-1- Georgia State UniversitySensorweb Research Laboratory CSC4220/6220 Computer Networks Dr. WenZhan Song Professor, Computer Science

-2- Georgia State UniversitySensorweb Research Laboratory Course roadmap Introduction Application Layer: WWW, FTP, , DNS, multimedia Transport Layer: reliable end-end data transfer principles, UDP, TCP Network Layer: IP addressing, routing and other issues Data Link Layer: framing, error control, flow control Medium Access Control (MAC) Layer: multiple-access, channel allocation Physical Layer: wired, wireless, satellite Other Topics: network security, social issues, hot topics, research directions

-3- Georgia State UniversitySensorweb Research Laboratory Data Link Layer Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies

-4- Georgia State UniversitySensorweb Research Laboratory Link Layer: Introduction Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame, encapsulates datagram “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

-5- Georgia State UniversitySensorweb Research Laboratory Data Link Layer Road Map Data link layer design issues Framing Error Control Example data link protocols HDLC and PPP Multiple Access Protocols Static channel allocation Dynamic channel allocation LAN technologies and their MAC protocols Ethernet WiFi and WiMax

-6- Georgia State UniversitySensorweb Research Laboratory Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders Sharing broadcast channels: multiple access channels Determine who gets to use the channel when there is competition for it

-7- Georgia State UniversitySensorweb Research Laboratory Functions of the Data Link Layer Relationship between packets and frames.

-8- Georgia State UniversitySensorweb Research Laboratory Services Provided to Network Layer Various services Unacknowledged connectionless service Without ack, no logical connection Appropriate when error rate is low, real-time traffic (voice, late data worse than bad data) Most LANs Acknowledged connectionless service No logical connection, but individually acked Useful over unreliable channels (wireless) Acknowledged connection-oriented service Guarantee frame received only once and in right order Three phases: connection established, frames transmitted, connection released

-9- Georgia State UniversitySensorweb Research Laboratory Adaptors Communicating link layer implemented in “adaptor” (called NIC) Ethernet card, PCMCI card, card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving node adapter is semi-autonomous link & physical layers sending node frame rcving node datagram frame adapter link layer protocol

-10- Georgia State UniversitySensorweb Research Laboratory Framing Encapsulate datagram into frame, adding header and trailer Break the bit stream up into discrete frames and compute the checksum of each frame Relationship between packets and frames

-11- Georgia State UniversitySensorweb Research Laboratory Framing – Character Count A character stream. (a) Without errors. (b) With one error.

-12- Georgia State UniversitySensorweb Research Laboratory Framing (2) – Flag Bytes (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing. Actually, after insert ESC control byte, it also inverts the fifth bit of (or XOR 0x20 with) the following data byte. See wiki or textbook p491. For simplicity, many textbooks do not mention it.wiki

-13- Georgia State UniversitySensorweb Research Laboratory Framing (3) – Bit Flags Delimit frame at both ends using May close one frame and open another Receiver hunts for flag sequence to synchronize Bit stuffing used to avoid confusion with data containing inserted after every sequence of five 1s If receiver detects five 1s it checks next bit If 0, it is deleted If 1 and seventh bit is 0, accept as flag If sixth and seventh bits 1, sender is indicating abort

-14- Georgia State UniversitySensorweb Research Laboratory Framing (4) – Bit Flags Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing.

-15- Georgia State UniversitySensorweb Research Laboratory Error Control Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors: error-detection codes signals sender for retransmission or drops frame Error Correction: receiver identifies and corrects bit error(s) without resorting to retransmission error-correction codes

-16- Georgia State UniversitySensorweb Research Laboratory Error Detection and Correction EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction

-17- Georgia State UniversitySensorweb Research Laboratory Error-Detection & Correction Code Single Bit Parity: Detect single bit errors Only odd bit errors detection Examine: Number of 1s in the code is even Consider a code with only 4 valid codewords: They have Hamming diatance 5 with each other, hence can correct double errors. Example: must be Rules To correct d errors, you need a distance 2d+1 code To detect d errors, you need a distance d+1 code Hamming distance of x and y is XOR(x,y)

-18- Georgia State UniversitySensorweb Research Laboratory Error-Correction Code Theoretical lower limit A code with m message bits and r check bits which will allow all single errors to be corrected 2 m legal messages – legal codewords For each legal message, there is n illegal codewords Total number of bit pattern is 2 n Therefore, (n+1)2 m  2 n n=m+r, (m+r+1)  2 r => the limit can be achieved by Hamming code Hamming Code It can correct any single bit error bits in power of 2 positions (1,2,4,8,) are check bits, the rest are m data bits Check bit forces the parity of some collection of bit “1” to be even To see which check bits the data bit in position k contributes to, rewrite k as a sum of powers of 2. 11=1+2+8, 29= Hamming code

-19- Georgia State UniversitySensorweb Research Laboratory Hamming Code Example If a 12-bit hamming code 0x9B8 = arrives at receiver, is there any bit error? If so, which bit is wrong? Chec k bit Checked Data bitsBits Sequence (Yes) (No) (No) (Yes) = =4+1 6 =4+2 7 = = = = = Hence bit 6 = 2+4 is wrong, so correct hamming code should be 0x9F8 =

-20- Georgia State UniversitySensorweb Research Laboratory Correct burst errors Use of Hamming code to correct k burst errors by send k ASCII character together, column by column. (Read Tanenbaum textbook page 195)

-21- Georgia State UniversitySensorweb Research Laboratory Error-Detection Codes Note When adding numbers, a carryout from the most significant bit needs to be added to the result Example: add two 16-bit integers wraparound sum checksum Internet checksum

-22- Georgia State UniversitySensorweb Research Laboratory Error-Detection Codes view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that exactly divisible by G (modulo 2) receiver knows G, divides by G. If non-zero remainder: error detected! can detect all burst errors less than r+1 bits, and odd number of bit errors widely used in practice (ATM, HDCL) Cyclic Redundancy Check

-23- Georgia State UniversitySensorweb Research Laboratory CRC Example Want: D. 2 r XOR R = nG equivalently: D. 2 r = nG XOR R equivalently: if we divide D. 2 r by G, want remainder R R = remainder[ ] If G has r+1 bit, R has r bit D.2rGD.2rG Operation is XOR, not +, or -!!! Notice: x^3+1 is a generator! Left shift r bit

-24- Georgia State UniversitySensorweb Research Laboratory Another Example Calculation of the polynomial code checksum. G CRC-32 =

-25- Georgia State UniversitySensorweb Research Laboratory CRC generator preference Generator G(x) with degree k can detect: All single-bit errors, as long as the x k and x 0 terms have nonzero coefficients. All double-bit errors, as long as G(x) has a factor with at least three terms. Any odd number of errors, as long as G(x) contains the factor (x+1). Any “burst” error(i.e., sequence of consecutive errored bits) with the length less than (including equal) k bits. Burst errors of larger than k bits can also be detected with high probability 1-1/2 k ) * Refer textbook page

-26- Georgia State UniversitySensorweb Research Laboratory Flow Control Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow Transmission time Time taken to emit all bits into medium Propagation time Time for a bit to traverse the link

-27- Georgia State UniversitySensorweb Research Laboratory Model of Frame Transmission X

-28- Georgia State UniversitySensorweb Research Laboratory Data Link Layer Road Map Data link layer design issues Framing Error Control Example data link protocols HDLC and PPP Multiple Access Protocols Static channel allocation Dynamic channel allocation LAN technologies and their MAC protocols Ethernet WiFi and WiMax

-29- Georgia State UniversitySensorweb Research Laboratory Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!

-30- Georgia State UniversitySensorweb Research Laboratory HDLC (High-Level Data Link Control) Frame format for bit-oriented protocols. Data link layer protocol used in IBM mainframe world, X.25 network, etc  Using slide window protocol (GBN, SR, or variations) for flow control and reliable data transfer

-31- Georgia State UniversitySensorweb Research Laboratory HDLC (High-Level Data Link Control) Control field of (a) An information frame. (b) A supervisory frame ACK or RECEIVE READY NAK or REJECT: follow GBN protocol RECEIVE NOT READY: ask sender to stop SELECTIVE REJECT: follow SR protocol (c) An unnumbered frame used for control purpose or carry data when unreliable connectionless service needed ACKed/NAKed frame#

-32- Georgia State UniversitySensorweb Research Laboratory The Data Link Layer in the Internet A home personal computer acting as an internet host.

-33- Georgia State UniversitySensorweb Research Laboratory PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address

-34- Georgia State UniversitySensorweb Research Laboratory PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

-35- Georgia State UniversitySensorweb Research Laboratory PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

-36- Georgia State UniversitySensorweb Research Laboratory PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error detection Not bit-oriented, e.g., bit-stuffing, it uses byte-stuffing!

-37- Georgia State UniversitySensorweb Research Laboratory Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern Q: is received data or flag? Sender: adds (“stuffs”) extra byte after each data byte Receiver: two bytes in a row: discard first byte, continue data reception single : flag byte

-38- Georgia State UniversitySensorweb Research Laboratory Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

-39- Georgia State UniversitySensorweb Research Laboratory PPP Data Control Protocol Before exchanging network-layer data, data link peers must LCP: configure PPP link (max. frame length, authentication) NCP: learn/configure network layer information for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address LCP configuration

-40- Georgia State UniversitySensorweb Research Laboratory Reading Assignment Chapter – 5.6