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Komunikasi Data dan Jaringan Komputer (Bagian 2)

Komunikasi Data dan Jaringan Komputer (Bagian 2). Dr. Tb. Maulana Kusuma mkusuma@staff.gunadarma.ac.id http://staffsite.gunadarma.ac.id/mkusuma. Program X. Data. Program Y. AH. Data. Application. Application. Presentation. PH. Data unit. Presentation. Session. SH. Data unit.

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Komunikasi Data dan Jaringan Komputer (Bagian 2)

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  1. Komunikasi Data dan Jaringan Komputer(Bagian 2) Dr. Tb. Maulana Kusuma mkusuma@staff.gunadarma.ac.id http://staffsite.gunadarma.ac.id/mkusuma Magister Teknik Elektro

  2. Program X Data Program Y AH Data Application Application Presentation PH Dataunit Presentation Session SH Data unit Session Transport TH Data unit Transport Network NH Data unit Network Data link LH Data unit LT Data link Physical Bits Physical Physical transmission medium Review of OSI Networking Model Magister Teknik Elektro

  3. Data Link Layer • Means of activating, maintaining and deactivating a reliable link • Error detection and control • Higher layers may assume error free transmission Magister Teknik Elektro

  4. Introduction • The PDU at the Data Link Layer (DL-PDU) is typically called a Frame. A Frame has a header, a data field, and a trailer • Example: Magister Teknik Elektro

  5. Framing • Problem: Identify the beginning and the end of a frame in a bit stream • Solution (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110") • Problem: • The sequence '01111110' must not appear in the data of the frame Magister Teknik Elektro

  6. Bit-oriented framing and bit stuffing • 'Bit stuffing': If the sender detects five consecutive '1‘ it adds a '0' bit into the bit stream. The receiver removes the '0' from each occurrence of the sequence '111110' • Note: The flags itself are not bit-stuffed. Magister Teknik Elektro

  7. Flow control • Flow Control is a technique for speed-matching of transmitter and receiver. Flow control ensures that a transmitting station does not overflow a receiving station with data • We will discuss two protocols for flow control: • Stop-and-Wait Protocol • Sliding Window Protocol • For the time being, we assume that we have a perfect channel between sender and receiver (no errors) Magister Teknik Elektro

  8. Stop-and-wait flow control • Simplest form of flow control • In Stop-and-Wait flow control, the receiver indicates its readiness to receive data for each frame • Operations: 1. Sender: Transmit a single frame 2. Receiver: Transmit acknowledgment (ACK) 3. Go to 1. Magister Teknik Elektro

  9. Analysis of stop-and-wait Magister Teknik Elektro

  10. Analysis of stop-and-wait • Transmission delayis the time that the sender needs to transmit a frame • Transmission delay is dependent on the size of a frame and the maximum data rate • Example: Frame Size = 1000 bit Data rate of network = 1 Mbps Transmission delay = 1000 bit / 1 Mbps = 1 ms Magister Teknik Elektro

  11. Analysis of stop-and-wait • Propagation delayis the time that a transmitted bit needs to travel from sender to the receiver • Propagation delay is only dependent on the speed of the transmission medium and the distance between sender and receiver. Speed of light: 300000 km/sec, Speed in guided media (approx.): 200000 km/sec • Example: Distance = 1000 km Propagation delay = 1000 km / (200000 km/sec) = 5 ms Magister Teknik Elektro

  12. Sliding window flow control • Major Drawback of Stop-and-Wait Flow Control: • Only one frame can be in transmission at a time • Sliding Window Flow Control • Allows transmission of multiple frames • Assigns each frame a k-bit sequence number • Range of sequence number is [0..2k-1], i.e., frames are counted modulo 2k Magister Teknik Elektro

  13. Operation of sliding window • Sending Window: • At any instant, the sender is permitted to send frames with sequence numbers in a certain range • The range of sequence numbers is called the sending window Magister Teknik Elektro

  14. Operation of sliding window • Receiving Window: • The receiver maintains a receiving window corresponding to the sequence numbers of frames that are accepted Magister Teknik Elektro

  15. Operation of sliding window • Operations at the sender: Magister Teknik Elektro

  16. Operation of sliding window • Operations at the sender: Magister Teknik Elektro

  17. Operation of sliding window • Operations at the receiver Magister Teknik Elektro

  18. Operation of sliding window • Operations at the receiver Magister Teknik Elektro

  19. Operation of sliding window • How is “flow control” achieved? • Receiver can control the size of the sending window • By limiting the size of the sending window data flow from sender to receiver can be limited • Interpretation of ACK N message: • Receiver acknowledges all packets until (but not including) sequence number N Magister Teknik Elektro

  20. Analysis of sliding window Magister Teknik Elektro

  21. Error control Two basic approaches to handle bit errors: • Error-detecting codes plus retransmission (Automatic Repeat reQuest / ARQ) • Used if retransmission of corrupted data is feasible • Receiver detects error and requests retransmission of a frame. • Error-correcting codes • Used if retransmission of the data is not possible • Data are encoded with sufficient redundancy to correct bit errors • Examples: Hamming Codes, Reed Solomon Codes, etc. Magister Teknik Elektro

  22. Error detection techniques • Error Detection Techniques: • Parity Checks • Cyclic Redundancy Check (CRC) Magister Teknik Elektro

  23. Parity checks • General Method: • Append a parity bit to the end of each character in a frame such that the total number of '1' in a character is: • even (even parity) or • odd (odd parity) • Example: With ASCII code, a parity bit can be attached to an 7-bit character • ASCII "G" = 1 1 1 0 0 0 1 • with even parity = • with odd parity = Magister Teknik Elektro

  24. Cyclic-Redundancy Codes • General Method: • The transmitter generates an n-bit check sequence numberfrom a given k-bit framesuch that the resulting (k+n)-bit frame is divisible by some number • The receiver divides the incoming frame by the same number • If the result of the division does not leave a remainder, the receiver assumes that there was no error Magister Teknik Elektro

  25. Cyclic-Redundancy Codes • CRC is used by all advanced data link protocols, for the following reasons: • Powerful error detection capability • CRC can be efficiently implemented in hardware Magister Teknik Elektro

  26. Additional facts on CRC • CRC can be efficiently implemented in hardware by a set of XOR gates and a shift register • The following generator polynomials are widely used: CRC-12: P(x) = x12 + x11 + x3 + x2 + x + 1 CRC-16: P(x) = x16 + x15 + x2 + 1 CRC-CCITT: P(x) = x16 + x12 + x5 + 1 CRC-32: P(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 Magister Teknik Elektro

  27. ARQ error control • Two types of errors: • Lost frames • Damaged Frames • Most Error Control techniques are based on (1) Error Detection Scheme (e.g., Parity checks, CRC), and (2) Retransmission Scheme • Error control schemes that involve error detection and retransmission of lost or corrupted frames are referred to as Automatic Repeat ReQuest (ARQ)error control Magister Teknik Elektro

  28. ARQ error control • All retransmission schemes use all or a subset of the following procedures: • Receiver sends an acknowledgment (ACK) if a frame is correctly received • Receiver sends a negative acknowledgment (NAK) if a frame is not correctly received • The sender retransmits a packet if an ACK is not received within a timeout interval • All retransmission schemes (using ACK, NAK or both) rely on the use of timers Magister Teknik Elektro

  29. ARQ error control • Note: Once retransmission is used, a sequence number is required for every data packet to prevent duplication of packets • Both ACKs and NAKs can be sent as special frames, or be attached to data frames going in the opposite direction (Piggybacking) Magister Teknik Elektro

  30. ARQ schemes • The most common ARQ retransmission schemes: • Stop-and-Wait ARQ • Go-Back-N ARQ • Selective Repeat ARQ • The protocol for sending ACKs in all ARQ protocols are based on the sliding window flow control scheme Magister Teknik Elektro

  31. Stop-and-wait ARQ • Stop-and-Wait ARQis an addition to the Stop-and-Wait flow control protocol: • Frames have 1-bit sequence numbers (SN = 0 or 1) • Receiver sends an ACK (1-SN) if frame SN is correctly received • Sender waits for an ACK (1-SN) before transmitting the next frame with sequence number 1-SN • If sender does not receive anything before a timeout value expires, it retransmits frame SN Magister Teknik Elektro

  32. Stop-and-wait ARQ • Lost frame Magister Teknik Elektro

  33. Stop-and-wait ARQ • Lost ACK Magister Teknik Elektro

  34. Go-back-N ARQ • Go-Back-N uses the sliding window flow control protocol. If no errors occur the operations are identical to Sliding Window • Operations: • A station may send multiple frames as allowed by the window size • Receiver sends a NAKiif frame i is in error. After that, the receiver discards all incoming frames until the frame in error was correctly retransmitted • If sender receives a NAKiit will retransmit frame i and all packets i+1, i+2,... which have been sent, but not been acknowledged Magister Teknik Elektro

  35. Go-back-N ARQ • Lost frame Magister Teknik Elektro

  36. Go-back-N ARQ • Lost ACK Magister Teknik Elektro

  37. Details Go-back-N ARQ • Scenario 1: A transmits frame i, and B detects error in frame i, but has received frames i-1, i-2,... correctly ➨ B sends NAKi • Scenario 2: Frame i is lost or B does not recognize frame i Assume that A sends frame i+1 and B receives it ➨ B sends NAKi, or A will timeout and retransmit frame i and all subsequent frames Magister Teknik Elektro

  38. Details Go-back-N ARQ • Scenario 3: B receives frame i and sends ACK(i+1)which is lost ➨ B may send an ACK(i+k)later which also acknowledges all frames < i+k (ACKs are “cumulative”) or A retransmits frame i and all subsequent frames • Scenario 4: NAKiis lost ➨ A will eventually time out Magister Teknik Elektro

  39. Example of Go-back-N ARQ Magister Teknik Elektro

  40. Selective-repeat ARQ • Similar to Go-Back-N ARQ. However, the sender only retransmits frames for which a NAK is received • Advantage over Go-Back-N: • Fewer Retransmissions. • Disadvantages: • More complexity at sender and receiver • Each frame must be acknowledged individually (no cumulative acknowledgements) • Receiver may receive frames out of sequence Magister Teknik Elektro

  41. Selective-repeat ARQ • Lost frame Magister Teknik Elektro

  42. Example of Selective-repeat ARQ Magister Teknik Elektro

  43. Analysis of ARQ protocols • What is the efficiency of the discussed ARQ protocols? • A number of assumptions: • ACKs and NAKs are never lost, and frames are not dropped. • Sizes of ACKs, NAKs, and frame headers are negligible. Magister Teknik Elektro

  44. Error correction techniques • Forward error correction (FEC) • Hybrid-ARQ (H-ARQ) • Type-I H-ARQ • Type-II H-ARQ • Type-III H-ARQ Magister Teknik Elektro

  45. Networking • Point to point communication not usually practical • Devices are too far apart • Large set of devices would need impractical number of connections • Solution is a communications network Magister Teknik Elektro

  46. Simplified Network Model Magister Teknik Elektro

  47. Two types of networks at the data link layer • Broadcast Networks: All stations share a single communication channel • Point-to-Point Networks: Pairs of hosts (or routers) are directly connected • Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point Magister Teknik Elektro

  48. Networking • Computer network A collection of computing devices that are connected in various ways in order to communicate and share resources Usually, the connections between computers in a network are made using physical wires or cables However, some connections are wireless, using radio waves or infrared signals Magister Teknik Elektro

  49. Networking • The generic term node or host refers to any device on a network • Data transfer rate The speed with which data is moved from one place on a network to another • Data transfer rate is a key issue in computer networks Magister Teknik Elektro

  50. Switching Networks • Long distance transmission is typically done over a network of switched nodes • Nodes not concerned with content of data • End devices are stations • Computer, terminal, phone, etc. • A collection of nodes and connections is a communications network • Data routed by being switched from node to node Magister Teknik Elektro

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