1. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-1 Link & Physical Layers Computer Networks Shahrood University of Technology Department of Computer Engineering & IT

2. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-2 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

3. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-3 Some terminology:  hosts and routers are nodes (bridges and switches too)  communication channels that connect adjacent nodes along communication path are links  wired links  wireless links  LANs  PDU: frame, encapsulates datagram link layer has responsibility of transferring datagram from one node to adjacent node over a link. application transport network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical modem “link” Link Layer: Introduction

4. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-4 Layers Application Software Application protocols (softwares) Transport Protocols (softwares) Network(Internetwork) Protocols (softwares) Link & Physical Protocols (Software + Hardware) TCP/IP Protocol Stack Logical Link Control Protocols (software) Medium Access Control Protocols (Hardware) Physical Protocols (Hardware) Ethernet, Token Ring, Token Bus FDDI,...,

5. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-5  Datagram transferred by different link protocols over different links:  e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link  Each link protocol provides different services  e.g., may or may not provide reliable data transfer (rdt) over link tourism analogy:  trip from Tehran to Toos  taxi: Tehran to Mehrabad Airport  plane: Mehrabad to Mashhad  bus: Mashhad to Toos  tourist = datagram  taxi, plane, bus = communication link  transportation mode = link layer protocol  tour agent = routing algorithm Link layer: context

6. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-6 IP Add: A LAN Add: B ppp, 33.3kbps IP Add: IR11 LAN Add;LR12 IP Add: IR13 LAN Add: LR13 client server Src IP Add=A Dst IP Add=S Src LAN Add=B Dst LAN Add=LR12 client to router1 Frame: router1 router2 router3 router4 Ethernet, 10Mbps IP Add: IR22 LAN Add: LR22 IP Add: S LAN Add: T Src IP Add=A Dst IP Add=S Src LAN Add=LR13 Dst LAN Add=LR22 router1 to router2 Frame: Src IP Add=A Dst IP Add=S Src LAN Add=LR42 Dst LAN Add=T router4 to server Frame: IP Add: IR42 LAN Add: LR42 Fast ethernet LAN Add: 48 bit(6 × 8bit) (example: 74-29-9C-E8-FF-55) Physical Frame Transfer

7. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-7  Framing, link access:  encapsulate datagram into frame, adding header, trailer  channel access if shared medium  ‘physical addresses’ used in frame headers to identify source, destination.  different from IP address!  Reliable delivery between adjacent nodes  we learned how to do this already (chapter 3)!  seldom used on low bit error link (fiber, some twisted pair)  wireless links: high error rates  Q: why both link-level and end-end reliability? Link layer headerLink layer trailerNetwork layer datagram Layer 2 PDU: Frame Link Layer Services

8. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-8 Link Layer Services (more)  Flow Control:  pacing between adjacent sending and receiving nodes  Error Detection:  errors caused by signal attenuation, noise.  receiver detects presence of errors:  signals sender for retransmission or drops frame  Error Correction:  receiver identifies and corrects bit error(s) without resorting to retransmission  Half-duplex and full-duplex  with half duplex, nodes at both ends of link can transmit, but not at same time

9. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-9 Adaptors Communicating  link layer implemented in “adaptor” (NIC)  Ethernet card, PCMCI card, 802.11 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 receiving node  adapter is semi-autonomous  link & physical layers sending node frame receiving node datagram frame adapter link layer protocol

10. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-10 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

11. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-11 Error Detection 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

12. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-12 Internet checksum Sender:  treat segment contents as sequence of 16-bit integers  checksum: addition (1’s complement sum) of segment contents  sender puts checksum value into UDP or TCP checksum field. Receiver:  compute checksum of received segment  check if computed checksum equals checksum field value:  NO - error detected  YES - no error detected. But maybe errors nonetheless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)

13. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-13 Checksumming: Cyclic Redundancy Check  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= add without carry)  receiver knows G, divides by G. If non-zero remainder: error detected!  can detect all burst errors less than r+1 bits  widely used in practice (ATM, HDCL) D: data bits to be sentR: CRC bits d [bits] r [bits] bit pattern mathematical formula : [D×2 r XOR R]

14. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-14 CRC Example want: D×2 r XOR R = n×G equivalently: D×2 r = n×G XOR R equivalently: if we divide D×2 r by G, want remainder R R = remainder[ ] D×2 r G D=101110, G=1001 r=3, D×2 r =101110000 D×2 r G = 101110000 1001 1001 101011 101 000 1010 1001 110 000 1100 1001 1010 1001 011 R

15. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-15 Ethernet CRC  Polynomial Presentation  Based on use of polynomial codes  Message frame and Generator thought of as binary polynomials  Example: 101101101 ~ x 8 + x 6 + x 5 + x 3 + x 2 + x 0  Ethernet CRC  It is a CRC-32: It is 33 bit code that is uses as Generator:  G(x) = x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11 + x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1

16. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-16 CRC Properties  Detect all single-bit errors if coefficients of x r and x 0 of G(x) are one  Detect all double-bit errors, if G(x) has a factor with at least three terms  Detect all number of odd errors, if G(x) contains factor (x+1)  Detect all burst of errors smaller than r bits

17. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-17 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

18. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-18 hub Client Printer Server Client LAN Switch Client Remote Access Server Modem pools Telephone Lines Router External Link Servers modem Client PPP for dial-up access point-to-point link between LAN switch and hosts.  point-to-point  PPP for dial-up access  point-to-point link between Ethernet switch and host Types of “links”- Point to Point

19. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-19  broadcast (shared wire or medium)  traditional Ethernet (coaxial bus, hub)  802.11 wireless LAN  upstream Hybrid Fiber Coaxial  Efficiency: Low Latency & High Throughput [in average] Types of “links”-Broadcast terminator

20. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-20 Ideal Broadcast Channel Access Protocol  Broadcast channel of rate R bps  When one node wants to transmit, it can send at rate R.  When M nodes want to transmit, each can send at average rate R/M  Fully decentralized:  no special node to coordinate transmissions  no synchronization of clocks, slots  Simple

21. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-21  Channel Partitioning  divide channel into smaller “pieces” (time slots, frequency, code)  allocate piece to node for exclusive use  Random Access  channel not divided, allow collisions  “recover” from collisions  “Taking turns” (Token/Polling)  tightly coordinate shared access to avoid collisions Broadcast Classes

22. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-22 Random Access Protocols  When node has packet to send  transmit at full channel data rate R.  no a priori coordination among nodes  two or more transmitting nodes -> “collision”,  random access MAC protocol specifies:  how to detect collisions  how to recover from collisions (e.g., via delayed retransmissions)  Examples of random access MAC protocols:  slotted ALOHA  ALOHA  CSMA, CSMA/CD, CSMA/CA

23. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-23 ALOHA (Pure, Slotted)  All frames same size  Slotted: Time is divided into equal size slots  1 slot = time to transmit 1 frame  Pure: Time remains continues  Slotted: Nodes start to transmit frames only at beginning of slots  Pure: Nodes start to transmit whenever a frame is made.  Slotted: Nodes are synchronized  Pure: Nodes are not synchronized  If 2 or more nodes transmit in slot, all nodes detect collision

24. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-24  Suppose N nodes with many frames to send, each transmits in slot with probability p  probability that first node has success in a slot = p(1-p) N-1  probability that any node has a success is = Np(1-p) N-1  For max efficiency with N nodes, find p m that maximizes Np(1-p) N-1  For many nodes, take limit of N p m (1- p m ) N-1 as N goes to infinity, gives 1/e = 0.37 At best: channel used for useful transmissions 37% of time! Slotted Aloha Efficiency

25. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-25 time t0t0 t 0 -TF t 0 +TF Vulnerable period frame transmission Suppose F : the average frame length, R : bandwidth, TF=F/R : frame time Transmit a frame at t=t 0 (and finish transmission of the frame at t 0 +TF ) Vulnerable period: if any other frames are transmitted during the period, the collision will occur. Therefore the probability of a successful transmission is the probability that there is no additional transmissions in the vulnerable period. Pure Aloha Mosel-1

26. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-26 Pure Aloha Mosel-2  Poisson model: probability of k frames transmission attempts in t time units. Time unit = 1 frame Time = F/R  G =Offered Load [Frames/Time unit]  infinite population model: (too many senders each with too many frames to transmit. t t t time …… The green frame dose not experience collision if during Vulnerable period no one tries to transmit a frame:

27. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-27 Throughput vs Offered Load- Pure Aloha  The throughput S is given by:

28. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-28 Slotted ALOHA Model  Synchronize the transmissions of stations  All stations keep track of transmission time slots and are allowed to initiate transmissions only at the beginning of a time slot.  Suppose a packet occupies one time slot  Vulnerable period is from t 0 -TF to t 0, i.e., TF seconds long.  Therefore, the throughput of the system is:

29. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-29 S-G Graphs-1 Slotted ALOHA (pure) Non-slotted ALOHA 0.37 0.18 0.5 G =Offered Load [Frame/Time unit] S =Throughput [Frame/Time unit]

30. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-30 S-G Graphs-2 Slotted ALOHA (pure) Non-slotted ALOHA G =Offered Load [Frame/Time unit] S =Throughput [Frame/Time unit] S=kG

31. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-31 Lack of collision control Collision Control Offered load Throughput Controlled Uncontrolled

32. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-32 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit:  If channel sensed idle: transmit entire frame  If channel sensed busy, defer transmission

33. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-33 CSMA Collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability ABC D time t0t0 t1t1 distance Collision Detection Times Frame transmission time terminator

34. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-34 collision occurs: D A finishes transmitting a frame, then receives D’s frame so A cannot detect the collision. F: Frame length [bit] R: Transmission Rate (Bandwidth) [bit/sec] —› TF=F/R d: A to D distance [m] V: signal propagation velocity in channel [m/sec] —› TAD= d/V TF ≥ 2×TAD —› F ≥ 2×R×TAD —› α =TAD/(F/R) ≤ 0.5 Collision detection condition: TF: Frame transmission Time [sec] TAD: A to D propagation time [sec] Then: TF≥ 2×TAD ABC D time t0t0 t1t1 distance Collision Detection Time=0 TAD TDA TF ‹ (TAD+TDA) = 2×TAD TF CSMA collisions

35. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-35 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA  collisions detected within short time  colliding transmissions aborted, reducing channel wastage  collision detection:  easy in wired LANs: measure signal strengths, compare transmitted, received signals  difficult in wireless LANs: receiver shut off while transmitting

36. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-36 “Taking Turns” MAC protocols channel partitioning MAC protocols:  share channel efficiently and fairly at high load  inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols  efficient at low load: single node can fully utilize channel  high load: collision overhead

37. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-37 “Taking Turns” MAC protocols Polling:  master node “invites” slave nodes to transmit in turn  concerns:  polling overhead  latency  single point of failure (master) Token passing (virtual ring):  control token passed from one node to next sequentially.  token message  concerns:  token overhead  high latency at low offered load  single point of failure (token)

38. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-38 Summary of MAC protocols Summary of MAC protocols  What do you do with a shared media?  Channel Partitioning, by time, frequency or code  Time Division,Code Division, Frequency Division  Random partitioning (dynamic),  ALOHA, S-ALOHA, CSMA, CSMA/CD  carrier sensing: easy in some technologies (wire), hard in others (wireless)  CSMA/CD used in Ethernet  Taking Turns  polling from a central site, token passing

39. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-39 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

40. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-40 Figure 6.11 IEEE 802 LAN standards MAC LLC Network Layer 802.2 Logical Link Control 802.3 CSMA-CD 802.5 Token Ring 802.11 Wireless LAN Other LANs Various Physical Layers Network Layer Data Link Layer Physical Layer  One LLC and several MACs, each MAC has an associated set of physical layers.  MAC provides connectionless transfer. Generally no error control because of relatively error free.  Ethernet consists of 802.2 + 802.3 + a physical layer

41. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-41 “dominant” LAN technology:  Cheap even for 100Mbs!  First widely used LAN technology  Simpler, cheaper than token LANs and ATM  Kept up with speed race: 10, 100, 1000, 10000, 40000 Mbps Metcalfe’s Ethernet Sketch, 1973, Xerox. Ethernet

42. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-42 Ethernet uses CSMA/CD  No slots  Adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense  Transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection  Before attempting a retransmission, adapter waits a random time, that is, random access.  Adapter keeps trying to transmit, that is, multiple access.

43. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-43 Ethernet CSMA/CD algorithm 1. Adaptor gets datagram and creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the m th collision, adapter chooses a K at random from {0,1,2,…,2 m -1}. Adapter waits K×512 bit times and returns to Step 2

44. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-44 Frame Ready for Transmission Sense Channel Channel Busy No Frame Transmission & Channel Sense Busy Collision Abort Transmission; Send Jam Signal(3Bytes) Frame successfully transmitted Yes Wait Inter-frame Gap 9.6 μs Increment Attempts N++ Too Many Attempts? Unsuccessful transmission, Excessive Collisions Ethernet MAC flow in Half-Duplex Mode-Transmission Inter-frame Gap allows receivers time to settle N=15 N<15 N<10 yesNo K=N K=10 Select A Random Integer R=(0 to 2 k -1) wait R×512 bit times Set Attempt N=0 Exponential backoff

45. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-45 Ethernet’s CSMA/CD (more) Bit time: 0.1 µsec for 10 Mbps Ethernet ; for K=1023, wait time is about 1023 ٭ (0.1 ٭ 512)≈50 msec Jam Signal: makes sure all other transmitters are aware of collision; 32 bits; Exponential Backoff:  Goal: adapt retransmission attempts to estimated current load  heavy load: random wait will be longer  first collision: choose K from {0,1}; delay is K x 512 bit transmission times  after second collision: choose K from {0,1,2,3}…  after next collision double K (and keep doubling on collisions until…..)  after 10 collisions, choose K randomly from {0,1,2,3,4,…,1023}

46. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-46 Ethernet comments  Exponential Back off: upper bounding at K = 1023(2 10 -1) limits max size.  could remember last value of K when we were successful (analogy: TCP remembers last values of congestion window size)  Q: why use binary back off rather than something more sophisticated such as TCP’s additive increase/multiplicative decrease (AIMD) -> simplicity (?)  note: Ethernet does multiplicative-increase-complete-decrease (why?) Increase the waiting probability range by 2 m Decreasing the rang completely after getting through

47. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-47 Ethernet MAC flow in Half-Duplex Mode-Receive Receive Process Channel Sense Idle Busy Start Receiving Channel Sense Received Frame too Small? (Jam Signal) Recognize Address? Valid Frame Check Sequence? Extra bits? Receive Alignment Error Receive Frame Check Error Successful Reception Busy Idle Yes No Yes No Yes No

48. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-48 Ethernet’s use of randomization More collisions Heavier Load, more nodes trying to send Randomize retransmissions over longer time interval, to reduce collision probability

49. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-49 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model Ethernet and Randomization Ethernet Model (throughput, response time, efficiency) 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

50. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-50 Ethernet Model  An analytical model for Ethernet is developed.  The model includes  Throughput,  Response Time and  Efficiency (Utilization)

51. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-51 Model Assumptions  Large number of active nodes, with each node having a large number of frames to send.  Fixed length frames.  The packets transmission probability is Poisson.  Poisson model: probability of k packets transmission attempts in t time units  infinite population model

52. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-52  Frame transmission time is unit of time (F/R).  Throughput S - number of frames successfully (without collision) transmitted per unit time.  Offered load G - number frames transmissions attempted per unit time.  Note: S <= G, S depends on G. Model Parameters

53. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-53 Throughput Model busy time is a random variable given by B idle time is a random variable given by I collision time is a random variable given by C time ……  The throughput S is given by:

54. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-54 Throughput vs Offered Load-Average Idle Time Because it is a Poisson Process then: I is a variable with an exponential distribution with expected value =1/ G Average (Accepted) idle time = G : Average offered load per unit time. 1/G : Average time between two consecutive transmission. Idle time:

55. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-55 Throughput vs Offered Load-Average Busy Time 1+α1+α α time no one transmits frame clear the channel successful frame transmission busy time length = B = (1+ α) Average (Accepted) busy time = A successful packet transmission:   = end-to-end propagation time/unit time  time during which collisions can occur Probability of successful transmission of a frame α

56. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-56 Throughput vs Offered Load-Average Collision Time Average (Accepted) collision time = A collision in the channel: transmission time α A frame enters into channel Any transmission causes collision time α Channel cleared Collision time length = α time αα Collision time length = 2α Channel cleared Collision time length = 1.5 α

57. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-57 Average Throughput  The throughput S is given by: frames/Unit time

58. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-58 Response Time Model Average response time: Unit time/frame Average response time is the time during which a frame gets through.  The throughput R is given by:

59. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-59 S-G Graphs-1 G =Offered Load [Frames/ unit time ] S =Throughput [Frames/ unit time ] capacity of CSMA/CD: maximum value of S over all values of G > α =0.1 means “10% of frame get transmitted before every one on the channel hears (detects) it”. α=0.01 α=0.02 α=0.05 α=0.1 α=0.2 > > > > >

60. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-60 S-G Graphs-2 G =Offered Load [Frames/ unit time ] S =Throughput [Frame/ unit time ] α=0.01 α=0.02 α=0.05 α=0.1 α=0.2 ALOHA S-ALOHA

61. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-61 Lack of collision control Collision Control Offered load Throughput Controlled Uncontrolled

62. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-62 R-G Graphs α = 0.01 α = 0.02 G =Offered Load [Frame/ unit time ] R = Response time [ unit time ] 1.02 1.01 1. 1 G =Offered Load [Frame/ unit time ] R = Response time [ unit time ] α = 0.1 α = 0.2 G =Offered Load [Frame/ unit time ] R = Response time [ unit time ] 1.2 α = 0.05 G =Offered Load [Frame/ unit time ] R = Response time [ unit time ] 1.05 α =0.01 α =0.02

63. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-63 Efficiency Model

64. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-64 A Discussion on Efficiency  Efficiency goes to 0 as d/v=t prop goes up (long distance)  Efficiency goes to 0 as F/R=t trans goes to 0 (high bandwidth)  People want: high bandwidth over long distances!  Recommendation: Don’t use Ethernet.

65. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-65 Efficiency2 G =Offered Load [Frames/ unit time ] α=0.01 α=0.02 α=0.05 α=0.1 α=0.2

66. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-66 Efficiency3 G =Offered Load [Frames/ unit time ] α=0.01 α=0.02 α=0.05 α=0.1 α=0.2

67. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-67 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

68. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-68 Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble:  7 bytes with pattern 10101010 followed by one byte with pattern 10101011  used to synchronize receiver, sender clock rates PADPAD 46 to 1500 Bytes 8B6B (2B) 4B Mini :6+6+2+46+4= 64 Bytes (512 bits) Max :6+6+2+1500+4= 1518 Bytes Ethernet Frame Structure-1

69. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-69  Addresses: 6 bytes  if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol  otherwise, adapter discards frame  Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)  CRC: checked at receiver, if error is detected, the frame is simply dropped Ethernet Frame Structure-2

70. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-70 Unreliable, connectionless service  Connectionless: No handshaking between sending and receiving adapter.  Unreliable: receiving adapter doesn’t send ACKs or NACKs to sending adapter  stream of datagrams passed to network layer can have gaps  gaps will be filled if application is using TCP  otherwise, application will see the gaps

71. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-71 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

72. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-72 LAN Addresses and ARP 32-bit IP address:  network-layer address  used to get datagram to destination IP network (recall IP network definition) LAN (or MAC or physical or Ethernet) address:  used to get datagram from one interface to another physically- connected interface (same network)  48 bit MAC address (for most LANs) burned in the adapter ROM

73. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-73 LAN Address  MAC address allocation administered by IEEE  Manufacturer buys portion of MAC address space (to assure uniqueness)  MAC flat address —› portability  can move LAN card from one LAN to another  IP hierarchical address NOT portable  depends on IP network to which node is attached

74. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-74 Ethernet Address Format  Every vendor (e.g., 3COM) is assigned a vendor block code.  Therefore, every globally administered address is globally unique.

75. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-75 LAN Addresses and ARP-1 1A-23-F9-CD-06-9B 8B-B2-2F-54-1A-0F 49-BD-D2-C7-56-2A 5C-66-AB-90-75-B1 61-BC-85-50-C1-7B B1-C6-A1-0B-B9-80 LAN 240.108.12.01 240.108.12.02 240.108.12.03 240.108.12.04 240.108.12.05 240.108.12.06 Each Adapter on LAN has unique LAN address Network Interface Card (Adaptor)

76. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-76 Recall earlier routing discussion Starting at A, given IP datagram addressed to B:  look up net. address of B, find B on same net. as A  link layer send datagram to B inside link-layer frame 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B datagram B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload frame source, dest address datagram source, dest address CRC frame datagram

77. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-77 ARP: Address Resolution Protocol  Each IP node (Host, Router) on LAN has ARP table  ARP Table: IP/MAC address mappings for some LAN nodes  TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B knowing B’s IP address? 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B

78. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-78 Operation of ARP FTP TCP IP Ethernet driver ARP TCP resolver IPARP Ethernet driver ARP hostname IP addr Establish connection with IP address Send IP datagram to IP address ARP request (Ethernet broadcast) (4) (5) (6) (3) (1) (2) (7) (8)(9) A B LAN

79. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-79 Routing to inside a LAN  A wants to send datagram to B, and A knows B’s IP address.  Suppose B’s MAC address is not in A’s ARP table.  A broadcasts ARP query packet, containing B's IP address  all machines on LAN receive ARP query  B receives ARP packet, replies to A with its (B's) MAC address  frame sent to A’s MAC address (unicast)  A caches (saves) IP-to- MAC address pair in its ARP table until information becomes old (times out)  soft state: information that times out (goes away) unless refreshed  ARP is “plug-and-play”:  nodes create their ARP tables without intervention from network administrator

80. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-80 Routing to another LAN LAN1 1A-23-F9-CD-06-9B 240.108.12.01 49-BD-D2-C7-56-2A 240.108.12.03 61-BC-85-50-C1-7B 240.108.12.02 LAN2 B1-C6-A1-0B-B9-80 40.211.7.200 33-5A-18-0E-CC-12 A B walkthrough: send datagram from A to B via R (assume A knows B’s IP address)  Two ARP tables in router, one for each IP network (LAN)  In ARP table at source, find R’s MAC address 49-BD-D2-C7-56-2A

81. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-81  A creates datagram with source A, destination B  A uses ARP to get R’s MAC address for 240.108.12.03  A creates link-layer frame with R's MAC address as destination, frame contains A-to-B IP datagram  A’s data link layer sends frame  R’s data link layer receives frame  R removes IP datagram from Ethernet frame, sees its destined to B  R uses ARP to get B’s physical layer address  R creates frame containing A-to-B IP datagram sends to B Routing to another LAN’

82. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-82 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

83. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-83  10: 10Mbps; 2: under 200 meters max cable length  thin coaxial cable in a bus topology  repeaters used to connect up to multiple segments  repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!  has become a legacy technology Ethernet Technologies: 10Base2

84. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-84  Hubs are essentially physical-layer repeaters:  bits coming in one link go out all other links  no frame buffering  no CSMA/CD at hub: adapters detect collisions  provides network management functionality Hubs hub node1 node2 node3 node4 node5 node6 To higher level hubs/switches

85. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-85  Used in 10BaseT, 10Base2  Each bit has a transition  Allows clocks in sending and receiving nodes to synchronize to each other  no need for a centralized, global clock among nodes!  This is physical-layer! Manchester Encoding

86. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-86 IEEE 802.3 10Mbps Mediums Coaxial Cable(50 Ohm) Unshielded twisted pair 850-nm optical fiber pair Baseband (Manchester) Manchester /On-Off Bus Star 500185100500 10030----33 1050.4 to 0.6 62.5/12.5 µm 10Base5 10Base2 10Base-T 10base-FP Transmission medium Signaling technology Topology Max segment length [m] Nodes per segment Cable diameter [mm]

87. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-87 2 pair, STP 2 pair, Category 5 UTP 2 optical fibers 4 pair, cat.3, 4 or 5 UTP MLT-3 4B5B, NRZI8B6T, NRZ 100 Mbps 100 200 400200 100Base-TX 100Base-FX 100Base-T4 Transmission medium Signaling technology Data rate Max segment length [m] Network span [m] IEEE 802.3 100Mbps Mediums STP: Shielded twisted pair; UTP: Unshielded twisted pair NRZ: NonReturn to Zero; NRZI: NRZ Invertwd

88. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-88 Gbit Ethernet  use standard Ethernet frame format  allows for point-to-point links and shared broadcast channels  in shared mode, CSMA/CD is used; short distances between nodes to be efficient  uses hubs, called here “Buffered Distributors”  Full-Duplex at 1 Gbps for point-to-point links  10 or 40 Gbps now !

89. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-89 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

90. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-90 Interconnecting LAN segments  Hubs  Bridges  Switches  Remark: switches are essentially multi-port bridges.  What we say about bridges also holds for switches!

91. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-91 Interconnecting with hubs  Backbone hub interconnects LAN segments  Extends max distance between nodes  But individual segment collision domains become one large collision domain  if a node in CS and a node EE transmit at same time: collision

92. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-92 Bridges  Link layer device  Stores and forwards Ethernet frames  Examines frame header and selectively forwards frame based on MAC destination address  When frame is to be forwarded on segment, uses CSMA/CD to access segment  Transparent  Hosts are unaware of presence of bridges  Plug-and-play, self-learning  bridges do not need to be configured

93. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-93 Bridges: Traffic Isolation  Bridge installation breaks LAN into LAN segments  Bridges filter packets:  Same-LAN-segment frames not usually forwarded onto other LAN segments  Segments become separate collision domains bridge collision domain collision domain = hub = host LAN (IP network) LAN segment

94. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-94 Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...

95. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-95 Self learning  A bridge has a bridge table  Entry in bridge table:  (Node LAN Address, Bridge Interface, Time Stamp)  Stale entries in table dropped (TTL can be 60 min)  Bridges learn which hosts can be reached through which interfaces  When frame received, bridge “learns” location of sender: incoming LAN segment  Records sender/location pair in bridge table

96. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-96 Filtering/Forwarding When bridge receives a frame: if entry found for destination then{ if destination on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

97. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-97 Bridge Example Suppose C sends frame to D and D replies back with frame to C.  Bridge receives frame from from C  notes in bridge table that C is on interface 1  because D is not in table, bridge sends frame into interfaces 2 and 3  Frame received by D

98. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-98 Bridge Learning: Example’  D generates frame for C, sends  Bridge receives frame  Notes in bridge table that D is on interface 2  Bridge knows C is on interface 1, so selectively forwards frame to interface 1  Bridge adds D on interface 2 of the table.

99. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-99 Interconnection Without Backbone  Not recommended for two reasons: - Single point of failure at Computer Science hub - All traffic between EE and SE must path over CS segment

100. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-100 Backbone Configuration Recommended !

101. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-101 Bridges Spanning Tree  For increased reliability, desirable to have redundant, alternative paths from source to destination.  With multiple paths, cycles result - bridges may multiply and forward frame forever  Solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

102. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-102 Some Bridge Features  Isolates collision domains resulting in higher total max throughput  Limitless number of nodes and geographical coverage  Can connect different Ethernet types  Transparent (“plug-and-play”): no configuration necessary

103. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-103 Bridges vs. Routers  Both store-and-forward devices  routers: network layer devices (examine network layer headers)  bridges are link layer devices  Routers maintain routing tables, implement routing algorithms  Bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

104. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-104 Routers vs. Bridges Bridges + and - + Bridge operation is simpler requiring less packet processing + Bridge tables are self learning - All traffic confined to spanning tree, even when alternative bandwidth is available - Bridges do not offer protection from broadcast storms

105. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-105 Routers vs. Bridges Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide protection against broadcast storms - require IP address configuration (not plug and play) - require higher packet processing  bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

106. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-106 Ethernet Switches  Essentially a multi-interface bridge  Layer 2 (frame) forwarding, filtering using LAN addresses  Switching: A-to-A’ and B-to-B’ simultaneously, no collisions  Large number of interfaces  Often: individual hosts, star- connected into switch  Ethernet, but no collisions!

107. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-107 Ethernet Switches’  Cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame  slight reduction in latency  Combinations of shared/dedicated, 10/100/1000 Mbps interfaces

108. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-108 Typical LAN (IP network) Dedicated Shared

109. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-109 hubsbridgesroutersswitches Traffic isolationnoyes plug & playyes noyes Optimal routingno yesno Cut throughyesno yes Summary comparison

110. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-110 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3 Links and Access Protocols 5.4 Ethernet 5.5 Ethernet Model 5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP 5.8 Ethernet Technologies 5.9 Hubs, bridges, and switches 5.10 Point to Point Protocol

111. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-111 Point to Point Link Layer 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, DHL connection, ISDN line  Popular point-to-point LLC protocols:  PPP (point-to-point protocol)  HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

112. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-112 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 de-multiplex upwards.  Bit transparency: must carry any bit pattern in the data field.  Error detection (no correction).  Connection live-ness: detect, signal link failure to network layer.  Network layer address negotiation: endpoint can learn/configure each other’s network address.

113. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-113 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!

114. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-114 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)

115. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-115 PPP Data Frame  info: upper layer data being carried  check: cyclic redundancy check for error detection

116. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-116 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 01111110 bytes in a row: discard first byte, continue data reception  single 01111110: flag byte

117. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-117 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

118. ali.dmohammadi @gmail.comLink & Physical Layers ali.dmohammadi @gmail.comLink & Physical Layers 5-118 PPP Data Control Protocol Before exchanging network-layer data, data link peers must  configure PPP link (max. frame length, authentication)  learn/configure network layer information  for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address