Data Link Layer (cont.) ( h h h ) (Sicherungsschicht) HDLC - 1
LOGICAL L LINK CONTROL MEDIUM ACCESS CONTROL PHYSICAL SIGNALING DATA LINK LAYER PHYSICAL LAYER ACCESS UNIT INTERFACE PHYSICAL MEDIA ATTACHMENT MEDIUM OSI-model for local l networks with partitioning of the link layer in media access control and logical link control, as defined by IEEE 802. HDLC - 2
Bit-Oriented Protocols: HDLC High Level Data Link Control Low overhead Bit streams are transmitted without t breaking it into characters. Bit streams that are not integral multiples of character lengths can be transmitted. Subsets: High Level Data Link Control (ISO standard): HDLC Synchronous DLC: SDLC HDLC Subsets: CCITT's Link Access Protocol - Balanced (LAP-B) Link Access Protocol for the ISDN D-channel (LAP-D) Link Access Protocol for Modems (LAP-M) HDLC - 3
HDLC Stations Two types of Stations Primary Secondary Types of Commands Commands: sent from primary to secondary Responses: sent from secondary to primary Only one primary can be active at any given time, while multiple secondaries can be active simultaneously HDLC - 4
HDLC Link Structures Unbalanced point-to-point link One primary, one secondary Unbalanced multi-point link One primary, multiple secondaries One-way controlled channel primary station commands answers " secondary station A secondary station B HDLC - 5
HDLC Link Structures (cont.) Balanced point-to-point link Both stations have primary/secondary status. While one station is transmitting, the other has to be a secondary. primary station A secondary station A symmetric channel commands answers commands answers secondary station B primary station B combined station both-way controlled channel commands answers combined station HDLC - 6
HDLC Modes of Operation Normal Response Mode (NRM) Used in unbalanced configurations In this unbalanced mode, secondaries can transmit only when instructed by the primary station The link may be point-to-point or multi-point primary station primary station 1) Poll-command 2) information i 3) receipt 1) select-command plus information i 2) receipt secondary station secondary station HDLC - 7
HDLC Modes of Operation (cont.) Asynchronous Response Mode (ARM) Used in unbalanced configurations. A secondary can transmit without being polled by the primary. Used in point-to-point configurations and duplex links. primary station answer command secondary station HDLC - 8
HDLC Modes of Operation (cont.) Asynchronous Balanced Mode (ABM) Used in balanced configurations where all stations are "equal" Duplex point-to-point links combined station 1) command 2) answer combined station combined station 1) command 2) answer combined station HDLC - 9
HDLC Frame Format frame F A C I FCS F opening flag 8 bits 01111110 address field 8 x n bits (n = 1 to k) control field 8 or 16 bits information field variable frame check field 16 or 32 bits closing flag 8 bits 01111110 span of frame check and zero insertion HDLC - 10
HDLC Frame Fields Flag Field Start and End Flag pattern (01111110) Zero insertion required for data transparency Address Field Contains the address of the secondary station Broadcast address (All 1's) Extended address option that can make the address as long as necessary (in increments of 8 bits) The last extended address octet starts with a 1, while the previous octets start with 0 Control Field Identifies frame types Contains Send and Receive sequence numbers Information Arbitrary length, arbitrary bit streams FCS 16-bit CRC (CCITT-16) FCS calculated for Address-Information fields without the inserted zeros (for data transparency) HDLC - 11
Basic Address Field 1 Octet 8 7 6 5 4 3 2 1 LSB transmitted first (LSB : Least Significant Bit) Address may be that of a secondary/combined d station. Global (all 1 s) or Null ( all 0 s) Extended Address Field 1 or more Octets LSB transmitted first 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 0 1 0... Extend Address to next Octet Terminate Address Field HDLC - 12
Information Frames HDLC Frame Types Carry data In duplex links, can use piggy-backed acknowledgements Identified by a 0 in the first bit of the control field Supervisory Frames Used for error and flow control Uses CRC-CCITT Contain sequence numbers Identified by a 10 in the first two bits of the control field Unnumbered Frames Used for connection establishment and disconnection No acknowledgements Identified by a 11 in the first two bits of the control field HDLC - 13
HDLC Control Field Bits: 1 2 3 4 5 6 7 8 0 SN P/F RN Information 1 0 Type P/F RN Supervisory 1 1 Type P/F Type Unnumbered HDLC - 14
BASIC CONTROL FIELD 1 OCTET 876 5 432 1 INFORMATION N(R) P/F N(S) 0 LSB TRANSMITTED FIRST COMMENTS N(R) = RECEIVE SEQUENCE COUNT = 0 TO 7 N(S) = SEND SEQUENCE COUNT = 0 TO 7 SUPERVISORY N(R) P/F SS 01 SS specifies RR, RNR, REJ, SREJ UNNUMBERED MMM P/F MM 1 1 MMM and MM specify the U command response P/F = POLL for command, F for response EXTENDED CONTROL FIELD 2 OCTETS 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 INFORMATION N(R) P/F N(S) 0 SUPERVISORY N(R) P/F 0 0 0 0 SS 01 UNNUMBERED 0 0 0 0 0 0 0 P/F MMM 0 MM 1 1 HDLC - 15
A I 3.5 P = 1 B N(S) = 3 N(R) = 3 N(R) = 5 RNR 3 RR 5 P = 1 F = 1? RNR 3 RR 5 RR 3 P = 1 F = 1 a) A N(S) = 1 N(R) = 5 N(S) = 2 N(R) = 5 I 1.5 I 2.5 REJ 1 I 1.5 FCS ERROR? I 2.5 b) HDLC - 16
The Poll/Final bit Depends on whether the transmission is from the primary or secondary If the frame is from a primary, the P/F bit is interpreted as the Poll bit If the frame is from a secondary, the P/F bit is interpreted as the Final bit, indicating the end of a sequence of frames A primary cannot send another poll to a specific secondary, until it has received the final frame HDLC - 17
Supervisory Frames Used for positive/negative acknowledgements. 2 S bits in the control byte identifying 4 kinds of supervisory frames: Receive Ready (RR) Receive Not Ready (RNR) Reject (REJ) Selective Reject (SREJ) RR Frame Used for acknowledging correctly received frames All frames with numbers less than N(R) have been received Also indicates that the receiver is ready to receive frames RNR Frame Used to indicate that the receiver is temporarily not ready May be used for flow control as a Wait-Acknowledgement (WACK) If a data frame is acknowledged by a WACK, the transmitter cannot send any more frames, until it receives an ACK to the transmitter's ENQ HDLC - 18
Supervisory REJ & SREJ Frames Both REJ and SREJ act as positive acknowledgements for all frames below number N(R) Frames with bad FCS are not necessarily rejected, but ignored REJ Frame Used as a negative acknowledgement The frame numbered N(R) and all frames after N(R) are to be retransmitted Go-Back-N ARQ operation SREJ Frame Used as a NAK for frame numbered N(R) Retransmission requested only for that frame Selective Repeat ARQ operation HDLC - 19
Unnumbered Frames Used for maintenance purposes Link start-up and shut-down Set operational modes No frame sequence numbers 5 Management bits in the control frame identify 32 different commands/responses X bits in the extended control field are reserved (set to 0) HDLC - 20
Unnumbered Commands and Responses Commands Set Asynchronous Response Mode (SARM) Set Asynchronous Response Mode Extended (SARME) Set Normal Response Mode (SNRM) Set Normal Response Mode Extended (SNRME) Set Asynchronous Balanced Mode (SABM) Set Asynchronous Balanced Mode Extended (SABME) Reset (RSET) Frame Reject (FRMR) Disconnect (DISC) Responses Unnumbered Acknowledgement (UA) Command Reject (CMDR) Frame Reject (FRMR) Disconnect Mode (DM) HDLC - 21
Link Access Procedure Balanced (LAP-B) Used to connect a computer to the data network, example: X.25 Balanced Mode, Connection-oriented, Point-to-point i duplex links SABM and SABME are the only two SET-commands allowed Addressing: DCE to DTE or DTE to DCE HDLC - 22
Example: Bit oriented protocol (HDLC) System A protocol System B interaction between actions interaction between network data link signals on the layer layer channel between A and B connect request connect confirm data request data confirm disconnect request disconnect confirm SABM UA information frame(s) acknowledge frame(s) DISC UA data link layer network layer connect indicate connect response data indicate data response disconnect indicate disconnect response HDLC - 23
Reliable data transmission with HDLC A confirms DLC A DLC B B initializes iti SABM data transmission UA A sends data A (0,0) B (0,0) A demands erroneous packet once more A has no data left to send A (1,1) A (2,1) REJ (1) B (1,1) B (2,2) 2) B (3,3) B receives UA and sends data A (3,1) B (1,3) (, B repeats starting with packet 1 RR (2) B (2,4) Formats: Sender (SN, RN), message (RN) Meaning of message: SABM Set Asynchronous Balanced Mode UA Unnumbered Acknowledge REJ Reject RR Receive Ready DISC Disconnect DM Disconnect Mode A confirms end of connection RR (3) RR (4) DM B (3,4) DISC B has no data left to send B demands end of connection Go-Back-N ARQ HDLC - 24
Logical Link Control (LLC) HDLC subset used with LANs Data link layer in LANs consists of two sublayers Logical Link Control (LLC) sublayer Medium Access Control (MAC) sublayer LLC supports connection-oriented and connectionless services. HDLC - 25
Limitations of HDLC Too many options Communicating HDLC devices will need to have the same options Only a single address field works fine with a single primary station HDLC came before OSI (layering is not totally aligned) Flags may have errors and synchronization may be lost Only one FCS for the entire frame NAKs are not sent immediately causing a lower transmission efficiency under high error rate situations HDLC - 26
Example protocol: PPP PPP is a form of HDLC, which is commonly used today When you dial up using your modem and connect your PC to your ISP's router/remote access server, the data link layer protocol run on the link is PPP PPP uses the ABM mode of HDLC and provides a connectionless unacknowledged service Since it is ABM, multipoint is not used; this means the address byte does not change. It is set to all 1's. This means all stations accept the frame. But since PPP has only two nodes, one node accepts any frame sent from the other node. Since it is unacknowledged, no supervisory frames are sent. So the control field is always 0000 0011. First two bits are 11 to represent unnumbered frames. Unnumbered frames must be allowed to carry data in addition to carrying SABM (Set ABM mode -open connection), SABME (E: Extended - control field is 16 bits and sequence numbers are 7 bits), unnumbered acks, etc. There are no "numbers" meaning no sequence numbers. The M bits of 0 must indicate a data type of the unnumbered frame. In PPP the header control field is shown as 0000 0011. PPP has one additional field over HDLC - it has a protocol type field to indicate what the type of higher layer protocols carried in the PPP frame. Also, PPP is byte-oriented oriented, unlike HDLC, which is bit-oriented; therefore bit stuffing is not used when the flag pattern appears in the payload; instead an escape character is used. HDLC - 27
Example protocol: PPP (cont..) HDLC - 28
Calculation of Error Probabilities Assumptions: Symmetric bit errors: equal probability of 0 1, 1 0 errors Independence assumption Bit error probability p Probability of correct bit transmission? 1-p Block of n bits (frame/packet) is corrupted, if one or more bits are corrupted, p(n) also known as PER P [i specific bits corrupted] = P [i bits corrupted] = p i = i p (1 p) n i n i p (1 p) i n i HDLC - 29
Calculation of Error Probabilities (cont.) p(n) = P [ 1 bit corrupted] = p i i 1 n p Probability that a block of n bits arrived with no errors, p 0 = P [0 bit corrupted] = correct transmission n 1( p) p(n) = 1 p 0 1 (1 p) n Approximation for small values of p: 1 (1 p ) n 1 (1 np ) np p 1 HDLC - 30
Calculation of Error Probabilities (cont.) Size of a packet F = 500 bytes Bit error probability bilit P = 10-6 Compute the probability of PER, p(n)? How many packets (frames) will be corrupted at 100 kbps? HDLC - 31
Calculation of Error Probabilities (cont.) Size of a frame F = 500 bytes (4000 bits) Bit error probability bilit P = 10-6 P(0)= (1-10 -6 ) 4000 = 0.9960 P(n)= 1-(1-10 -6 ) 4000 = 0.0040 4 out of every 1000 packets (frames) have errors At 100 kbps, this means 6 packets (frames) per minute are in error HDLC - 32
Error Handling Methods ARQ: Automatic ti Repeat Request Packets contain sequence number NS Correctly received packet: Receiver sends positive Acknowledgement ACK[NS] Erroneous packet: Receiver sends negative Acknowledgement NAK[NS] Sender must keep copy of packet until an ACK is received Error Detection with FCS, e.g. CRC (Cyclic Redundancy Check) Checking Frame Sequence 3 Schemes Stop-and-Wait Go Back N Selective Reject HDLC - 33
Stop-and-Wait Automatic Repeat Request Sequence: Packet, Acknowledgement, Packet, Ackknowledgement,... time of transmission sending window 5 5 6 receiving window 5 5 6 Round-Trip-Delay HDLC - 34
Throughput Throughput (Durchsatz): the amount of work that can be performed by a computer system or component in a given period of time [IEEE 90]. [IEEE 90] Institute of Electrical and Electronics Engineers. IEEE Standard Computer Dictionary: A Compilation of IEEE Standard Computer Glossaries. New York, NY: 1990. Throughput: The amount of data transferred from one place to another or processed in a specified amount of time. Data transfer rates for disk drives and networks are measured in terms of throughput. Typically, throughputs are measured in kbps, Mbps and Gbps. Often S (English) or D (German) is used as an abbreviation for throughput. HDLC - 35
Throughput (cont.) However, there is no standard, universally recognized definition of throughput and it is important to understand the definition of throughput hp t that t is being used in each situation. ti The throughput is often given in normalized form; that is, it is the rate at which user data is carried over the channel divided by the channel capacity. Sometimes this is referred to as normalized throughput, but often it is not. Another way of describing normalized throughput: portion of time used for transmitting useful information. Sometimes this is also called goodput. HDLC - 36
Stop-and-Wait-ARQ Formula? n: packet length in bits PER: Packet Error probability c: round trip delay (sec), delay between the end of the last packet and the start of the next one v: transmission speed in bit/sec D n * (1 PER n c * v ) Throughput? Low, especially for small packets and long propagation delays! e.g. not suitable for satellite links HDLC - 37
Normalized Throughput = Useful load of the channel (portion of time used for the transmission of information over some time T) n data : packet length in bits n ack : ACK lenghth in bits v: transmission speed in bit/sec t n: change over time /processing time t d : Propagation delay Transmission ss delay vs Propagation on delay? HDLC - 38
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Error Handling Methods ARQ: Automatic ti Repeat Request Go-Back-N-ARQ Continuous data flow is possible Data is sent until a NAK[NS e ] is received Sender restarts sending beginning g with packet NS e Receiver ignores all packets until packet NS e Sufficient buffer space is needed A positive acknowledgement ACK[NS] acknowledges all packets up to packet NS HDLC - 41
Go-Back-N- Automatic Repeat Request transmission time sending window 4 5 6 7 8 9 5 6 7 8 9 receiving window 4 5 5 6 Round-Trip-Delay for packet 5 HDLC - 42
Throughput? Go-Back-N-ARQ Higher, because data can be sent continuously Formula? n: packet length in bits PER: Packet Error Rate c: round trip delay, delay between end of last packet and the start of next one v: transmission speed in bit/sec D n n * (1 PER PER * c ) * v HDLC - 43
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Error Handling Methods ARQ: Automatic ti Repeat Request Selective Reject Data can be sent continuously All correctly received packets are acknowledged and if necessary buffered Erroneous packets are selectively requested by NAKs Buffering is needed to guarantee the right sequence of packets to higher layers HDLC - 46
Selective-Reject- Automatic Repeat Request transmission time sending window 4 5 6 7 8 9 5 10 11 8 12 receiving window 4 5 6 7 8 9 5 10 11 Round-Trip-Delay HDLC - 47
Throughput? Selective-Reject-ARQ Highest possible throughput, because data can be sent continuously and only erroneous packets have to be retransmitted Formula? n: packet length in bits PER: Packet Error probability c: round trip delay, delay between end of last packet and the start of the next one v: transmission speed in bit/sec D 1 PER HDLC - 48
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ARQ methods Sender Buffer Receiver Buffer Stop & Wait ARQ 1 PDU 1 PDU Go-Back-N ARQ W PDUs 1 PDU Selective Reject ARQ W PDUs W PDUs HDLC - 50
PACKET ERROR RATE vs THROUGHPUT HDLC - 51
ROUND TRIP DELAY vs THROUGHPUT HDLC - 52
TRANSMISSION SPEED vs THROUGHPUT HDLC - 53
Comparison FEC and ARQ ARQ: Automatic Repeat Request Pros: Cons: High throughput for good channel quality Guarantee for very low rest bit error rate No complex decoding Return channel necessary Throughput fluctuates with the channel quality Varying delay, not suitable for real time applications Additional buffers in sender and receiver necessary FEC: Forward Error Correction Pros: No return channel necessary Constant throughput independent of channel quality Cons: Constant delay between sender and receiver Low Throughput because of high redundancy Rest bit error rate depends on channel quality Coding and Decoding algorithms highly complex HDLC - 54
Error Clusters Errors are usually not independent Errors are correlated Clusters are one possible description yes error no cluster b1 cluster b2 bit timing definition of a cluster error HDLC - 55