Lecture Notes for Telecommunications

29 September 2004 - The Physical Layer

The Physical Layer

• The physical layer is responsible for moving bits from here to there.

  • The physical layer abstracts the transmission medium for the data-link layer.

• Given two systems connected by a medium, how can you send bits from one system to the other?

Connection Media

• Different media, with end-point electronics and optics, determine media properties.

  • And that effects how you send data.

• Of what does this intra-system medium consist?

  • Shoe leather (sneaker net).

  • Twisted pair (The category or cat cable).

  • Coaxial cable (coax).

  • Optical fiber.

  • Radio waves.

  • Infrared.

Physical-Layer Properties

• Encodings turn data (bits) into signal.

• Simultaneous data flow is determined by "duplexivity".

  • Half-duplex is one-way data flow.

  • Full-duplex is two-way data flow.

• Data rates measure information per unit time.

• Operating constraints under which the medium performs.

Link Examples

Medium	Bandwidth	Distance
Cat 5 twisted pair	10-1,000	100 m
50-ohm coax (thinwire)	10-100	200 m
75-ohm coax (thickwire)	10-100	500 m
Multimode fiber	100	2 km
Single-mode fiber	100-2,400	40 km

Physical-Layer Services

Service	Bandwidth
B-channel ISDN	64 Kbps
T1 (DS1)	1.544 Mbps
T3 (DS3)	44.736 Mbps
STS-3 (OC-3)	115.251 Mbps
STS-12 (OC-12)	722.08 Mbps
STS-24 (OC-24)	1.24416 Gbps
STS-48 (OC-48)	2.28832 Gbps

Data Rates

• The principle measure of media is data rates.

• But what's the units: bits or bauds or Hertz?

  • Alternatively, are we measuring data or signal or bandwidth?

• The bandwidth of the medium measured in Hertz.

• Signal modulations are measured in bauds.

  • The medium bandwidth limits the signal baud rate.

• A set of signal modulations represent a bit.

Signals and Uncertainty

• It begins with a wiggle.

  • A wiggle has phase, amplitude, and frequency.

• To communicate, we have to introduce uncertainty, and then resolve it.

• We can be uncertain about a wiggle's phase, amplitude, or frequency.

  • To keep it simple, we'll be uncertain about one thing at a time.

  • Modems are uncertain about phase and amplitude.

Signals and Modulation

• Adding uncertainty to a signal leads to either

  • phase shift keying (PSK),

  • amplitude shift keying (ASK), or

  • frequency shift keying (FSK).

The Bit Pipe

• The physical layer defines signal levels and timings so that it can deliver a stream of bits to the data-link layer.

• Signal bandwidth determines data-rate limits.

• Timing errors can occur because

  • medium noise or distortion, and

  • clock drift between source and destination.

Asynchronous Transmission

• Data transmission is synchronized through

  • A start bit.

  • A stop bit.

  • An idle state between stop and start bits.

• Timing starts at the start bit.

• Timing errors accumulate up to one-half the bit time over the character.

Asynchronous Analysis

• Simple timing mechanisms with inherent character framings.

• Idle time allows for data-rate adaption.

• Noisy media results in timing errors.

• Stop and start-bit overhead.

• Overall results in cheap and low-performance data transmission.

Synchronous Transmission

• High data-rate transmission technique.

• Uses continuous data transmission to maintain synch.

  • Idle characters are sent in the absence of data.

• Not signals, but signal transitions allow synchronization.

  • A minimum transmission frequency maintains synchronization.

Example Synchronous Encodings

• The signal carries the timing data too.

  • Non-return to zero (NRZ).

  • Non-return to zero inverted (NRZI).

  • Manchester encodings.

  • Block codes, such as 4B/5B.

Synchronous Transmission

• Ensuring signal transitions by means of

  • Source code restrictions.

  • Dedicated timing bits.

  • Bit insertion.

  • Data scrambling.

  • Forced bit errors.

  • Line coding to include transitions in the signals.

Synchronous Transmission

• The physical layer delivers a stream of bits to the data-link layer.

Ensuring Signal Transitions

• Source-code restrictions prevent transition-less data.

  • Cannot provide a clear channel.

• Dedicated timing bits.

  • Use some bit transmissions just for timing.

  • Example: Dataphone Digital Service.

    • Use every one of eight bits to guarantee a signal transition.

  • Example: Synchronous modems.

    • Periodically insert a SYNC character.

  • Ethernet uses a timing sequence preamble.

Bit Insertions

• Insert timing bits into the data-bit stream when necessary.

• Bit stuffing.

  • Break up patterns with extra bits, which must be removed at the destination.

• But extra bits means extra delay.

Ensuring Signal Transitions

• Data scrambling breaks up long runs of 0s or 1s.

  • Encryption or compression, for example.

  • This is a probabilistic solution.

• Force a deliberately bad bit to break up runs.

  • The bad bit will be discarded at the destination.

• Line coding.

  • Some line coding methods ensure regular transitions that can be used for clock recovery.

  • FDDI 4B/5B, for example.

This page last modified on 14 November 2004.