COMPUTER NETWORKS

Dr.Amruta Mohite

Dr.Amruta Mohite

PHYSICAL LAYER

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Physical Layer: Introduction

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• Transmitting raw bits over a “wire”
• Make sure a “1” bit is sent as a 1
• EE/ECE problem:
• How many volts represents a “1” or “0”?
• How long does a bit time last?
• How many pins does the connector have?
• How many wires does the transmission media have?
• Are pulses electrical or optical or waves?

Physical Layer : MULTIPLEXING

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• Under the simplest conditions, a medium can carry only one

signal at any moment in time.

• For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth.
• The current techniques that can accomplish this include frequency division multiplexing, time division multiplexing, and code division multiplexing.

Physical Layer:MULTIPLEXING

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• A multiplexer is a physical layer device that combines multiple

data streams into one or more output channels at the source.

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• Multiplexers demultiplex the channels into multiple data streams at the remote end and thus maximize the use of the bandwidth of the physical medium by enabling it to be shared by multiple traffic sources.

MULTIPLEXING: FDM

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• In FDM the frequency spectrum is divided among the logical channels, with each user having exclusive possession of some frequency band.

MULTIPLEXING: FDM

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• One problem with FDM is that a user is given all of the frequency

to use, and if the user has no data to send, bandwidth is wasted -

- it cannot be used by another user.

• AM radio broadcasting is example of FDM. The allocated

spectrum is about 1 MHz, roughly 500 to 1500 KHz.

• Different frequencies are allocated to different logical channels ( stations ),
• each operating in a portion of the spectrum, with the interchanel

separation great enough to prevent interference.

MULTIPLEXING: TDM

• Use time slicing to give each user the full bandwidth, but for only fraction of a second at a time (analogous to time sharing in operating systems).
• Again, if the user doesn't have data to sent during his time slice,

the bandwidth is not used (e.g., wasted).

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MULTIPLEXING: TDM

• Use time slicing to give each user the full bandwidth, but for only fraction of a second at a time (analogous to time sharing in operating systems).
• Again, if the user doesn't have data to sent during his time slice,

the bandwidth is not used (e.g., wasted).

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MULTIPLEXING: CDM

• Also known as code division multiple access(CDMA).
• An advanced technique that allows multiple devices to transmit on the same frequencies at the same time.
• Each mobile device is assigned a unique 64-bit code (chip

spreading code).

• To send a binary 1, mobile device transmits the unique code.
• To send a binary 0, mobile device transmits the inverse of code.

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MULTIPLEXING:Me/itS & Deme/itS

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TRANSMISSION MEDIA

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• The world of computer networks s data communications would

not exist if there were no medium by which to transfer data.

• How transmissions flow over media ?

• Simplex :
• Half-Duplex :

• Full-Duplex :

Only in one direction

Travels in either direction, but not both directions at the same time

Can travel in either direction simultaneously

• Two major categories of media include
1. Guided (a physical path)

2.

Unguided (waves propagated, but not in a directed manner)

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MAGNETIC MEDIA

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• Put files on tape, floppy disks
• Physically carry. Example:
• 8mm video tape holds 7 gigabytes
• box 20”x 20”x 20” holds 1000 tapes
• 24 hour delivery via FedEx
• 1000 x 7GB * 8 / (24 * 3600) = 648 Gbps
• 1000 times faster than high-speed ATM High delay in accessing data

TWISTED PAIR

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• Two copper wires are strung between sites
• “Twisted'' to reduce interference
• Can carry analog or digital signals
• Distances of several kilometers
• Data rates of several Mbps common
• wire thickness and length
• shielding to eliminate noise
• Good, low-cost communication

❖ Existing phone lines.

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UNSHIELDED TWISTED PAIR

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• Advantages

Inexpensive

Easy to terminate Widely used, tested

Supports many network types

• Dis-Advantages

Susceptible to interference damage Prone during installation Distance limitations

UNSHIELDED TWISTED PAIR

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BASEBAND COAXIAL

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• Copper core, insulating material (“coax”)
• Baseband indicates digital transmission : as opposed to broadband

analog

• To connect, need to touch core: T junction

• 10 Mbps is typical

Slide: 17

BROADBAND COAXIAL

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• Broadband means analog over coax
• telephone folks mean wider than 4 kHz
• Typically 300 MHz, data rate 150 Mbps
• Up to 100 km (metropolitan area!)
• Inexpensive technology used in cable TV
• Divide into MHz channels
• Amplifiers to boost, data only one-way!
• Dual cable systems (still, root must transmit)
• Midsplit systems divide into two

BROADBAND VS BASEBAND: Which is

Better?

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• Baseband:
• simple to install
• interfaces are inexpensive
• short range
• Broadband:
• more complicated
• more expensive
• more services (can carry audio and video)

OPTICAL FIBER

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• Hair-width silicon or glass
• Signals are pulses of light (digital)
• Ex: pulse means “1”, no pulse means “0”
• Glass “leaks” light?
• Three components required:
• Fiber medium: 100s miles, no signal loss
• Light source: Light Emitting Diode (LED), laser diode, current generates a pulse of light
• Photo diode light detector: converts light to electrical signals

OPTICAL FIBER

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OPTICAL FIBER

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OPTICAL FIBER

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• Advantages:
• Huge data rate (1 Gbps), low error rate
• Hard to tap (leak light), so secure .
• Thinner (per logical phone line) than coax
• No electrical noise (lightning) or corrosion (rust)
• Dis-Advantages:
• Difficult to tap, really point-to-point technology [ training or expensive

tools or parts are required ]

• One way channel [ Two fibers needed for full duplex communicati_So_ln_id]_{e: 23}

OPTICAL FIBER :USES

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• long-haul trunks-- increasingly common in telephone

network (Sprint ads)

• metropolitan trunks-- without repeaters (have 8 miles in length)
• rural exchange trunks-- link towns and villages
• local loops-- direct from central exchange to a subscriber

(business or home)

• local area networks-- 100Mbps ring networks

Slide: 24

WIRELESS

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is controlled by accurate

• Radio transmits at 10KHz to 1KHz
• Microwaves transmit at 1GHz to 500GHz
• Infrared transmits at 500GHz to 1THz
• Radio transmission may include:
• Narrow band
• High-powered
• Frequency hopping spread spectrum(hop timing)

• Direct-sequence-modulation spread spectrum (uses multiple

frequencies at the same time, transmitting data in „chips‟ at high s_Sp_le_ie_ded_:) ₂₆

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