Chapter Five

Understanding the Physical Layer

Objectives

• Here you will see how data is encoded for transmission over media.

• You’ll learn some different communications mechanisms.

• Some of the media we discussed in Chapter 2 will be covered in greater detail.

Reviewing the Functions of the Physical Layer

• Converts the data into bits to send over the medium

• Defines bit encoding, synchronization, and timing

• Defines the physical media and connectors used on the network

Bounded Physical Signaling

• Basically two forms of media– Copper– Fiber optics

• Copper-based signaling involves altering electrical signals

• Fiber optics involves sending timed bursts of light

Data Encoding over Copper

• Digital data needs to be converted to an analog electrical signal.

• Timing is critical so that a device that sends a series of 20 0s isn’t read as having sent 22 0s.

• Synchronization is critical so that lost packets aren’t ignored.

Some Encoding Mechanisms

• Return to Zero (RTZ)

• Alternate Mark Inversion (AMI)

• High Density Bipolar Order Three Encoding (HDB3)

• Manchester

Return to Zero

• Also called pulse signaling

• Now considered obsolete

• Is either a signal or there isn’t

• Presence of a signal interpreted as 1

• Lack of a signal interpreted as 0

Alternate Mark Inversion

• Similar to RTZ except:– A 1 was either positive or negative voltage– Only lack of voltage interpreted as 0

• A good clocking signal hard to maintain so fast throughput not possible

• Now obsolete

HBD3

• Another variation on RTZ that limits the number of 0s in a string

• After a fourth consecutive 0, a violation bit inserted to break the string

• Improves clocking to some extent, but still only useful for low-speed devices

• Does see limited use

Manchester Encoding

• Voltage is used to encode both 0s and 1s.

• A movement toward positive voltage from center is interpreted as a 1.

• Movement toward negative is interpreted as 0.

• A coinciding signal called the digital phase loop locked signal (DPLL) keeps time.

• Most current technologies use variations on Manchester.

Bit Timing and Synchronization

• Asynchronous communication

• Synchronous communication

Asynchronous Communication

• Data transmitted a byte at a time

• May or may not use parity for error detection (but not correction)

• A start bit marks the beginning

• One (or two) stop bits mark the end

• Good for short bursts of data

Understanding Parity

• A byte of data consists of 8 bits plus a parity bit.

• Parity counts the number of 1s in the 8 bits of data.

• If an even number of 1s is detected, a 1 is stored in the parity bit (a 0 if odd).

• On the receiving end, all nine bits are counted.

• An odd number of 1s MUST be detected or a nonmaskable interrupt is generated and the system halts.

Synchronous Communication

• Data transmitted in packets– Header contains protocol and addressing

information– Payload contains user data– Trailer contains error correction information

• Essential for transmitting larger files

Synchronous Error Correction

• Checksum

• Cyclical redundancy check– Regardless of which method is used, if a packet

is determined to be good, an acknowledgment (ACK) packet is issued to the transmitting computer.

Checksum

• All the 1s in the packet are counted and the value is stored in the trailer.

• On the receiving end, the 1s are counted again and compared to value in the trailer.

• If the two values do not match, the receiving computer issues a NACK (no acknowledgement) packet and the data is retransmitted.

CRC

• The data in the packet is treated as a long string of 0s and 1s that represent a VERY large number.

• A mathematical calculation is performed on that number and the results stored in the trailer.

• On the receiving end, the same calculation is performed.

• If the results don’t match, a NACK is issued.

Fiber Optics

• Uses either LED emitters or laser emitters

• Good for extremely long ranges (2KM and up)

• Difficult to hack into without detection

• Not susceptible to environmental interference

Types of Fiber

• Loose tube– Several strands of cable are packed into a single

insulator.– A steel wire provides extra tensile strength.– Interstitial filling provides protection against stress.

• Tight buffered– A single fiber is encased in several layers of protection.

The Optical Transmitter

• Light emitting diode (LED) or laser diode (LD) provides light source.

• Most modern transmitters use pulse width modulation.– 0s and 1s are differentiated by how long a pulse

of light lasts.

Single Mode versus Multimode

• Single mode fiber sends only one signal over one strand of wire.

• Multimode fiber sends several signals over a single strand.– Wavelength division multiplexing separates the

signals into separate channels.– Different light wave frequencies are bounced at

different angles within fiber.

Fiber Optics Connectors

• ST connector

• Subminiature assembly

• Mechanical transfer registered jack

Wireless Signaling

• Radio– 802.11b and 802.11g for private networks

• 802.11b provides up to 11Mb/s in 5Ghz band.• 802.11g provides up to 54Mb/s in 2.4Ghz band.• A wireless access point (WAC) acts as the epicenter

of the network.

– Spread spectrum allows for greater security, but requires licensing

Microwave

• Terrestrial– Line of sight, limited by distance of horizon– Can be extended by putting repeaters at high

elevations• Satellite

– Kind of expensive (not everyone has a spare satellite in their back pocket)

– But provides virtually global coverage