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Data Link Layer (Layer 2, L2)

Data Link Layer (Layer 2, L2). 염익준. Cables. Used to connect nodes in the same room or building. Leased Lines. Used to connect site to site. Last-Mile Links. POTS (Plain Old Telephone Service): 56 Kbps ISDN (Integrated Services Digital Network): 64-128 Kbps

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Data Link Layer (Layer 2, L2)

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  1. Data Link Layer (Layer 2, L2) 염익준

  2. Cables • Used to connect nodes in the same room or building.

  3. Leased Lines • Used to connect site to site.

  4. Last-Mile Links • POTS (Plain Old Telephone Service): 56 Kbps • ISDN (Integrated Services Digital Network): 64-128 Kbps • xDSL (Digital Subscriber Line): 16 Kbps – 55.2 Mbps • Asymmetric DSL (ADSL): Upstream (16 – 640 Kbps) and Downstream (1.544 – 8.448 Mbps) • Very-high rate DSL (VDSL): 12.96 – 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 802.11a, b, g • Wireless MAN – IEEE 802.16, WiBro • Piconet – Bluetooth (upto 1 Mbps)

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

  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.

  9. Framing Bits Node A Adaptor Adaptor Node B Frames

  10. Sentinel approach Byte-counting approach 8 8 8 8 8 16 Header Body CRC ETX STX SYN SYN SOH Byte-Oriented Protocols

  11. Bit-Oriented Protocols • A frame is a collection of bits. • HDLC (High-Level Data Link Control) • 01111110 is used for distinguishing both the beginning and the end of a frame. 8 16 16 8 Beginning Ending Header Body CRC sequence 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. 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)). Stop and Wait (1/2)

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

  17. 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 Sliding Window Sender Receiver … …

  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. 64 48 48 16 32 Src Dest Preamble Type Body CRC addr addr

  21. Ethernet Address • 48 bit • Unique address for each adaptor • 8:0:2b:e4:b1:2 -> 00001000 00000000 00101011 11100100 …. • 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 802.11 WLAN Application Layer Transport Layer Network Layer Date Link Layer Physical Layer IEEE 802.11

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

  25. Technical Issues • Unstable channel • Hidden terminal problem • Exposed terminal problem

  26. 802.11 MAC Contention-free Delivery Contention-based Delivery Point Coordination Function (PCF) Distributed Coordination Function (DCF)

  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 A Super Frame Contention-free Period Contention Period

  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 DIFS Contention Window PIFS SIFS Busy Transmission

  29. Medium idle? Wait IFS Wait IFS Still idle? Still idle? Exponential backoff while medium idle Transmit frame Transmit frame DCF Logic Wait for frame to transmit Wait until current transmission ends No Yes No Yes No Yes

  30. 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

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