Chapter 1. Introduction to Data Communications

Chapter 1. Introduction to Data Communications

Physical and Data Link Layers Copyright 2005 John Wiley & Sons, Inc 3-1 Physical Layer - Overview Includes network hardware and circuits Network circuits: physical media (e.g., cables) and special purposes devices (e.g., routers and hubs). Types of Circuits Network Layer Data Link Layer Physical Layer Physical circuits connect devices & include actual wires such as twisted pair wires

Logical circuits refer to the transmission characteristics of the circuit, such as a T-1 connection refers to 1.5 Mbps Can be the same or different. For example, in multiplexing, one wire carries several logical circuits Copyright 2005 John Wiley & Sons, Inc 3-2 Types of Data Transmitted Analog data Produced by telephones Sound waves, which vary continuously over time Can take on any value in a wide range of possibilities Digital data Produced by computers, in binary form, represented as a series of ones and zeros Can take on only 0 ad 1 Copyright 2005 John Wiley & Sons, Inc

3-3 Data Type vs. Transmission Type Analog Digital Transmission Transmission Analog Radio, Data Broadcast TV PCM & Video standards using codecs

Digital Data Modem-based communications LAN cable standards Copyright 2005 John Wiley & Sons, Inc 3-4 Digital Transmission: Advantages Produces fewer errors Easier to detect and correct errors, since transmitted data is binary (1s and 0s, only two distinct values)) Permits higher maximum transmission rates e.g., Optical fiber designed for digital transmission More efficient Possible to send more digital data through a given circuit

More secure Easier to encrypt Simpler to integrate voice, video and data Easier to combine them on the same circuit, since signals made up of digital data Copyright 2005 John Wiley & Sons, Inc 3-5 Point-to-Point Configuration Used when computers generate enough data to fill the capacity of the circuit Each computer has its own circuit to any other computer in the network (expensive) Copyright 2005 John Wiley & Sons, Inc 3-6 Multipoint Configuration Used when each computer does not need to continuously use the entire capacity of the circuit

+ Cheaper (no need for many wires) and simpler to wire - Only one computer can use the circuit at a time Copyright 2005 John Wiley & Sons, Inc 3-7 Data Flow (Transmission) data flows move in one direction only, (radio or cable television broadcasts) data flows both ways, but only one direction at a time (e.g., CB radio) (requires control info) data flows in both directions at the same time Copyright 2005 John Wiley & Sons, Inc

3-8 Communications Media Physical matter that carries transmission Guided media: Transmission flows along a physical guide (Media guides the signal)) Twisted pair wiring, coaxial cable and optical fiber cable Wireless media (aka, radiated media) No wave guide, the transmission just flows through the air (or space) Radio (microwave, satellite) and infrared communications Copyright 2005 John Wiley & Sons, Inc 3-9 Twisted Pair (TP) Wires Commonly used for telephones and LANs Reduced electromagnetic interference Via twisting two wires together

(Usually several twists per inch) TP cables have a number of pairs of wires Telephone lines: two pairs (4 wires, usually only one pair is used by the telephone) LAN cables: 4 pairs (8 wires) Also used in telephone trunk lines (up to several thousand pairs) Shielded twisted pair also exists, but is more expensive Copyright 2005 John Wiley & Sons, Inc 3 - 10 Coaxial Cable (protective jacket ) Wire mesh ground Less prone to interference than TP (due to

(shield) used mostly for CATV More expensive than TP (quickly disappearing) Copyright 2005 John Wiley & Sons, Inc 3 - 11 Fiber Optic Cable Light created by an LED (light-emitting diode) or laser is sent down a thin glass or plastic fiber Has extremely high capacity, ideal for broadband Works better under harsh environments Not fragile, nor brittle; Nit heavy nor bulky More resistant to corrosion, fire, etc., Fiber optic cable structure (from center): Core (v. small, 5-50 microns, ~ the size of a single hair) Cladding, which reflects the signal Protective outer jacket

Copyright 2005 John Wiley & Sons, Inc 3 - 12 Optical Fiber Excessive signal weakening and dispersion (different parts of signal arrive at different times) Center light likely to arrive at the same time as the other parts Copyright 2005 John Wiley & Sons, Inc 3 - 13 Wireless Media Radio Wireless transmission of electrical waves over air Each device has a radio transceiver with a specific frequency Low power transmitters (few miles range) Often attached to portables (Laptops, PDAs, cell phones) Includes

AM and FM radios, Cellular phones Wireless LANs (IEEE 802.11) and Bluetooth Microwaves and Satellites Infrared invisible light waves (frequency is below red light) Requires line of sight; generally subject to interference from heavy rain, smog, and fog Used in remote control units (e.g., TV) Copyright 2005 John Wiley & Sons, Inc 3 - 14 Microwave Radio High frequency form of radio communications Extremely short (micro) wavelength (1 cm to 1 m) Requires line-of-sight Perform same functions as cables Often used for long distance, terrestrial transmissions (over 50 miles without repeaters) No wiring and digging required Requires large antennas (about 10 ft) and high towers

Posses similar properties as light Reflection, Refraction, and focusing Can be focused into narrow powerful beams for long distance Copyright 2005 John Wiley & Sons, Inc 3 - 15 Satellite Communications in a geosynchronous orbit A special form of microwave communications Long propagation delay Due to great distance between ground station and satellite (Even with signals traveling at light speed) Copyright 2005 John Wiley & Sons, Inc Signals sent from

the ground to a satellite; Then relayed to its destination ground station 3 - 16 Digital Transmission of Digital Data Computers produce binary data Standards needed to ensure both sender and receiver understands this data Coding: language that computers use to represent letters, numbers, and symbols in a message Signaling (aka, encoding): language that computers use to represent bits (0 or 1) in electrical voltage Bits in a message can be send in A single wire one after another (Serial transmission) Multiple wires simultaneously (Parallel transmission) Copyright 2005 John Wiley & Sons, Inc 3 - 17

Coding A character a group of bits Letters (A, B, ..), numbers (1, 2,..), special symbols (#, $, ..) 1000001 Main character codes in use in North America ASCII: American Standard Code for Information Interchange Originally used a 7-bit code (128 combinations), but an 8-bit version (256 combinations) is now in use EBCDIC: Extended Binary Coded Decimal Interchange Code An 8-bit code developed by IBM Copyright 2005 John Wiley & Sons, Inc 3 - 18 Transmission Modes Parallel mode Uses several wires, each wire sending one bit

at the same time as the others A parallel printer cable sends 8 bits together Computers processor and motherboard also use parallel busses (8 bits, 16 bits, 32 bits) to move data around Serial Mode Sends bit by bit over a single wire Serial mode is slower than parallel mode Copyright 2005 John Wiley & Sons, Inc 3 - 19 Manchester Encoding Used by Ethernet, most popular LAN technology Defines a bit value by a mid-bit transition A high to low voltage transition is a 0 and a low to high mid-bit transition defines a 1 Data rates: 10 Mb/s, 100 Mb/s, 1 Gb/s, .. 10- Mb/s one signal for every 1/10,000,000 of a second (10 million signals (bits) every second)

Less susceptible to having errors go undetected No transition en error took place Copyright 2005 John Wiley & Sons, Inc 3 - 20 Digital Transmission Types Unipolar Bipolar NRZ Bipolar RZ Manchester Copyright 2005 John Wiley & Sons, Inc 3 - 21 Analog Transmission of Digital Data A well known example Using phone lines to connect PCs to Internet

PCs generates digital data Phone lines use analog transmission technology Modems translate digital data into analog signals Internet M Telephone Network Phone line PC M Digital data Analog transmission Central Office (Telco) Copyright 2005 John Wiley & Sons, Inc

3 - 22 Telephone Network Originally designed for human speech (analog communications) only POTS (Plain Old Telephone Service) Enables voice communications between two telephones Human voice (sound waves) converted to electrical signals by the sending telephone Signals travel through POTS and converted back to sound waves Sending digital data over POTS Use modems to convert digital data to an analog format One modem used by sender to produce analog data Another modem used by receiver to regenerate digital data Copyright 2005 John Wiley & Sons, Inc 3 - 23 Sound Waves and Characteristics 90o

Amplitude Height (loudness) of the wave Measured in decibels (dB) 0o 180o Frequency: 270o Number of waves that pass in a second Measured in Hertz (cycles/second) Wavelength, the length of the wave from crest to crest, is related to frequency Phase: Refers to the point in each wave cycle at which the wave begins (measured in degrees) (For example, changing a waves cycle from crest to trough corresponds to a 180 degree phase shift).

Copyright 2005 John Wiley & Sons, Inc 360o 3 - 24 Wavelength vs. Frequency speed = frequency * wavelength v=f v = 3 x108 m/s = 300,000 km/s = 186,000 miles/s Example: if f = 900 MHz = 3 x108 / 900 x 10 3 = 3/9 = 0.3 meters Copyright 2005 John Wiley & Sons, Inc

3 - 25 Modulation odification of a carrier waves fundamental characteristics in order to encode information Carrier wave: Basic sound wave transmitted through the circuit (provides a base which we can deviate) asic ways to modulate a carrier wave: Amplitude Modulation (AM) Also known as Amplitude Shift Keying (ASK) Frequency Modulation (FM) Also known as Frequency Shift Keying (FSK) Phase Modulation (PM) Also known as Phase Shift Keying (PSK) Copyright 2005 John Wiley & Sons, Inc 3 - 26 Amplitude Modulation (AM) Changing the height of the wave to encode data One bit is encoded for each carrier wave

change A high amplitude means a bit value of 1 Low amplitude means a bit value of 0 More susceptible noise than the other modulation methods Copyright 2005 John Wiley & Sons, Inc 3 - 27 Frequency Modulation (FM) Changing the frequency of carrier wave to encode data One bit is encoded for each carrier wave change Changing carrier wave to a higher frequency encodes a bit value of 1 No change in carrier wave

frequency means a bit value of 0 Copyright 2005 John Wiley & Sons, Inc 3 - 28 Phase Modulation (PM) Changing the phase of the carrier wave to encode data One bit is encoded for each carrier wave change Changing carrier waves phase by 180o corresponds to a bit value of 1 No change in carrier waves phase means a bit value of 0 Copyright 2005 John Wiley & Sons, Inc 3 - 29

Bit Rate vs. Baud Rate bit: a unit of information baud: a unit of signaling speed Bit rate (or data rate): b Number of bits transmitted per second Baud rate (or symbol rate): s number of symbols transmitted per second General formula: b=sxn where b = Data Rate (bits/second) s = Symbol Rate (symbols/sec.) n = Number of bits per symbol Copyright 2005 John Wiley & Sons, Inc Example: AM n=1 b=s Example: 16-QAM

n=4 b=4xs 3 - 30 Bandwidth of a Voice Circuit Difference between the highest and lowest frequencies in a band or set if frequencies Human hearing frequency range: 20 Hz to 14 kHz Bandwidth = 14,000 20 = 13,800 Hz Voice circuit frequency range: 0 Hz to 4 kHz Designed for most commonly used range of human voice Phone lines transmission capacity is much bigger 1 MHz for lines up to 2 miles from a telephone exchange 300 kHz for lines 2-3 miles away Copyright 2005 John Wiley & Sons, Inc 3 - 31 Data Capacity of a Voice Circuit

Fastest rate at which you can send your data over the circuit (in bits per second) Calculated as the bit rate: b = s x n Depends on modulation (symbol rate) Max. Symbol rate = bandwidth (if no noise) Maximum voice circuit capacity: Using QAM with 4 bits per symbol (n = 4) Max. voice channel carrier wave frequency: 4000 Hz = max. symbol rate (under perfect conditions) Data rate = 4 * 4000 16,000 bps Copyright 2005 John Wiley & Sons, Inc 3 - 32 Modem - Modulator/demodulator Device that encodes and decodes data by manipulating the carrier wave V-series of modem standards (by ITU-T) V.22 An early standard, now obsolete Used FM, with 2400 symbols/sec 2400 bps bit rate

V.34 One of the robust V standards Used TCM (8.4 bits/symbol), with 3,428 symbols/sec multiple data rates(up to 28.8 kbps) Includes a handshaking sequence that tests the circuit and determines the optimum data rate Copyright 2005 John Wiley & Sons, Inc 3 - 33 Data Compression in Modems Used to increase the throughput rate of data by encoding redundant data strings Example: Lempel-Ziv encoding Used in V.44 Creates (while transmitting) a dictionary of two-, three-, and four-character combinations in a message Anytime one of these patterns is detected, its index in dictionary is sent (instead of actual data) Average reduction: 6:1 (depends on the text) Provides 6 times more data sent per second Copyright 2005 John Wiley & Sons, Inc

3 - 34 Digital Transmission of Analog Data Analog voice data sent over digital network using digital transmission Requires a pair of special devices called Codec - Coder/decoder A device that converts an analog voice signal into digital form Also converts it back to analog data at the receiving end Used by the phone system Copyright 2005 John Wiley & Sons, Inc 3 - 35 PCM - Pulse Code Modulation phone switch (DIGITAL) local loop Analog

transmission DS-0: Basic digital communications unit used by phone network Corresponds to 1 digital voice signal trunk Central Office (Telco) To other switches Digital transmission convert analog signals to digital data using PCM (similar to PAM) 8000 samples per second and 8 bits

per sample (7 bits for sample+ 1 bit for control) 64 Kb/s (DS-0 rate) Copyright 2005 John Wiley & Sons, Inc 3 - 36 ADPCM Adaptive Differential Pulse Code Modulation Encodes the differences between samples The change between 8-bit value of the last time interval and the current one Requires only 4 bits since the change is small Only 4 bits/sample (instead of 8 bits/sample), Requires 4 x 8000 = 32 Kbps (half of PCM) Makes it possible to for IM to send voice signals as digital signals using modems (which has <56 Kbps) Can also use lower sampling rates, at 8, 16 kbps Lower quality voice signals. Copyright 2005 John Wiley & Sons, Inc

3 - 37 V.90 and V.92 Modems Combines analog and digital transmission Uses a technique based on PCM concept Recognizes PCMs 8-bit digital symbols (one of 256 possible symbols) 8,000 per second Results in a max of 56 Kbps data rate (1 bit used for control) V.90 Standard Based on V.34+ for Upstream transmissions (PC to Switch) Max. upstream rate is 33.4 Kbps V.92 Standard (most recent) Uses PCM symbol recognition technique for both ways Max. upstream rate is 48 kbps Very sensitive to noise lower rates Copyright 2005 John Wiley & Sons, Inc 3 - 38

Multiplexing Breaking up a higher speed circuit into several slower (logical) circuits Several devices can use it at the same time Requires two multiplexer: one to combine; one to separate Main advantage: cost Fewer network circuits needed Categories of multiplexing: Frequency division multiplexing (FDM) Time division multiplexing (TDM) Statistical time division multiplexing (STDM) Wavelength division multiplexing (WDM) Copyright 2005 John Wiley & Sons, Inc 3 - 39 Frequency Division Multiplexing Makes a number of smaller channels from a larger frequency band 3000 Hz available bandwidth

Used mostly by CATV Host computer FDM Guardbands needed to separate channels To prevent interference between channels Unused frequency bands ,wasted capacity FDM circuit Four terminals Dividing the circuit horizontally Copyright 2005 John Wiley & Sons, Inc

3 - 40 Time Division Multiplexing Dividing the circuit vertically Allows multiple channels to be used by allowing the channels to send data by taking turns 4 terminals sharing a circuit, with each terminal sending one character at a time Copyright 2005 John Wiley & Sons, Inc 3 - 41 Comparison of TDM Time on the circuit shared equally Each channel getting a specified time slot, (whether it has any data

to send or not ) More efficient than FDM Since TDM doesnt use guardbands, (entire capacity can be divided up between channels) Copyright 2005 John Wiley & Sons, Inc 3 - 42 Statistical TDM (STDM) Designed to make use of the idle time slots (In TDM, when terminals are not using the multiplexed circuit, timeslots for those terminals are idle.) Uses non-dedicated time slots Time slots used as needed by the different terminals Complexities of STDM Additional addressing information needed Since source of a data sample is not identified by the time slot it occupies Potential response time delays (when all terminals try to use the multiplexed circuit intensively)

Requires memory to store data (in case more data come in than its outgoing circuit capacity can handle) Copyright 2005 John Wiley & Sons, Inc 3 - 43 Digital Subscriber Line (DSL) Became popular as a way to increase data rates in the local loop. Uses full physical capacity of twisted pair (copper) phone lines (up to 1 MHz) Instead of using the 0-4000 KHz voice channel 1 MHz capacity split into (FDM): a 4 KHz voice channel an upstream channel a downstream channel May be divided further (via TDM) to have one or more logical channels Requires a pair of DSL modems

One at the customers site; one at the CO site Copyright 2005 John Wiley & Sons, Inc 3 - 44 Data Link Layer - Introduction Network Layer Responsible for moving messages from one device to another Data Link Layer Controls the way messages are sent on media Physical Layer Organizes physical layer bit streams into coherent messages for the network layer Major functions of a data link layer protocol Media Access Control Controlling when computers transmit Error Control Detecting and correcting transmission errors Message Delineation Identifying the beginning and end of a message

Copyright 2005 John Wiley & Sons, Inc 3 - 45 Media Access Control (MAC) Controlling when and what computer transmit Important when more than one computer wants to send data (at the same time over the same circuit); e.g., Point-to-point half duplex links computers to take turns Multipoint configurations Ensure that no two computers attempt to transmit data at the same time Main approaches Controlled access Contention based access Copyright 2005 John Wiley & Sons, Inc 3 - 46 Contention

Transmit whenever the circuit is free Collisions Occurs when more than one computer transmitting at the same time Need to determine which computer is allowed to transmit first after the collision Used commonly in Ethernet LANs Copyright 2005 John Wiley & Sons, Inc 3 - 47 Relative Performance Depends on network conditions Work better for networks with high traffic volumes Work better for smaller networks with low usage

Cross-over point: About 20 computers Copyright 2005 John Wiley & Sons, Inc When volume is high, performance deteriorates (too many collisions) Network more efficiently used 3 - 48 Error Control Handling of network errors caused by problems in transmission Network errors

e.g., changing a bit value during transmission Controlled by network hardware and software Human errors: e.g., mistake in typing a number Controlled by application programs Categories of Network Errors Corrupted (data changed) Lost data Copyright 2005 John Wiley & Sons, Inc 3 - 49 Sources of Errors and Prevention Source of Error What causes it How to prevent it More important mostly on analog

Line Outages Faulty equipment, Storms, Accidents (circuit fails) White Noise (hiss) (Gaussian Noise) Movement of electrons (thermal energy) Increase signal strength (increase SNR) Impulse Noise Sudden increases in electricity (e.g., lightning, power surges) Shield or move the wires Cross-talk

Multiplexer guard bands are too small or wires too close together Increase the guard bands, or move or shield the wires Echo Poor connections (causing signal to be reflected back to the source) Fix the connections, or tune equipment Attenuation Gradual decrease in signal over distance (weakening of a signal) Intermodulation Noise Signals from several circuits

combine Use repeaters or amplifiers Move or shield the wires Jitter Analog signals change (small changes in amp., freq., and phase) Tune equipment Harmonic Distortion Amplifier changes phase (does not correctly amplify its input signal) Tune equipment (Spikes)

Copyright 2005 John Wiley & Sons, Inc 3 - 50 Error Detection Sender calculates an Error Detection Value (EDV) and transmits it along with data Receiver recalculates EDV and checks it against the received EDV Mathematical calculations Data to be transmitted Mathematical calculations ?

= EDV Larger the size, better error detection (but lower efficiency) If the same No errors in transmission If different Error(s) in transmission Copyright 2005 John Wiley & Sons, Inc 3 - 51 Error Detection Techniques Parity checks Longitudinal Redundancy Checking (LRC) Polynomial checking Checksum Cyclic Redundancy Check (CRC)

Copyright 2005 John Wiley & Sons, Inc 3 - 52 Parity Checking One of the oldest and simplest A single bit added to each character Even parity: number of 1s remains even Odd parity: number of 1s remains odd Receiving end recalculates parity bit If one bit has been transmitted in error the received parity bit will differ from the recalculated one Simple, but doesnt catch all errors If two (or an even number of) bits have been transmitted in error at the same time, the parity check appears to be correct Detects about 50% of errors Copyright 2005 John Wiley & Sons, Inc 3 - 53

LRC - Longitudinal Redundancy Checking Adds an additional character (instead of a bit) Block Check Character (BCC) to each block of data Determined like parity but, but counting longitudinally through the message (as well as vertically) Calculations are based on the 1st bit, 2nd bit, etc. (of all characters) in the block 1st bit of BCC number of 1s in the 1st bit of characters 2nd bit of BCC number of 1s in the 2ndt bit of characters Major improvement over parity checking 98% error detection rate for burst errors ( > 10 bits) Less capable of detecting single bit errors Copyright 2005 John Wiley & Sons, Inc 3 - 54

Polynomial Checking Adds 1 or more characters to the end of message (based on a mathematical algorithm) Two types: Checksum and CRC Checksum Calculated by adding decimal values of each character in the message, Dividing the total by 255. and Saving the remainder (1 byte value) and using it as the checksum 95% effective Cyclic Redundancy Check (CRC) Computed by calculating the remainder to a division problem: Copyright 2005 John Wiley & Sons, Inc 3 - 55 Error Correction Once detected, the error must be corrected Error correction techniques Retransmission (a.k.a, Backward error correction)

Simplest, most effective, least expensive, most commonly used Corrected by retransmission of the data Receiver, when detecting an error, asks the sender to retransmit the message Often called Automatic Repeat Request (ARQ) Forward Error Correction Receiving device can correct incoming messages itself Copyright 2005 John Wiley & Sons, Inc 3 - 56 Automatic Repeat Request (ARQ) Process of requesting that a data transmission be resent Main ARQ protocols Stop and Wait ARQ (A half duplex technique) Sender sends a message and waits for acknowledgment, then sends the next message Receiver receives the message and sends an acknowledgement, then waits for the next message

Continuous ARQ (A full duplex technique) Sender continues sending packets without waiting for the receiver to acknowledge Receiver continues receiving messages without acknowledging them right away Copyright 2005 John Wiley & Sons, Inc 3 - 57 Data Link Protocols Classification Asynchronous transmission Synchronous transmission Differ by Message delineation Frame length frame k-1 frame k frame k+1

Frame field structure Copyright 2005 John Wiley & Sons, Inc 3 - 58 Asynchronous Transmission Sometimes called start-stop transmission Used by the receiver for separating characters and for synch. Each character is sent independently Sent between transmissi ons (a

series of stop bits) Used on point-to-point full duplex circuits (used by Telnet when you connect to Unix/Linux computers) Copyright 2005 John Wiley & Sons, Inc 3 - 59 Asynchronous File Transfer Used on Point-to-point asynchronous circuits Typically over phone lines via modem Computer to computer for transfer of data files Characteristics of file transfer protocols Designed to transmit error-free data Group data into blocks to be transmitted (rather sending character by character) Popular File transfer Protocols Xmodem, Zmodem, and Kermit Copyright 2005 John Wiley & Sons, Inc

3 - 60 File Transfer Protocols Xmodem One of the oldest async file transfer protocol Uses stop-and-wait ARQ. Start of Header SOH Packet # Packet # compl. Checksum (128 bytes) Xmodem-CRC: uses 1 byte CRC (instead of checksum) Xmodem-1K: Xmodem-CRC + 1024 byte long message field Zmodem Uses CRC-32 with continuous ARQ Dynamic adjustment of packet size (based on circuit)

Kermit Very flexible, powerful and popular Typically uses CRC-24 and 1K size, but adjustable Copyright 2005 John Wiley & Sons, Inc 3 - 61 Synchronous Transmission Data sent in a large block Called a frame or packet Typically about a thousand characters (bytes) long Includes addressing information Especially useful in multipoint circuits Includes a series of synchronization (SYN) characters Used to help the receiver recognize incoming data Synchronous transmission protocols categories Bit-oriented protocols: SDLC, HDLC Byte-count protocols: Ethernet

Byte-oriented protocols: PPP Copyright 2005 John Wiley & Sons, Inc 3 - 62 HDLC High-Level Data Link Control Formal standard developed by ISO Same as SDLC, except Longer address and control fields Larger sliding window size And more Basis for many other Data Link Layer protocols LAP-B (Link Accedes Protocol Balanced) Used by X.25 technology LAP-D (Link Accedes Protocol Balanced) Used by ISDN technology LAP- F (Used by Frame Relay technology) Copyright 2005 John Wiley & Sons, Inc

3 - 63 Ethernet (IEEE 802.3) Most widely used LAN protocol, developed jointly by Digital, Intel, and Xerox, now an IEEE standard Uses contention based media access control Byte-count data link layer protocol No transparency problem uses a field containing the number of bytes (not flags) to delineate frames Error correction: optional Copyright 2005 John Wiley & Sons, Inc 3 - 64 Ethernet (IEEE 802.3) Frame Used by Virtual LANs; (if no vLAN, the field is omitted If used, first 2 bytes is set to: 24,832 (8100H)

Used to hold sequence number, ACK/NAK, etc., (1 or 2 bytes) 00 01 10 11 Data (43 - 1497 bytes) Repeating pattern of 1s and 0s (1010101010) (number of bytes in the message field)

Used to exchange control info (e.g., type of network layer protocol used) Copyright 2005 John Wiley & Sons, Inc 3 - 65 Point-to-Point Protocol (PPP) Byte-oriented protocol developed in early 90s Commonly used on dial-up lines from home PCs Designed mainly for point-to-point phone line (can be used for multipoint lines as well) Specifies the network layer protocol used (e.g, IP, IPX) (up to 1500 bytes) Copyright 2005 John Wiley & Sons, Inc 3 - 66

Data Link Protocol Summary Protocol Size Error Detection Retransmission Media Access Asynchronous Xmission 1 Parity Continuous ARQ Full Duplex XMODEM

132 8-bit Checksum Stop-and-wait ARQ Controlled Access XMODEM-CRC 132 8-bit CRC Stop-and-wait ARQ Controlled Access XMODEM-1K 1028 8-bit CRC

Stop-and-wait ARQ Controlled Access ZMODEM * 32-bit CRC Continuous ARQ Controlled Access KERMIT * 24-bit CRC Continuous ARQ

Controlled Access SDLC * 16-bit CRC Continuous ARQ Controlled Access HDLC * 16-bit CRC Continuous ARQ Controlled Access Token Ring

* 32-bit CRC Stop-and wait ARQ Controlled Access Ethernet * 32-bit CRC Stop-and wait ARQ Contention SLIP *

None None Full Duplex PPP * 16-bit CRC Continuous ARQ Full Duplex File Transfer Protocols Synchronous Protocols * Varies depending on message length. Copyright 2005 John Wiley & Sons, Inc

3 - 67 Transmission Efficiency An objective of the network: Move as many bits as possible with min errors higher efficiency and lower cost Factors affecting network efficiency: Characteristics of circuit (error rate, speed) Speed of equipment, Error control techniques Protocol used Information bits (carrying user information) Overhead bits ( used for error checking, frame delimiting, etc.) = Total number of info bits to be transmitted Total number of bits transmitted Copyright 2005 John Wiley & Sons, Inc 3 - 68

Throughput A more accurate definition of efficiency Total number of information bits received per second; takes into account: Overhead bits (as in transmission efficiency) Need to retransmit packets containing errors Complex to calculate; depends on Transmission efficency Error rate Number of retransmission Transmission Rate of Information Bits (TRIB) Used as a measurement of throughput Copyright 2005 John Wiley & Sons, Inc 3 - 69

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