OSI Model _Open System Interconnection Model_ Application
One of the features of computer networks is to group the flow of information transmitted by adding control information, and send the packet to the destination correctly. Adding control information packet are two: one is used to verify whether the receiver correctly received the error control information; the other is the sender of the packet specified in the receiver's address information. Thus the network must have the error control and addressing functions. Also required when multiple nodes simultaneously send the packet, the network must also be some sort of conflict arbitration process to decide who to send, who after the delivery. All of these data packets with control information in a network nodes through a feature called the right to send forward data link control DLC.
Data Link Control OSI Model (Open System Interconnection Model) Application Layer 7 Protocol Application Layer 7 Peer Processor Presentation Layer 6 Protocol Presentation Layer 6 Session Layer 5 Protocol Session Layer 5 Transport Layer 4 Protocol Transport Layer 4 Network Layer 3 Protocol Network Layer 3 Data Link Layer 2 Protocol Data Link Layer 2 Physical Layer 1 Protocol Physical Layer 1 Peer Protocol Data Link Control Connectionless without reliability Establish connection is not necessary Acknowledgment is not need Connectionless with reliability Establish connection is not necessary Acknowledgment is need Connection-oriented with reliability Establish connection is necessary Acknowledgment is need Data Link Control Frame Synchronization Data are sent in blocks called frames. The beginning and end of each frame must be recognizable Flow Control The sending station must not send frames at a rate faster then the receiving station can absorb them. Error control Any bit errors introduced by the transmission system must be corrected Addressing On a multipoint line, such as a local area network (LAN), the identity of the two stations involved in a transmission must be specified Data Link Control Control and data on same link It is usually not desirable to have a physically separate communications path for control information. Accordingly, the receiver must be able to distinguish control information from the data being transmitted. Link management The initiation, maintenance, and termination of a sustained data exchange requires a fair amount of coordination and cooperation among stations. Procedures for the management of this exchange are required Link management Flow Control Error Detection Error Control Data Link Control Flow Control Flow control is a technique for assuring that a transmitting entity does not overwhelm a receiving entity with data. Buffer Size Data Link Control Stop-and-Wait Flow Control simplest form of flow control. Only one frame Transmission Time Transmission : The time it take for a station to transmit a frame that is normalized to one take for a station to transmit a frame that is normalized to one Propagation delay Propagation delay :: The time it takes for a bit to travel from sender to receiver that is expressed as “a” The time it takes for a bit to travel from sender to receiver that is expressed as “a” Data Link Control Stop-and-Wait Flow Control propagation time less than transmission time The frame is sufficiently long that the first bits of the frame have arrived at the destination before the source has completed the transmission of the frame. propagation time greater than transmission time The sender completes transmission of the entire frame before the leading bits of that frame arrive at the receiver Transmission Time Transmission : The time it take for a station to transmit a frame that is normalized to one take for a station to transmit a frame that is normalized to one Propagation delay Propagation delay :: The time it takes for a bit to travel from sender to receiver that is expressed as “a” The time it takes for a bit to travel from sender to receiver that is expressed as “a” Data Link Control Sliding-windows flow control Multiple Frame Full-duplex link 3-bit field Sequence number of the next frame Data Link Control Each time a frame is sent, the shade window shrinks Each time a frame is sent, the shade window shrinks each time an acknowledgment is received the shade window grows each time an acknowledgment is received the shade window grows ady 3 Next 7 Frames Receive-Re RNR 5 :: RNR 5 received 4 but unable to received 4 but unable to accept any more accept any more Example of a sliding-window protocol Data Link Control Error Detection Pb : Probability of a single bit error or bit error rate P1 : Probability that a frame arrives with no bit errors. P2 : Probability that a frame arrives with one or more undetected bit errors P3 : Probability that a frame arrives with one or more detected bit errors but no undetected bit errors F : the number of bit per frame P = (1 − Pb ) F 1 P2 = 1 − P1 Data Link Control Error Detection and Correction 1. ใหผูรับตรวจสอบความผิดพลาดและแกไขขอมูลที่สงใหถูกตองดวยตัวเอง 2. ใชเทคนิคการสะทอนกลับ (Echoplex) โดยผูรับจะสงขอมูลที่ไดรับกลับไป ที่ผูสง ถาขอมูลตรงกับแสดงวาการสงในครั้งนั้นถูกตอง ่ 3. การตรวจสอบแบบอัตโนมัติ โดยการเพิมขอมูลสําหรับใชในการตรวจสอบ เพิ่มเขาไปในนอกเหนือจากขอมูลที่สง Frame Check Sequence Frame Check Sequence (FCS) (FCS) Data Link Control Error Detection Parity Check Cyclic Redundancy Check (CRC) Data Link Control Parity Check The simplest error-detection scheme is to append a parity bit to the end of a block of data. Synchronous transmission XOR Even parity G (1110001) + Odd parity Odd parity G (11100011) (11100011) Asynchronous transmission G (11000011) (11000011) ERROR G (11000001) (11000001) NO ERROR Data Link Control Block Check Character P 7 6 5 4 3 2 1 Character 1 1 0 0 0 0 0 1 1 1 1 0 0 0 0 1 0 2 XOR 0 1 0 0 0 0 1 1 3 1 1 0 0 0 1 0 0 4 0 1 0 0 0 1 0 1 5 0 1 0 0 0 1 1 0 6 1 1 0 0 0 1 1 1 7 1 1 0 0 1 0 0 0 8 0 1 0 0 1 0 0 1 9 0 1 0 0 1 0 1 0 10 1 1 0 0 1 0 1 1 11 0 1 0 0 1 1 0 0 12 1 1 0 0 1 1 0 1 13 1 1 0 0 1 1 1 0 14 1 0 0 0 1 1 1 1 BCC Data Link Control Cyclic Redundancy Check (CRC) or Polynomial Code One of the most common, and one of the most powerful. 110001 1 x X55 + 1 x X44 + 0 x X33 + 0 x X22 + 0 x X11 + 1 x X00 X55 + X44 + 1 Transmitter Frame Check Sequence) Generate an n-bit sequence ((Frame Check Sequence) k+n bit Modulo 2 Arithmetic Uses binary addition with no carries or XOR 1111 11001 T = (k + n)-bit frame to be transmitted with n < k +1010 x 11 M = k-bit message, the first k bits of T 0101 101011 F = n-bit FCS, the last n bits of T P = pattern of n + 1 bits E = error Data Link Control CRC Calculating 1101010110 Q P 110101 101000110100000 2nM 110101 111011 Message (M) = 1010001101 (10-bits) 110101 111010 110101 Pattern (P) = 110101 (6-bits) 111110 110101 To be calculated 101100 FCS (R) = 110101 (5-bits) 110010 110101 Transmitter 01110 R Frame Check Sequence) Generate an n-bit sequence ((Frame Check Sequence) k+n bit Transmitter Receiver Remainder 101000110101110 101000110101110 101000110101110 101000110101110 00000 Data Link Control Cyclic Redundancy Check (CRC) • All single-bit errors • All double-bit errors, as long as P(X) has at least three 1s • Any odd number of errors, as long as P(X) contains a factor (X-1) • Any burst error for which the length of the burst is less than the length of the divisor polynomial; that is, less than or equal to the length of the FCS • Most larger burst errors CRC-16 CRC-16 = X16 = X16 + + X15 X15 + + X2 + 1 X2 + 1 CRC-CCITT = X16 CRC-CCITT = X16 + + X12 X12 + + X5 + 1 X5 + 1 CRC-32 CRC-32 = X32 = X32 + + X26 X26 + + X25 + X23 + X22 + X16 + X12 + X25 + X23 + X22 + X16 + X12 + X11 X11 + + X10 X10 + + X8 + X7 + X5 + X4 + X2 + 1 X8 + X7 + X5 + X4 + X2 + 1 Data Link Control Error Control ERROR Lost Frame Damaged Frame A frame fails to arrive A recognizable frame at the other side. does arrive, but some of the bits are in error. Data Link Control Error detection Positive acknowledgment The destination returns a positive acknowledgment to successfully received, error-free frames. Retransmission after timeout The source retransmits a frame that has not been acknowledged after a predetermined amount of time. Negative acknowledgment and retransmission The destination returns a negative acknowledgment to frames in which an error is detected. The source retransmits such frames. Data Link Control ARQ : Automation Repeat Request Stop-and-Wait ARQ Data Link Control ARQ : Automation Repeat Request Go-Back-N ARQ Pipelining Damaged Frame Damaged RR Data Link Control ARQ : Automation Repeat Request Selective-Reject ARQ Data Link Control HDLC : High-level Data Link Control Basic Characteristics Three type of stations Two link configuration Three data transfer modes Three types of stations Primary Station For controlling the operation of the link called “command” Secondary Station Operates under the control of the primary station called “responses” Combined Station Two link configurations Unbalanced configuration One primary and one/more secondary station and supports both full- duplex and half-duplex transmission. Balanced configuration Two combined stations and supports both full-duplex and half-duplex transmission. Data Link Control HDLC : High-level Data Link Control Basic Characteristics Multidrop Line Synchronous TxRx Three data transfer modes Normal response mode (NRM) Used unbalanced configuration. Primary initiate data transfer to a secondary but a secondary may only transmit data in response to a command from the primary Asynchronous balanced mode (NRM) Used with a balanced configuration. Either combined station may initiate transmission without receiving permission from the other combined station. Asynchronous response mode (NRM) Used with an unbalanced configuration. The secondary may initiate transmission without explicit permission of the primary. The primary still retains responsibility for the line, including initialization, error recovery, and logical disconnection. Data Link Control HDLC : High-level Data Link Control : Frame Structure Frame Format Flag Field : unique pattern 01111110 Bit Stuffing Data transparency Data Link Control HDLC : High-level Data Link Control : Frame Structure Data Link Control HDLC : High-level Data Link Control : Frame Structure Frame Format Address Field : uniformity if all bit is 1, it means broadcast communication Extended address field Data Link Control HDLC : High-level Data Link Control : Frame Structure Frame Format 0 :: primary poll to secondary 0 primary poll to secondary 1 :: primary receive data 1 primary receive data 8-bit control field format Sliding-windows Sliding-windows Data Link Control HDLC : High-level Data Link Control : Frame Structure 8-bit control field format Sliding-windows Sliding-windows 00 00 RR RR Receive ready Receive ready 01 01 REJ REJ Reject Reject 10 10 RNR RNR Receive not ready Receive not ready 11 11 SREJ SREJ Selective reject Selective reject Data Link Control Initialization Three phase of HDLC operation Data Transfer Disconnect