1. Chapter Five Understanding the Physical Layer

2. 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.

3. 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

4. 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

5. 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.

6. Some Encoding Mechanisms Return to Zero (RTZ) Alternate Mark Inversion (AMI) High Density Bipolar Order Three Encoding (HDB3) Manchester

7. 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

8. 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

9. 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

10. 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.

11. Bit Timing and Synchronization Asynchronous communication Synchronous communication

12. 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

13. 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.

14. 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

15. 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.

16. 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.

17. 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.

18. 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

19. 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.

20. 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.

21. 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.

22. Fiber Optics Connectors ST connector Subminiature assembly Mechanical transfer registered jack

23. 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

24. 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