signals.

� At higher speeds there is a coding step, then a signalling step where electrical pulses are put on a copper cable or light pulses are put on a fibre optic cable.

R e c o g

n iz

in g F

ra m

e S

ig n a

NRZ - non return to zero

� A very simple signalling system

� 1 is high voltage, 0 is low voltage

� Voltage does not have to return to zero during each bit period

NRZ problems

� A long string of 1s or 0s can let sender and receiver get out of step with their timing

� Inefficient, subject to interference

� Straightforward NRZ is not used on any kind of Ethernet, though it could be used if combined with a coding step

Manchester encoding

� Voltage change in the middle of each bit period

� Falling voltage means 0, Rising voltage means 1

� Change between bit periods is ignored.

Manchester encoding

� The transition (up or down) matters, not the voltage level

� The voltage change in the middle of each bit period allows the hosts to check their timing

� 10 Mbps Ethernet uses Manchester encoding (on UTP or old coaxial cables)

� Not efficient enough for higher speeds

Two steps

� Ethernet varieties of 100Mbps and faster use a coding step followed by converting to signals.

� Bits are grouped then coded.

� E.g. bits 0011 could be grouped and coded as 10101 (4-bit to 5-bit, 4B/5B). Each possible 4-bit pattern has its own code.

� This adds overhead but gives advantages

Advantages of group and code

� Control codes such as “start”, “stop” can have codes that are not confused with data

� Codes are designed to have enough transitions to control timing

� Codes balance number of 1s and 0s – minimise amount of energy put into system

� Better error detection – invalid codes are recognised

100 Mbps Ethernet on UTP

� 100 Mbps Ethernet uses 4B/5B encoding first

� It then uses MLT-3 to put the bits on the cable as voltage levels

� 1 means change, 0 means no change

100 Mbps Ethernet on fiber

� 100BaseFX Ethernet uses 4B/5B encoding first

� It then uses NRZI (inverted) encoding to put flashes of LED infra red light on a multimode fiber optic cable

� 1 means change, 0 means no change

Gigabit Ethernet on UTP

� Uses a complicated coding step followed by a complicated scheme of putting signals on the wires, using 4 wire pairs.

Digital Bandwidth

� The amount of data that could flow across a network segment in a given length of time.

� Determined by the properties of the medium and the technology used to transmit and detect signals.

� Basic unit is bits per second (bps)

� 1 Kbps = 1,000 bps, 1Mbps = 1,000,000 bps 1 Gbps = 1,000,000,000 bps

Throughput and Goodput

� Throughput is the actual rate of transfer of bits at a given time

� Varies with amount and type of traffic, devices on the route etc.

� Always lower than bandwidth

� Goodput measures usable data transferred, leaving out overhead. (headers etc.)

Bandwidth, Throughput, and Goodput

Media

� Copper cable (twisted pair and coaxial)

� Fiber optic cable

� Wireless

Physical Media: Characteristics

Coaxial cable

� Central conductor

� Insulation

� Copper braid acting as return path for current and also as shield against interference (noise)

� Outer jacket

Connectors for coaxial cable

Coaxial cable

� Good for high frequency radio/video signals

� Used for antennas/aerials

� Used for cable TV and Internet connections, often now combined with fibre optic.

� Formerly used in Ethernet LANs – died out as UTP was cheaper and gave higher speeds

Unshielded twisted pair (UTP) cable

� 8 wires twisted together into 4 pairs and with an outer jacket.

� Wires have color-coded plastic jackets

� Commonly used for Ethernet LANs

Basic Characteristics of UTP cable

RJ45 connectors

Plugs on patch

cables (crimped)

Sockets to terminate installed cabling (punch down)

Straight through cable

� Both ends the same

� Connect PC to switch or hub

� Connect router to switch or hub

� Installed cabling is straight through

Crossover cable

� Wire 1 swaps with 3

� Wire 2 swaps with 6

� Connect similar devices to each other

� Connect PC direct to router

Rollover cable

� Cisco proprietary

� Wire order completely reversed

� Console connection from PC serial port to router – to configure router

� Special cable or RJ45 to D9 adaptor.

UTP cable

� EIA/TIA sets standards for cables

� Category 5 or higher can be used for 100Mbps Ethernet. Cat 5e can be used for Gigabit Ethernet if well installed.

� We have Cat 5e. A new installation now would have Cat 6.

� The number of twists per metre is carefully controlled.

Shielded twisted pair (STP)

� Wires are shielded against noise

� Much more expensive than UTP

� Might be used for 10 Gbps Ethernet

Noise

� Electrical signals on copper cable are subject to interference (noise)

� Electromagnetic (EMI) from device such as fluorescent lights, electric motors

� Radio Frequency (RFI) from radio transmissions

� Crosstalk from other wires in the same cable or nearly cables

Avoiding noise problems

� Metal shielding round cables

� Twisting of wire pairs gives cancelling effect

� Avoiding routing copper cable through areas liable to produce noise

� Careful termination – putting connectors on cables correctly

Fiber optic cable

� Transmits flashes of light

� No RFI/EMI noise problem

� Several fibers in cable

� Paired for full duplex

Single mode fiber optic

� Glass core 8 – 10 micrometres diameter

� Laser light source produces single ray of light

� Distances up to 100km

� Photodiodes to convert light back to electrical signals

Multimode fiber optic

� Glass core 50 – 60 micrometres diameter

� LED light source produces many rays of light at different angles, travel at different speeds

� Distances up to 2km, limited by dispersion

� Photodiode receptors

� Cheaper than single mode

Characteristics of Fiber Optic Cable

Fiber optic connectors

Straight tip (ST) connector single mode

Subscriber connector (SC) multimode

Single mode lucent connector Multimode lucent connector

Duplex multimode lucent connector (LC)

Which cable for the LAN?

100km or 2km

No noise problems

Within/between buildings

More expensive

Harder to install

Max 100 m length

Noise problems

Within building only

Cheaper

Easier to install

Fiber opticUTP copper

Testing cables

Fluke NetTool for twisted pair cables

Optical Time Domain Reflectometer (OTDR) for fiber

optic cables

Wireless

� Electromagnetic signals at radio and microwave frequencies

� No cost of installing cables

� Hosts free to move around

Wireless access point Wireless adaptor

Wireless problems

� Interference from other wireless communications, cordless phones, fluorescent lights, microwave ovens…

� Building materials can block signals.

� Security is a major issue.

Wireless networks

� IEEE 802.11 - Wi-Fi for wireless LANs. Uses CSMA/CA contention based media access

� IEEE 802.15 - Bluetooth connects paired devices over 1 -100m.

� IEEE 802.16 - WiMAX for wireless broadband access.

� Global System for Mobile Communications (GSM) - for mobile cellular phone networks.