Data link layer layer 2 l2
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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

Data Link Layer (Layer 2, L2)

염익준


Cables

Cables

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


Leased lines

Leased Lines

  • Used to connect site to site.


Last mile links

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)


Wireless links

Wireless Links

  • Global links – PCS, GSM

  • Wireless LAN – IEEE 802.11a, b, g

  • Wireless MAN – IEEE 802.16, WiBro

  • Piconet – Bluetooth (upto 1 Mbps)


Encoding nrz

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


Nrzi and manchester coding

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


4b 5b coding

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.


Framing

Framing

Bits

Node A

Adaptor

Adaptor

Node B

Frames


Byte oriented protocols

Sentinel approach

Byte-counting approach

8

8

8

8

8

16

Header

Body

CRC

ETX

STX

SYN

SYN

SOH

Byte-Oriented Protocols


Bit oriented protocols

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


Error detection na ve approach

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?


Error detection

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


Reliable transmission

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))


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)).

Stop and Wait (1/2)


Stop and wait 2 2

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


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

Sliding Window

Sender

Receiver


Ethernet ieee 802 3

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.


Physical properties of ethernet

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,…


Ethernet frame format

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


Ethernet address

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.


Ethernet transmitter algorithm

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


Ieee 802 11 wlan

IEEE 802.11 WLAN

Application Layer

Transport Layer

Network Layer

Date Link Layer

Physical Layer

IEEE 802.11


Ieee 802 11 wlan1

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


Technical issues

Technical Issues

  • Unstable channel

  • Hidden terminal problem

  • Exposed terminal problem


802 11 mac

802.11 MAC

Contention-free

Delivery

Contention-based

Delivery

Point Coordination

Function (PCF)

Distributed Coordination

Function (DCF)


Access mode

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


Dcf access control

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


Dcf logic

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


Backoff timer

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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