Data Link Layer Protocols

Data Communication and Ema sonv il: Networ q@hcmut.edu king Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT Cont  Chapter 3: Data Link Layer Protocolsent Flow Control Error Control Connection Management Data Link Protocols  Chapter 4: Netwo Industrial rks CAN Networks MODBUS Networks Applications Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT Framing  The data link layer needs to s that each frame is fro anot pack bits into frames, o distinguishable m her Variabl Frami Fixed‐Size Framing e‐Size Telecomm Dept Faculty of EEE ng DCN‐ 2013 HCM UT Flow and Error  The most important Control responsibilities of the data link layer are flow control and error control Collectively, these functions are known as data link control Flow control refers to a set of implemented in b usi on o th com procedures used to restrict program langua software ydata ng that e f the e mon Telecomm DCN‐ the amount of sender ming ges Dept 2013 Faculty of send before HCM can EEE UT Error  Forward Error Control (FEC) Control Usually used in real‐time application (e.g voice, video) Requ Accept Error Detection + ARQ (Automatic errors if can not correct est) Retransmission List some FEC methods? Require absolutely correct data transmission Used in non real‐time application (e.g email, file Telecomm DCN‐ transmission) Dept 2013 Facultybasic of HCM 2 ARQ protocols: EEE UT P  P:  Notations I‐ fram ACK frame e or NAK frame S Primary, sending data sendi ACK frames ng back to  S: Secondary, receiving data frames  and  Primary ACK (Acknowledge) frame: I frame: data/Information frame  response/reply sent from S back to I(N): Data frame with sequence N SP,back confi receiv acorrup I‐ (err confirm receiving a good I‐frame NAK toTelecomm P,(Negative rmAcknowledge) ing frame: ted response/reply fram orsent DCN‐ from Dept 2013 frame) e Faculty of HCM EEE UT  Idl R Princi P can have only ple: I‐frame e Q  outstanding (awaiting for an ACK) at a time  On receipt of an error‐free ACK frame, P transmits  another I‐frame If S receives an I‐frame or P receives an ACK containing transmission  error: the frame is discarded  If P does not receive an ACK within a Telecomm given timeout Dept Faculty of interval: EEE DCN‐ 2013 HCM UT  Implicit retransmission  P has to wait a  timeout  If S receives a interval before  corrupted I‐ retransmission return frame: What is the an  NAK to P drawback of this If P receives an ACK,  mechanism? How stop the to improve timer, and transmit Explicit the next Telecomm frame retransmission Dept Faculty of If P receives a NAK, EEE Idle RQ DCN‐ 2013 HCM UT Idle  Advantages : RQ Send and Wait Small buffers: S (Stop both Pand andWait) keeps only frame  Disadvantages: Low link utilization Used N(R): in seque transmission of receive nce Telecomm Dept printable number Faculty of EEE DCN‐ 2013 HCM UT  Link utilization :  In case BER=0: Idle‐RQ T ix = U T T ix=+2T1p Tt =  Total average1+2T delayp /Tix 1+2a ≈ ix per frame? Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT      High‐level Data Link Control (HDLC) is a bit‐oriented protocol for communication over point‐to‐point and multipoint links HDLC Support full‐duplex, transparent mode operation  Variations: LAPB, LAPD,LAPM Used in Frame Relay, PPP Primary station   Control the operation of the data link Telecomm Dept Send the command frames Secondary Faculty ofstation EEE Controlled by Primary station DCN‐ 2013 HCM UT 5 HDLC  Operational • modes: Used in unbalanced configuration • Slave stations (secondaries) can transmit only  •Normal Response when specially instructed Mode (NRM)  Asynchronous by the master (primary) station Used in unbalanced configuration Response The linkMode may be(ARM) point‐to‐point, multipoint Allow a secondary to initiate a • (only primary allowed) transmission without receiving  Asynchronous Mode (ABM) permission fromBalanced the primary • Normally Mainly used point‐to‐point usedoninduplex point‐to‐point links configuration and duplex links • Each station has an equal status and Telecomm DCN‐ performs both primary and Dept 2013 Faculty of HCM secondary functions EEE UT • •  frame  HD LC Frame formats: both data and control messages are carried in a standard format Flag Field  determine the beginning and  (0x7E):  the end of a frame  Receiver must hunt this value • If the next bit is bit 0, it for • synchronization If the next bit is 7t bit is bit • will be removed the next bit is bit 7th bit is bit 1: continue Bit If stuffing if there ish a1, and bit 0: Flag – 1, and If number of bits 1=15: DCN‐ bits “1”: Dept 2013 Faculty of idle channel HCM EEE UT  Address Field: HDLC Depending on operational mode In NRM mode, multidrop line: each station has unique address If the primary wants to connect with the slave, it will put the slave 8 bit in length, can be o 7bit addresswiinha this address field Certain Last th fir bi equ t expanded with multiple f s addresses known octet ll ve e st as t al o Telecomm DCN‐ group address can be assigned to more Dept 2013 Faculty of HCM than one EEE UT HDLC Control field: There are  Unnumbered frames (U—frame): used for such functions asin linkHDLC: setup frame types  and disconnection They not contain any acknowledgement  information Information frame (I‐frame): carry the actual information I‐frames can be used to piggyback acknowledgement information if the operational mode is ABM or ARM Telecomm DCN‐ Supervisory frame (S‐frame): are used for Dept 2013 Faculty HCM errorof and flow control and EEE UT HD Control field: 1 byte or bytes in LC length type (S,I,U) P/F: Select thefrom a secondary 1• Command: or 2Poll first bitor P, bits request the response depending • Response: onbit F, indicating this is the context: define the frame response to asequence command N(S): send number N(R): receive 0 sequence number Telecomm Dept Faculty of EEE 1 DCN‐ 2013 HCM UT HDLC Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT HDLC  cont bi fiel definiti Extende rol t d ons d Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT  Information an U‐ field: Contained in I‐ frame Variable length Multiple of bytes HDLC d fram e CRC  FCS field:o CRC 16 r 32 Error detection Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT  Link Management HDLC Setup connection Disconnect ion Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT  Data  V(S) indicates the next send sequence transfer: number N(S), which is allocated to an I‐frame transmitted by that  station V(R) is the send sequence number of the next in‐sequence  I‐frame expected by that station  At receiver, if N(S)= V(R): receive correct sequenced frame Bit P means: this Telecomm frame needs an Dept response from the Faculty of receiver (by bit F) EEE HD LC DCN‐ 2013 HCM UT  Data transfer: HDLC  Negative Acknowledg ement  Go‐back‐N  Using REJ/RR Telecomm Dept Faculty of EEE DCN‐ 2013 HCM UT 6  Piggyback  Example: an exchange using piggybacking Node A without errors:  begins the exchange of information with an I‐frame followed by another I‐ frame Node B piggybacks its acknowledgment of both frames onto an I‐frame of its own Node B’s first I‐ frame is also numbered [N(S) field] and contains a in its N(R) field, acknowledging the receipt of A’s Telecomm frames and and indicating that Dept it expects frame Faculty of EEE to arrive next Node B transmits its HDLC DCN‐ 2013 HCM UT  HD Piggyback with Example: an exchange in errors: which a frame is LC lost Node B sends three data   frames (0, 1, and 2), but frame is lost When node A receives frame 2, it discards it  and sends a REJ frame for frame Note that the protocol being used is Go‐Back‐N with the special use of an REJ Telecomm frame as a NAK Dept Faculty of frame The NAK frame does EEE DCN‐ 2013 HCM UT  So oth cas me er es: Telecomm Dept Faculty of EEE HDLC DCN‐ 2013 HCM UT ...Cont  Chapter 3: Data Link Layer Protocolsent Flow Control Error Control Connection Management Data Link Protocols  Chapter 4: Netwo Industrial rks CAN Networks... most important Control responsibilities of the data link layer are flow control and error control Collectively, these functions are known as data link control Flow control refers to a set of implemented... protocol Determine the 2.1 velocity of i m an link utilization for /s d propagation of the s  twisted the following types1of data link pair cable link A km in length assuming a data satell lin