Data Link Layer 10/28/2013. Data Link Layer. Adaptors Communicating. Data Link Layer. Session Transport Network Data Link Physical

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1 /28/23 Data Link Layer Department f Cmputer and IT Engineering University f Kurdistan Data Cmmunicatin Netwtks (Graduate level) Data Link Layer By: Dr. Alireza Abdllahpuri 2 Data Link Layer Applicatin Presentatin Sessin Transprt Netwrk Data Link Physical Functin: Send blcks f data (frames) between physical devices Regulate access t the physical media Key challenge: Hw t delineate frames? Hw t detect errrs? Hw t perfrm media access cntrl (MAC)? Hw t recver frm and avid cllisins? 3 Adaptrs Cmmunicating sending nde datagram frame adapter link layer implemented in adaptr (NIC) Ethernet card, PCMCI card, 82. card sending side: encapsulates datagram in a frame adds errr checking bits, rdt, flw cntrl, etc. frame link layer prtcl adapter receiving nde receiving side lks fr errrs, rdt, flw cntrl, etc extracts datagram, passes t receiving nde adapter is semiautnmus link & physical layers 4

2 /28/23 Hp-t-hp delivery Link-Layer Services Service prvided by the data link layer fr netwrk layer: Unacknwledged cnnectinless service Acknwledged cnnectinless service Acknwledged cnnectin-riented service 5 6 Link-Layer Duties Framing Encapsulating packet int frame, adding header, trailer Using MAC addresses, rather than IP addresses Errr detectin and/r crrectin Errrs caused by signal attenuatin, nise. Receiver detecting presence f errrs Receiver crrecting errrs withut retransmissin Flw cntrl Pacing between adjacent sending and receiving ndes Media access cntrl (MAC) Pacing between adjacent sending and receiving ndes Framing 7 8 2

3 /28/23 Framing Break sequence f bits int a frame Typically implemented by the netwrk adaptr. Byte Cunting (character cunt) Data Sender: insert length f the data in bytes at the beginning f each frame Receiver: extract the length and read that many bytes Hw t define the brders? what if the cunt field gets crrupted? 9 Byte Oriented: Byte stuffing START DLE DLE Data DLE END Add START and END sentinels t the data Prblem: what if END appear in the data? END Add a special DLE (Data Link Escape) character befre END What if DLE appears in the data? Add DLE befre it. Used by Pint-t-Pint prtcl, e.g. mdem, DSL, cellular Bit Oriented: Bit Stuffing Data Add sentinels t the start and end f data Bth sentinels are the same Example: in High-level Data Link Prtcl (HDLC) Sender: insert a after each in data Knwn as bit stuffing Receiver: after seeing in the data remve the (it was stuffed) lk at ne mre bit end f frame errr! Discard the frame Disadvantage: 2% verhead at wrst 2 3

4 /28/23 Bit Stuffing- example Physical Layer Cding Vilatins Only applicable if physical layer cding has sme redundancy The cmbinatins f lw-lw and high-high which are nt used fr data may be used fr marking frame bundaries. 3 e.g., in Manchester cding 4 Errr Cntrl 5 Transmissin Errrs Transmissin errrs are caused by: Thermal nise {Shannn} impulse nise (e..g, arcing relays) signal distrtin during transmissin (attenuatin) crsstalk vice amplitude signal cmpressin (cmpanding) quantizatin nise (PCM) jitter (variatins in signal timings) receiver and transmitter ut f synch. 6 4

5 /28/23 Packet crruptin Types f errrs: single bit; nly bit in the data unit has changed burst errr; 2 r mre bits in the data unit have changed Example: less likely. s f burst impulse nise 2 bps data rate 2 bits f errrs Errr Detectin Errr detectin Transmit extra (redundant) infrmatin Use redundant infrmatin t detect errrs Extreme case: send tw cpies f the data Trade-ff: accuracy vs. verhead Techniques fr detecting errrs Parity checking Checksum Cyclic Redundancy Check (CRC) 7 8 Errr detectin Errr detectin techniques Redundancy: adding extra bits fr detecting errrs at the destinatin 9 2 5

6 /28/23 Parity bits Idea: add extra bits t keep the number f s even Example: 7-bit ASCII characters + parity bit Detects -bit errrs and sme 2-bit errrs Nt reliable against bursty errrs Tw Dimensinal Parity Parity bit fr each clumn Can detect all -, 2-, and 3-bit errrs, sme 4-bit errrs 4% verhead Parity bit fr each rw Parity bit fr the parity byte 2 22 Tw Dimensinal Parity Examples Tw Dimensinal Parity Examples Odd number f s Odd number f s Odd Number f s Odd number f s Odd Number f s Cannt be detected Odd number f s

7 /28/23 Errr detectin techniques 25 Checksums Idea: Add up the bytes in the data Include the sum in the frame START Data Checksum END Use nes-cmplement arithmetic Lwer verhead than parity: 6 bits per frame But, nt resilient t errrs Why? + = Used in UDP, TCP, and IP 26 Checksum s cmplement. The data unit is divided int k sectins, each f n bits 2. All sectins are added using s cmplement 3. The sum is cmplemented 4. The checksum is sent with data Example: IP header 27 Suppse the fllwing blck f 6 bits is t be sent using a checksum f 8 bits. The numbers are added using ne s cmplement Sum Checksum The pattern sent is 28 7

8 /28/23 Checksum (cnt d) Errr detectin techniques Perfrmance Can we use this technique t detect an errr if 2 crrespnding ppsite bits in 2 data segments becme crrupted? An example: Transmitted sequence: Received sequence: 29 3 Cyclic Redundancy Check (CRC) Uses field thery t cmpute a semi-unique value fr a given message Much better perfrmance than previus appraches Fixed size verhead per frame (usually 32-bits) Quick t implement in hardware Only in 2 32 chance f missing an errr with 32-bit CRC Details are in the bk/n Wikipedia Cyclic Redundancy Check (CRC) Represent a (n+)-bit message as an n-degree plynmial M(x) E.g., M(x) = x 7 + x 5 + x 3 + x 2 + x Chse a divisr k-degree plynmial G(x) Cmpute reminder R(x) f M(x)*x k / G(x), i.e., cmpute A(x) such that M(x)*x k = A(x)*G(x) + R(x), where degree(r(x)) < k Let T(x) = M(x)*x k R(x) = A(x)*G(x) Then T(x) is divisible by G(x)

9 /28/23 CRC check Based n binary divisin CRC added t the data unit s that the resulting data becmes exactly divisible by a predetermined number CRC exactly less bit than the divisr CRC generatr Mdul-2 divisin. A string f n zers added t the data unit 2. New data divided by divisr 3. Replace s by CRC CRC generatr (cnt d) CRC perfrmance Name Plynmial Applicatin CRC-8 x 8 + x 2 + x + ATM header CRC- x + x 9 + x 5 + x 4 + x 2 + ATM AAL ITU-6 x 6 + x 2 + x 5 + HDLC ITU-32 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 + LANs 35 CRC can detect. all burst errrs that affect dd number f bits 2. all burst errrs f length degree f plynmial 3. with high prbability burst errrs > degree f plynmial Example: CRC-2 (degree f 2). OK 2. OK % 36 9

10 /28/23 Errr crrectin. By retransmissin flw and errr cntrl prtcls 2. Frward Errr Crrectin (FEC) require mre redundancy bits shuld lcate the invalid bit r bits n-bit cde wrd cntains m data bits + r redundancy bits n=m+r m+r+ bits discverable by r bits m r r Example: 2 m + r + Fr m=5 data bits can r be 3? Cde-wrd 37 Hamming distance The Hamming distance is the number f bits that have t be changed t get frm ne bit pattern t anther. Example: & have a hamming distance f 2 Fr any cding whse members have a Hamming distance f tw, any ne bit errr can be detected. Why? 38 Hamming distance Errr detectin/crrectin (a) A cde with pr distance prperties x x x x x x x (b) x = cdewrds = nn-cdewrds A cde with gd distance prperties x x x x x x x T detect d single bit errrs, yu need a d+ cde distance. T crrect d single bit errrs, yu need a 2d+ cde distance. In general, the price fr redundant bits is t expensive t d errr crrectin fr netwrk messages. Netwrk prtcls use errr detectin and ARQ. 39 4

11 /28/23 Hamming cde Example: 7-bit ASCII cde + 4 bit redundancy Hamming cde (cnt d) Example f redundancy bit calculatin (even parity) Each r bit is the parity bit fr ne cmbinatin f data bits r: bits:, 3, 5, 7, 9, r2: bits: 2, 3, 6, 7,, r3: bits 4, 5, 6, 7 r4: bits 8, 9,, Σ Σ Σ Σ Σ Σ Σ Σ Σ Σ Σ Hamming cde (cnt d) Errr detectin using Hamming cde Flw Cntrl nce the bit is identified the receiver can reverse its value 43 44

12 /28/23 Flw Cntrl Categries f Flw Cntrl The prcess f managing the rate f data transmissin between tw ndes t prevent a fast sender frm verwhelming a slw receiver Transmissin Link Buffer Stp-and-Wait Autmatic Repeat request Simplest flw and errr cntrl mechanism The sending device keeps a cpy f the last frame transmitted until it receives an acknwledgement identificatin f duplicate transmissin (lst r delayed ACK) A damaged r lst frame is treated in the same way A Simplex Stp-and-Wait ARQ. Nrmal peratin 2. The frame is lst 3. The ACK is lst 4. The ACK is delayed Timers intrduced Psitive ACK sent nly fr frames received safe & sund

13 /28/23 Stp-and-Wait ARQ(Nrmal peratin) Stp-and-Wait ARQ(Lst r damaged frame) The sender will nt send the next piece f data until it is sure that the current ne is crrectly received sequence number necessary t check fr duplicated packets N NACK when packet is crrupted duplicate ACKs instead rundtrip time + prcessing in the receiver Shuld be as shrt as pssible. Why? 49 5 Stp-and-Wait ARQ (Lst ACK) Stp-and-Wait ARQ (Delayed ACK) Imprtance f frame numbering Imprtance f ACK numbering Numbering frames prevents the retaining f duplicate frames 5 Numbered acknwledgments are needed if an acknwledgment is delayed and the next frame is lst. 52 3

14 /28/23 Duplex Stp-and-Wait ARQ Piggybacking cmbine data with ACK (less verhead saves bandwidth) RTT Perfrmance f Stp-and-wait ARQ d trans = (Frame size)/bandwidth d ptp = (Speed f signal)/ (Channel length) Utilizatin = d trans /( d trans + 2 d prp ), errr free case r Utilizatin = (-P E )d trans /( d trans + 2 d prp, ) errr case Stp-and-wait peratin Example: Gbps link, 5 ms prp. delay, 8 bit frame: sender receiver first bit transmitted, t = last bit transmitted, t = L / R Sliding windw (Pipelined) prtcls Pipelining: sender allws multiple, in-flight, yet-t-beacknwledged frames range f sequence numbers must be increased buffering at sender and/r receiver RTT first bit arrives last bit arrives, send ACK ACK arrives, send next frame, t = RTT + L / R very lw d trans L 8bits = = R bps 9 = 8micrsecnds U sender = L / R RTT + L / R = = Tw generic frms f pipelined prtcls: G-Back-N, Selective repeat 56 4

15 /28/23 Sliding Windw Prtcls Sequence numbers sent frames are numbered sequentially number f frames stred in the header if the number f bits in the header is m than sequence number ges frm t 2 m - Sliding windw t hld the unacknwledged utstanding frames frame acknwledged frames sequence number 57 Questin What is the cnsequence f the windw size in the receiver n the rder f the received packets? Answer: Windw size means that the packets are received in rder This is nt the case fr the larger windw size 58 Sliding Windw: Retransmissin Strategies G-back-N (Cntrl variables) Hw t deal with errrs? If the ACK is nt received then the sender must g back and retransmit earlier frames (frm the lst frame) (G-Back-N) S- hlds the sequence number f the recently sent frame SF hlds sequence number f the first frame in the windw SL hlds the sequence number f the last frame R sequence number f the frame expected t be received Sender nly retransmits frames fr which a NAK is received (the lst r crrupted frames) (Selective repeat)

16 /28/23 The name f G-back-N: why? Re-sending frame when the frame is damaged the sender ges back and sends a set f frames starting frm the last ne ACKn d the number f retransmitted frames is N Example: The windw size is 4. A sender has sent frame 6 and the timer expires fr frame 3 (frame 3 nt ACKn d). The sender ges back and re-sends the frames 3, 4, 5 and 6. G-back-N (nrmal peratin) Hw many frames can be transmitted withut acknwledgment? ACK nt necessary if ACK2 is sent expected sequence number 6 62 G-back-N (damaged r lst frame) G-back-N (sender windw size) damaged frames are discarded! Why are crrectly received packets nt buffered? What is the disadvantage f this? sequence number

17 /28/23 G-back-N (drawback) Inefficient all ut f rder received packets are discarded This is a prblem in a nisy link many frames must be retransmitted -> bandwidth cnsuming Slutin re-send nly the damaged frames Selective Repeat ARQ Prcessing at the receiver mre cmplex The windw size is reduced t 2 m- (at mst) Bth the transmitter and the receiver have the same windw size Receiver expects frames within the range f the sequence numbers Selective Repeat ARQ avid unnecessary retransmissins Selective Repeat ARQ (lst frame) Selective Repeat

18 /28/23 Selective Repeat ARQ (sender windw size) Sliding windw- analysis Frame transmissin time is nrmalized t a value f ; thus, the prpagatin time is a Sliding windw- analysis The thrughput n the line depends n bth the windw size W and the value f a Sliding windw- utilizatin a =d prp /d trans Errr free channel Selective repeat with errr G-back-N with errr Assume a frame is in errr with prbability P

19 /28/23 Sequence number and windw size Stp-and-wait utilizatin (example) Stp-and-wait ARQ G-back-N Selective repeat Transmitter windw size W W Receiver windw size W Hw many sequence numbers required? 2 +W In SR prtcl with the windw size f 5, hw many bits required t numbering the sequence f frames? 2*5= sequence numbers, we need 4 bits 2W A channel has a bit rate f 4 kbps and a prpagatin delay f 2 ms. Fr what range f frame sizes des Stpand-Wait give an efficiency (link utilizatin) > 5%? Efficiency will be 5% when the time t transmit the frame equals the rundtrip prpagatin delay. At a transmissin rate f 4 bits/ms, 6 bits takes 4 ms. Fr frame sizes abve 6 bits, stp and wait is reasnably efficient Sliding windw utilizatin (example) A 3 km lng T trunk is used t transmit 64 byte frame s using selective-repeat prtcl. If the prpagatin speed is 6 µsec/km, hw many bits shuld the sequence numbers be? T perate efficiently, the sequence space (actually, the send windw size) must be large enugh t allw the transmitter t keep transmitting until the first acknwledgement has been received. The prpagatin time is 8 ms. At T speed, which is.536 Mbps (excluding the header bit), a 64 byte frame takes.3 msec. Therefre, the first frame fully arrives 8.3 msec after its transmissin was started. The acknwledgement takes anther 8 msec t get back, plus a small (negligible) time fr the acknwledgement t arrive fully. In all, this time is 36.3 msec. The transmitter must have enugh windw space t keep ging fr 36.3 msec. A frame takes.3 ms, s it takes 2 frames t fill the pipe. Seven bit sequence numbers are needed 75 Questins 76 9