Layer 1 Physical Layer. Transmission Media. Twisted Pair

Transcription

1 Layer Physical Layer Chapter 2: Computer Networks 2.: Physical Layer and Data Link Layer 2.2: Examples for Local Area Networks 2.3: Examples for Wide Area Networks 2.4: Wireless Networks Chapter 4: Application Protocols Chapter 3: Internet Protocols Computer Networks OSI Reference Model Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Connection parameters mechanical electric and electronic functional and procedural More detailed: Physical transmission medium (Copper cable, optical fiber, radio,...) Pin usage in network connectors Representation of raw bits (Code, voltage, etc.) Data rate Control of bit flow: serial or parallel transmission of bits synchronous or asynchronous transmission simplex, half-duplex or full-duplex transmission mode Page Page 2 Transmission Media Twisted Pair Twisted Pair Copper conductor Several media, varying in transmission technology, capacity, and bit error rate (BER) Satellites Interior insulation Radio connections Braided outer conductor Glass core Coaxial cable Protective outer insulation Optical fiber Glass cladding Plastic Characteristics: Data transmission through electrical signals Problem: electromagnetic signals of the environment can disturb the transmission within copper cables Solution: two insulated, twisted copper cables Twisting reduces electromagnetic interference with environmental disturbances Simple principle (costs and maintenance) Well known (e.g. telephony) Can be used for digital as well as analogous signals Bit error rate ~ -5 Twisted Pair: 4 pairs of twisted copper cables in one outer insulation are named Twisted Pair cable! Insulation Copper core Page 3 Page 4

2 Coaxial Cable Structure Insulated copper cable as center conductor Braided outer conductor reduces environmental disturbances Interior insulation separates center and outer conductor Braided outer conductor Copper conductor Interior insulation Protective outer insulation Characteristics: Higher data rates over larger distances than twisted pair: -2 GBit/sec up to km Better shielding than for twisted pair, resulting in better signal quality Bit Error Rate ~ -9 Optical Fiber Characteristics: Nearly unlimited data rate (theoretically up to 5. GBit/s) over very large distances Wavelength in the range of microns (determined by availability of light emitters and attenuation of electromagnetic waves: range of the wavelength around.85µm,.3µm and.55µm are used) Insensitive to electromagnetic disturbances Good signal-to-noise-ratio Bit Error Rate: ~ -2 Early networks were build with coaxial cable, in the last ten years however it was more and more replaced by twisted pair. Page 5 Page 6 Optical Transmission Optical Fiber Structure of an optical transmission system Optical source (converts electrical into optical signals; normally in the form light pulse ; no light pulse ) Communication medium (optical fiber) Detector (converts optical into electrical signals) electrical signal optical source optical signal optical fiber optical detector Physical principle: Total reflection of light at another medium electrical signal Structure of a fiber Core: optical glass (extremely thin) Internal glass cladding Protective plastic covering The transmission takes place in the core of the cable: Core has higher refractive index, therefore the light remains in the core Ray of light is reflected instead of transiting from medium to medium 2 Refractive index is material dependent A cable consists of many fibers Medium 2 Medium Refractive index: Indicates refraction effect relatively to air optical source (LED, Laser) Medium 2 Medium (core) Medium 2 Page 7 Page 8

3 Problems with Optical Fiber The ray of light is increasingly weakened by the medium! Absorption can weaken a ray of light gradually Impurities in the medium can deflect individual rays Dispersion (less bad, but transmission range is limited) Rays of light are spreading in the medium with different speed: - Ways (modes) of the rays of light have different length (depending on the angle of incidence) - Rays have slightly different wavelengths (and propagation speed) Refractive index in the medium is not constant (effect on speed) Here only a better quality of radiation source and/or optical fiber helps! kurzes, Electrical starkes input signal Signal Optical Glasfaser Fiber langes, Electrical schwaches output signal Signal Page 9 Encoding of Information Shannon: The fundamental problem of communication consists of reproducing on one side exactly or approximated a message selected on the other side. Objective: useful representation (encoding) of the information to be transmitted Encoding categories Source encoding (Layer 6 and 7) Channel encoding (Layer 2 and 4) Cable encoding (Layer ) Encoding of the original message E.g. ASCII-Code (text), tiff (pictures), PCM (speech), MPEG (video), Representation of the transmitted data in code words, which are adapted to the characteristics of the transmission channel (redundancy). Protection against transmission errors through error-detecting and/or -correcting codes Physical representation of digital signals Page Baseband and Broadband The transmission of information can take place either on the baseband or on broadband. This means: Baseband The digital information is transmitted over the medium as it is. For this, encoding procedures are necessary, which specify the representation of resp. (cable codes). Broadband The information is transmitted analogous (thereby: larger range), by modulating it onto a carrier signal. By the use of different carrier signals (frequencies), several information can be transferred at the same time. While having some advantages in data communications, broadband networks are rarely used baseband networks are easier to realize. But in optical networks and radio networks as well as for Cable TV this technology is used. Cable Code: Requirements How can digital signals be represented electrically? As high robustness against distortion as possible T 2T 3T 4T 5T 6T 7T t Transmission T 2T 3T 4T 5T 6T 7T Efficiency: as high data transmission rates as possible by using code words binary code: +5V/- 5V? ternary code: +5 V/V/- 5V? quaternary code: 4 states (coding of 2 bits at the same time) Synchronization with the receiver, achieved by frequent changes of voltage level regarding to a fixed cycle Avoiding direct current: positive and negative signals should alternatively arise t Page Page 2

4 NRZ: Non Return to Zero Simple approach: Encode as positive tension (+5V) Encode as negative tension (- 5V) Differential NRZ Differential NRZ: similar principle to NRZ Encode as tension level change Encode as missing tension level change +5V +5V -5V -5V Advantage: Very simple principle The smaller the clock pulse period, the higher the data rate Disadvantage: Loss of clock synchronization as well as direct current within long sequences of or Very similar to NRZ, but disadvantages only hold for sequences of zeros. Remark: In some implementations (e.g. CISCO) the level change is effected in the middle of the bit (not at the beginning of the bit) but this is no principal difference. Page 3 Page 4 Manchester Code Differential Manchester Code For automatic synchronization, with each code element the clock pulse is transferred. Used is a tension level change in the middle of each bit: encode as tension level change of positive (+5V) to negative (-5V) encode as tension level change of negative (- 5V) to positive (+5V) +5V -5V Advantages Clock synchronization with each bit, no direct current End of the transmission easily recognizable Disadvantage Capacity is used only half! Variant of the Manchester Code. Similar as it is the case for the Manchester code, a tension level change takes place in the bit center, additionally a second change is made: Encode as missing level change between two bits Encode as level change between two bits +5V -5V Page 5 Page 6

5 Sicherungsebene 4B/5B Code 4B/5B Code Table Disadvantage of the Manchester code: 5% efficiency, i.e. B/2B Code (one bit is coded into two bits) An improvement is given with the 4B/5B Code: four bits are coded in five bits: 8% efficiency Functionality: Level change with, no level change with (differential NRZ code) Coding of hexadecimal characters:,,, 9, A, B,, F (4 bits) in 5 bits, so that long zero blocks are avoided Selection of the most favorable 6 of the possible 32 code words (maximally 3 zeros in sequence) Further 5 bit combinations for control information Expandable to B/B Codes? Page 7 Decimal Data Transmitted Symbol Assignment Quiet -line state (status) Invalid 2 Invalid 3 Invalid 4 Halt -line state (status) 5 Invalid 6 Invalid 7 R-Reset (logical )-control (control) 8 Invalid 9 Data Data Data 2 Invalid 3 T-Ending delimiter (control) 4 Data 5 Data Worst case: 6 Invalid 7 K-starting delimiter (control) 8 Data 9 Data 2 Data 3 Zeros 2 Data 22 Data 23 Data 24 J-starting delimiter (control) 25 S - set (logical ) - control (control) 26 Data 27 Data 28 Data 29 Data 3 Data Chapter 2.: Physical 3 Layer and Data Link Layer Idle-line state (status) Page 8 Layer 2: Division into two Parts LLC: Frame Construction Logical Link Control (LLC) (Layer 2b) Organization of the data to be sent into frames Guarantee (if possible) an error free transmission between neighboring nodes through: Detection (and recovery) of transfer errors Flow Control (avoidance of overloading the receiver) Buffer Management Medium Access Control (MAC) (Layer 2a) Control of the access to the communication channel in broadcast networks Data Link Layer LLC MAC (Medium MAC Access Control) ISO/OSI 82.3 CSMA/CD (Ethernet) 82.4 Token Bus IEEE 82.2 Logical Link Control 82.5 Token Ring 82.6 DQDB Existing Reale Networks Netze ANSI X3T9.5 FDDI... ATM Forum... ATM LAN Emulation Page 9 Organization of a message into uniform units (for simpler transmission) Well-defined interface to the upper layer (layer 3) Marking of the units: Error check Header Data Trailer (checking sequence for the frame) Control information (addresses, frame numbers, ) (Physical) mark the frame by: Start and end flags Start flag and length Code injuries FCS = Frame Checking Sequence Next task of the LLC layer: protected transmission of the frames to the communication partner. The transmission over layer is not necessarily free of errors! Question: how can errors be recognized and repaired? Page 2

6 Error-detecting and -correcting Codes From the data, compute a short checksum and send it together with the data to the receiver. The receiver also computes a checksum from the received data and compares it with those of the sender. Simplest procedure - parity bit: count the number of s: Sender: PB: sends: -Bit errors are detected Receiver: PB computes: 2-Bit errors are not detected Corrections are not possible! Variant: double parity Improvement of the parity bit procedure by further parity bits. For this, several blocks of bits are grouped and treated together: Sender: Receiver: An incorrect bit can be identified and corrected by this procedure. Error-correcting Codes Error correction Error correction generally means: Transmission of redundancies: Length of the transmission: n bit (2 n possible binary sequences) Message length: m (<n) bit (2 m permissible code words) k parity bits (k = n - m) Form a sphere of code words around each message Hamming Distance D: Number of places, in which two binary sequences differ Then: D 2t + code is t-error-correcting D t + code is t-error-detecting That means: for error-correction, a relatively large overhead has to be transmitted and it is not guaranteed, that the identified correction really is the right one. Page 2 Page 22 Hamming Code Hamming Code Goal: Use of several parity bits, each of them considering several bits (overlapping). Errors can be identified and corrected by combining the parity bits. The Hamming code is the minimal code of this category. Idea: Representation of each natural number by sum of powers to two. In a code word w = z,, z n the parity bits are placed exactly at the k positions, for them the index is a power of two. At the remaining m = n - k positions the data bits are placed. Each of the k additional bits is a parity bit for all places, for which the representation in powers of two contains the position of the additional bit. Page 23 ASCII-Code H A M M I N G Parity bit : Data bit 3, 5, 7, 9, 3 = + 2 Parity bit 2: Data bit 3, 6, 7,, 5 = + 4 Parity bit 4: Data bit 5, 6, 7 6 = Parity bit 8: Data bit 9,, 7 = = + 8 Problem with Hamming code: errors involving several following bits are usually wrongly corrected = = Codeword Receiver: examine parity bits if necessary, sum up indices of the incorrect parity bits index of the incorrect bit -bit errors can definitely be identified and corrected Page 24

7 Hamming Code Error Detection with Cyclic Codes Transmission error Receiver computes parity bits: Summing up the indizes, 2 and 4, bit 7 is detected as false Problem: how to recognize errors in several bits, especially sequences of bit errors? The use of simple parity bits is not suitable. However, in data communication (modem, telephone cables) such errors arise frequently. Weaknesses: 2-bit errors are not corrected (or wrongly corrected!) 3-bit errors are not recognized a) Bit 4 and bit inverted: parity bits, 2, 4, 8 are wrong bit 5 is to be corrected, but does not exist b) Bit 2 and bit 4 inverted parity bits 2, 4 wrong bit 6 is falsely recognized as incorrect c) Bits, 8, 9 inverted all parity bits are correct no error is recognized Page 25 Most often used: Polynomial Codes Idea: a k-bit PDU (a k-,, a ) is seen as a polynomial a k- x k- + + a with the coefficients and. Example: is seen as x 6... x x = x 6 + x 5 + x 2 + For computations, polynomial arithmetic modulo 2 is used, i.e. addition and subtraction without carriage. Both operations become Exclusive-OR operations. Page 26 Error Detection with Cyclic Codes CRC - Example Idea for error detection: Sender and receiver agree upon a generator polynomial G(x) = x r + + x. The first and the last coefficient have to be. The sender interprets a data block of length m as polynomial M(x). The sender extends M(x), i.e. adds redundant bits in a way that the extended polynomial M (x) is divisible by G(x). (Redundancy = remainder R(x) by division of the sequence with G(x)) The receiver divides the received extended M(x) by G(x). If the remainder is, there was (probably!) no error, otherwise some error occurred. Name: Cyclic Redundancy Checksum (CRC) Note: also the parity bit can be seen as CRC, with generator polynomial x +! Data to be transmitted: Generator polynomial: x 4 + x + Sender: : = = x 3 + = R(x) CRC =, sending Receiver: Note: usually, the extra positions are preset with zeros but in some cases, e.g. Ethernet, uses the inverted bits as presets. : = Data received correctly Page 27 Page 28

8 Computation of CRC: Shift Registers Implementation by Shift Registers: XOR for substraction AND for applying substraction: first register = : no substraction first register = : substraction Generator polynomial x 4 + x +: R R R R R Shift Registers - Example Data to be transmitted: Generator polynomial: x 4 + x + Simplified realization: R R R R R When no more input is given in the leftmost register, the other registers contain the CRC result. Page 29 Page 3 Shift Registers - Example Shift Registers - Example Page 3 Page 32

9 Shift Registers - Example Shift Registers - Example x x x x x x x x x x x x x x x Page 33 Page 34 CRC is not perfect CRC: Characteristics Receiver: : = Error detected Receiver: : = Error not detected Common generator polynomials: CRC-6: G(x) = x 6 + x 5 + x 2 + CRC CCITT: G(x) = x 6 + x 2 + x 5 + Ethernet: G(x) = x 32 + x 26 + x 23 + x 22 + x 6 + x 2 + x + x + x 8 + x 7 + x 5 + x 4 + x 2 + x + Error detection (for 6-bit generator polynomials): all single bit errors all double bit errors all three-bit errors all error samples with odd number of bit errors all error bursts with 6 or fewer bits % of all 7-bit error bursts % of all error bursts with length 8 bits Remaining error rate <.5 * -5 block error rate Page 35 Page 36

10 CRC is not perfect CRC: Error Correction.) There are still remaining some error-combinations which can occur undetected the probability is very small, but in principle it is possible 2.) Assume, the receiver only gets part of the original message: Header / Data FCS Header / Data The receiver interprets the end of the message as FCS and tries to check the rest of the received bits as sent data. Probability for assumed FCS = FCS for assumed data : /2 r for r-bit FCS, because each of the r bits is correct with probability ½. This problem can be eliminated by using a fixed byte structure for the data frame or a length field in the header. 3.) An error could consist of adding a multiple of G(x) to M (x) In exceptional cases even errors are correctable by CRCs. Example: ATM (Asynchronous Transfer Mode) Data units have fixed length 5 byte header + 48 byte data The last header byte is a checksum for the header Generator polynomial G(x) = x 8 + x 2 + x + It is even possible to correct a -bit error, due to: there are 4 possible -bit errors in the 4-bit header and those lead on 4 different non-zero remainders. Correction is not assigned with e.g. Ethernet: an Ethernet frame has a length between 64 and 52 byte. Page 37 Page 38 Error Protection Mechanisms Flow Control: Send and Wait Error correction: FEC (Forward Error Correction) Use of error-correcting codes Falsified data in most cases can be corrected. Uncorrectable data are simply dismissed. Feedback from the receiver to the sender is not necessary. Suitable for transmissions tolerating no transmission delays (video, audio) as well as for coding resp. protecting data on CDs or DVDs. Error detection: ARQ (Automatic Repeat Request) Use of error-detecting codes (CRC) Errors are detected, but cannot be corrected. Therefore, falsified data must be requested again. Introduction of flow control: number the data blocks to be sent acknowledgement of blocks by the receiver incorrectly transferred blocks are repeated Suitable for transmissions which do not tolerate errors (files). Page 39 Simple procedure: The sender sends a data block and waits, until an acknowledgement of the receiver arrives or a timeout is reached. Incorrect blocks are repeated, otherwise the next block is sent. Disadvantage: large waiting periods between the transmission of single blocks. Thus much transmission capacity is wasted. Transmitter Receiver Waiting period ACK 2 F time out ACK: Acknowledgment, i.e. everything ok 2 2 ACK 3 3 Page 4

11 Flow control: Sliding Window Introduction of a transmission window Common procedure to avoid long waiting periods of the sender Sender and receiver agree upon a transmission window. If W is the window size, it means: the sender may send maximally W messages without an acknowledgement of the receiver. The messages are sequentially numbered in the frame header (,, 2,, MODULUS-,, ; whereby W < MODULUS). The sender may send sequentially numbered messages up to W, without getting an acknowledgement for the first frame. The receiver confirms by acknowledgements (ACK). The sender moves the window as soon as an ACK arrives. Advantages of the procedure: The sender can take advantage of the network capacity A sender who is too fast for the receiver is slowed down (the receiver only can read data from the network slowly, thus it rarely sends ACKs) Sending and receiving speed are adapted Page 4 Sliding Window Example (for 3-bit sequence/acknowledgement number) with 3 bits for sequence/acknowledgement number, m = 8 possible combinations Stations agree upon a window size W with W < m, e.g. W = 7 The window limits the number of unconfirmed frames allowed at one time (here max. 7, because of W = 7) With receipt of an acknowledgement, the window is shifted accordingly Frames are numbered sequentially modulo m (for m = 8 thus numbers from to 7) 7 7 ACK 6 ACK 2 ACK Station Station sendet sends Station Station receives erhält Station Station verschiebt slides Station Station receives erhält Station Station verschiebt slides Frames - Quittung Fenster um, Quittung, 2 Fenster um 2, frames - 6 acknowledgement sendet window Frame by 7 acknowledgement sendet window Frame by, 2 Time and sends,2 and sends frame 7 frames, Page Maximum Window Size with Sliding Window There is a reason why window size W has to be smaller than MODULUS: Sequence numbers e.g. have 3 bits: 2 3 = 8 sequence numbers (,, 7) I.e. MODULUS = 8 Assume the window size to be W = 8. A sends frames to B. A has recently received an ACK for frame 2; A is allowed to send 8 more frames Elimination of Errors: Go-back-n The sender sends data blocks continuously (within the transmission window). The receiver answers: ACK j : everything up to block j is correct REJ j /NACK j : up to block j- everything is correct, block j is incorrect Go-back-n: with a REJ j, starting from block j everything is transferred again Disadvantage: With a REJ j the transmitter must repeat all blocks starting from j Advantage: the receiver needs only one buffer place A receives ACK 2. There are two possibilities: Case : B only has received 2 and retransmits the (old) number ACK number. Everything in between has been lost. Example of W = 5; MODULUS = 8 Source Case 2: B has received and confirms the (new) number 2 (all ACKs may have been lost or forgotten in between) A does not know whether case or 2 holds: the ACK is ambiguous! W must be less than 8 (W < MODULUS in general). Destination ACK ACK REJ 2 ACK 2 Page 43 Page 44

12 High Level Data Link Control (HDLC) Protocol for layer 2: HDLC Frame identification: mark the beginning and end with a flag: Flag may never occur within a frame Used for this purpose: Bitstuffing Sender inserts a zero after each sequence of five ones. The receiver removes this zero. Sender: Receiver: Page 45 HDLC Frame For synchronization on layer Header Data Trailer (6) 6 8 Address Control DATA FCS Flag D/C N(S) P/F N(R) Flag In the idle state, only s are sent. The receiver recognizes a transmission with the first. Address contains an identifier to inform the receiver about what to do, e.g. means IP protocol used for processing the data. The checksum is computed by using a CRC. Since CRC conducts no error correction, flow control is necessary. Control contains the sequence number N(S) for the message; at the same time, in the opposite direction a message can be acknowledged using the acknowledgement number N(R) ( Piggybacking ). Bits D/C: DATA () resp. control () P/F: Poll/final for the coordination of several senders Page 46 Medium Access Control (MAC) MAC - Reservation Protocols Controlling the competitive access of several users to a shared medium Simplest procedures: firm assignment of a limited capacity Time Division Multiple Access (TDMA) Frequency Frequency Division Multiple Access (FDMA) Frequency User User 2 User 3 Time Time Each user gets the entire transmission capacity for fixed time intervals (Baseband transmission) Each user gets a fixed portion of the transmission capacity (a frequency range) for the whole time (Broadband transmission) Page 47 Communication follows a two-phase schema (alternating phases): In the reservation phase the sender makes a reservation by indicating the wish to send data (or even the length of the data to be sent) In the transmission phase the data communication takes place (after successful reservation) Advantage: very efficient use of the capacity Disadvantage: Delay by two-phase procedure; further, often a master station is needed, which cyclically queries all other stations whether they have to send data. This master station assigns sending rights. Techniques for easy reservation without master station: Explicit reservation Implicit reservation Page 48

13 Explicit Reservation Uses two frame types: reservation frame (very small) in the first phase data frame (constant length) in the second phase Variant : without contention Only suitable for small number of users Each user i is assigned the i-th slot in the reservation frame. If it wants to send data, it sets the i-th bit in the reservation frame to. After the reservation phase, all stations having set their reservation bit can send their data in the order of their bits in the reservation frame. reservation frame data frames of stations having reserved Explicit Reservation Uses two frame types: reservation frame (very small) in the first phase data frame (constant length) in the second phase Variant 2: with contention For higher number of users The reservation frame consists of a limited number of contention slots (smaller than the number of participating stations) Users try to get a contention slot (and by that make a reservation for a data slot) by random choice, writing their station number into a slot If there is no collision in the reservation phase, a station may send. reservation frame with contention slots data frames of stations having reserved This procedure is called Bitmap Protocol Page 49 Page 5 Implicit Reservation Implicit Reservation No reservation slots, only data slots of certain length. A window consists of N data slots, windows a cyclically repeated The duration of the window must be longer than the round-trip time Procedure: A station which wants to send observes N slots without doing anything and marks the slots as follows:, if the slot if empty or collided, if the slot is used by somebody else In the following window the station randomly chooses one of the slots marked with (Simplification: choose the first slot marked with ) Two cases: conflict: try again successful transmission: slot reserved for the station as long as it sends data. If the station is not using its slot in one window, the reservation is dismissed. Example: 8 data slots, Stations A - F Reservation Slots: Window Window 2 A C D A B A B/D F ACDABA-F Window 3 A C A B A AC-ABA-- Window 4 A B/F B A F A---BAF- Window 5 A B A F D A---BAFD Window 6 A C E E B A F D t Stations observing 8 slots Collision within slot 7; the other slots are reserved Page 5 Page 52

14 MAC Decentralized Protocols Best known protocol has the name ALOHA Developed on the Hawaiian islands: stations are connected by satellite Very simple principle, no coordination: Stations are sending completely uncoordinated, all using the same frequencies When two (or more) stations are sending at the same time, a collision occurs: both messages are destroyed. Problem: collisions occur even with very small overlaps! Vulnerability period: 2 times the length of a frame When a collision occurs, frames are repeated after a random time Problem: since traffic runs over a satellite, a sender only hears after very long time, whether the transmission was successful or not. Collision MAC Decentralized Protocols Problem with ALOHA: even small laps already lead to transmission conflicts. Therefore often collisions arise, causing many repetitions: No guaranteed response times Low throughput Improvement: Slotted ALOHA (first version was pure ALOHA ) The whole time axis is divided into time slots (similar to TDMA, but time slots are not firmly assigned to stations) The transmission of a block starts at the beginning of a time slot fewer collisions, vulnerability period of one frame length But: the stations must be synchronized! Collision Sender A Sender A Sender B Sender B Sender C t Sender C t Page 53 Page 54 How to estimate the Efficiency of ALOHA? Randomness Which possible states do we have in ALOHA? thinking users backlogged users common channel total traffic G no conflict conflict G: average number of frames per time unit S: throughput, i.e. rate of successful transmissions Relation between G and S? Depends significantly on the traffic structure. Model A: only one sender. No collisions, so S = G is possible Model B: many users, each of them inactive most time new arrivals are totally random Suppose that the total traffic is absolutely random. What is the biggest randomness possible? arrival Random arrivals: t consider time interval of length h. Observe arrivals in that interval. h Arrivals have a certain intensity G (average rate per time unit) Randomness : probability of an arrival in a (small) time interval h is proportional o(h): disturbance function with to the intensity G o( h) for h to h h... to G h For very small h, only or arrivals are possible: Prob(exactly one arrival within interval length h) = G h + o(h) Prob(no arrivals within interval length h) = G h + o(h) Page 55 Page 56

15 Application for ALOHA access protocol Throughput vs. Offered Traffic These conditions/requirements lead to the Poisson distribution. Consider the total number i of events in an interval of length T i ( G T) G T Prob(exactly i arrivals in [;T]) = e Back to ALOHA: relation between S and G Suppose that G follows the poisson distribution, i.e. G is totally random. i! S = G Prob(no collision occurs) = G Prob(nobody else started in my vulnerability period) = G Prob(no arrival in interval of length T = 2) for pure ALOHA G Prob(no arrival in interval of length T = ) for slotted ALOHA Prob( arrivals in [;T]) = e -G T = G e -2G for pure ALOHA G e -G for slotted ALOHA Note: randomness (i.e. Poisson) is not valid for high traffic and if waiting time after retransmission is short Page 57 Analytical computation: Both, Pure ALOHA and Slotted ALOHA cannot achieve a high throughput But: simple principle, no coordination necessary between the stations Page 58 MAC Decentralized Protocols MAC Coordination by using a Token Variant of ALOHA for networks with small range exists Similar to ALOHA: no coordination of the stations But: each station which wants to send first examines whether already another station is sending If no sending takes place, the station begins to send (Carrier Sense Multiple Access, CSMA, see chapter 2.2) Note: this principle only works within networks having a short transmission delay using the principle within satellite systems is not possible because there would be no chance to know whether a conflict occurred before end of the transmission Advantages: simple, because no master station and no tokens are needed; nevertheless good utilization of the network capacity Disadvantage: no guaranteed medium access, a large delay up to beginning a transmission is possible Introduction of a token (determined bit sequence) Only the holder of the token is allowed to send Token is cyclically passed on between all stations particularly suitable for ring topologies Token Ring (4/6/ Mbit/s, see chapter 2.2) Characteristics: Guaranteed accesses, no collisions Very good utilization of the network capacity, high efficiency Fair, guaranteed response times Possible: multiple tokens But: complex and expensive Passing on of the token Page 59 Page 6