Data Link Layer (Layer 2, L2) 염익준

2 Cables Used to connect nodes in the same room or building. CableTypical BandwidthsDistances Cat-5 twisted pair Thin-net coax Thick-net coax Multimode fiber Single-mode fiber Mbps 100 Mbps Mbps 100 m 200 m 500 m 2 km 40 km

3 Leased Lines Used to connect site to site. ServiceBandwidth DS1 (or T1) DS3 STS-1 (or OC1) STS-3 STS-12 STS-48 STS Mbps Mbps Mbps Mbps Mbps Gbps Gbps

4 Last-Mile Links POTS (Plain Old Telephone Service): 56 Kbps ISDN (Integrated Services Digital Network): Kbps xDSL (Digital Subscriber Line): 16 Kbps – 55.2 Mbps –Asymmetric DSL (ADSL): Upstream (16 – 640 Kbps) and Downstream (1.544 – Mbps) –Very-high rate DSL (VDSL): – 55.2 Mbps Symmetric Running over much shorter distances (1000 to 4500 feet) Cable modem –Using cable network –40 Mbps downstream on a single CATV channel (6 MHz)

5 Wireless Links Global links – PCS, GSM Wireless LAN – IEEE a, b, g Wireless MAN – IEEE , WiBro Piconet – Bluetooth (upto 1 Mbps)

6 Encoding - NRZ Signalling component Signal Bits Node Adaptor Bits NRZ Problems of NRZ –Consecutive 0s – cannot distinguish with dead links –Consecutive 1s - Baseline wander –Clock recovery

7 NRZI and Manchester Coding NRZI (Non Return to Zero Inverted) –Transition to encode 1 –Stay to encode 0 –Solving the problem of consecutive 1s Manchester encoding –Transmitting the exclusive-OR of the NRZ-encoded data and the clock –Bit rate is half the baud rate. *baud rate: the rate at which the signal changes

8 4B/5B Coding Insert extra bits to break up long sequence of 0s or 1s. Every 4 bits are encoded in a 5 bit code. 5 bit codes are selected so that –No more than one leading 0 –No more than two tailing 0s Resulting that no more than three consecutive 0s. Transmitted using the NRZI. 4 bit data5 bit code

9 Framing Frames Bits Adaptor Node BNode A

10 Byte-Oriented Protocols Sentinel approach Byte-counting approach SYN HeaderBody SYN SOH STX ETX CRC

11 Bit-Oriented Protocols A frame is a collection of bits. HDLC (High-Level Data Link Control) – is used for distinguishing both the beginning and the end of a frame. HeaderBody CRC Beginning sequence Ending sequence

12 Error-Detection: Naïve approach Send a message twice Compare two copies at the receiver –If different, some errors exist How many bits of error can you detect? What is the overhead?

13 Error Detection Problem: detect bit errors in packets (frames) Solution: add extra bits to each packet Goals: –Reduce overhead, i.e., reduce the number of redundancy bits –Increase the number and the type of bit error patterns that can be detected Examples: –Two-dimensional parity –Checksum –Cyclic Redundancy Check (CRC) –Hamming Codes

14 Reliable Transmission Overhead for error correction is too large. Corrupt frames are discarded -> Link-level packet loss. Reliable transmission is implemented by acknowledgment and timeout. (called automatic repeat request (ARQ))

15 Stop and Wait (1/2) The simplest ARQ scheme A sender sets a timer before sending a packets. The sender sends the next packet if it receives an ACK before the timer is expired. Otherwise, the sender retransmits the packet. Potential to cause duplicate copies of a packet (refer to Figure (c) and (d)).

16 Stop and Wait (2/2) Use 1 bit sequence number to distinguish packets. The main shortcoming is low utilization. Frame 0 ACK 0 SenderReceiver Frame 1 ACK 1 Frame 0 ACK 0

17 Sliding Window Window is defined as “the number of frames to be sent without ACK.” Trying to keep the pipe full. Sequence number is needed to identify packets. (be careful not to be wrapped up) ACK –Cumulative –Negative –Selective SenderReceiver … …

18 Ethernet (IEEE 802.3) Developed in the mid 1970s at the Xerox Palo Alto Research Center. Successful example of CSMA/CD (Carrier Sense Multiple Access with Collision Detect) technology.

19 Physical Properties of Ethernet Implemented on a coaxial cable of up to 500m. Can be extended by repeaters (up to four repeaters). An Ethernet is limited to supporting a maximum of 1024 hosts. Terminator are used to absorb the signal and keep it from bouncing back and interfering with trailing signals. Manchester encoding scheme is used. 10Base2, 10Base5, 10BaseT, 100BaseT,…

20 Ethernet Frame Format The 64 bit preamble alternating 0s and 1s allows the receiver to synchronize with the signal. Each frame contains up to 1500 bytes of data. –for bounding delay A frame must contain at least 46 bytes of data to detect collision. Dest addr CRCPreamble Src addr TypeBody 1648

21 Ethernet Address 48 bit Unique address for each adaptor 8:0:2b:e4:b1:2 -> …. An Ethernet adaptor receives all frames and accepts –Frames addressed to its own address –Frames addressed to the broadcast address (consisting of all 1s) –Frames addressed to a multicast address (the first bit set to 1 but is not the broadcast address), if it has been instructed to listen to that address. –All frames, if it has been placed in promiscuous mode.

22 Ethernet Transmitter Algorithm When the adaptor has a frame to send and the line is idle, it transmits the frame immediately. When an adaptor has a frame to send and the line is busy, it waits for the line to go idle, and then transmits immediately (1-persistent). If collision is detected, the sender transmits a 32-bit jamming sequence and then stops the transmission. Once an adaptor has detected a collision and stopped its transmission, it waits a certain amount of time and tries again. Exponential backoff

23 IEEE WLAN Physical Layer Date Link Layer Network Layer Transport Layer Application Layer IEEE

24 IEEE WLAN Data Link Layer Logical Link Layer (802.2) Interface with upper layer, framing, error control MAC Layer (802.11) CSMA/CA FHSS DSSS a OFDM b HR/DDSS Physical Layer

25 Technical Issues Unstable channel Hidden terminal problem Exposed terminal problem

MAC Point Coordination Function (PCF) Distributed Coordination Function (DCF) Contention-free Delivery Contention-based Delivery

27 Access Mode Point Coordination Function (PCF): –for delay sensitive service –use polling for channel access control –not widely deployed Distributed Coordination Function (DCF): –for best-effort data service –use CSMA/CA for channel access control –may use RTS/CTS scheme Contention-free PeriodContention Period A Super Frame

28 DCF Access Control SIFS (Short Inter Frame Spacing) –Highest priority, for ACK, CTS, Polling response PIFS (PCF IFS) –Medium priority, for time-bounded service using PCF DIFS(DCF IFS) –Lowest priority, for asynchronous data service BusyTransmission DIFS SIFS PIFS Contention Window

29 Backoff Timer Randomize interframe space to avoid collision backoff time = slot time * random no. Pick a random integer between 0 to CW CW is initially set to 7 for each collision occur, CW = 2*CW+1 increased upto 255