Data Link Layer (Layer 2, L2)Data Link Layer (Layer 2, L2)
염익준
2
CablesCables
• Used to connect nodes in the same room or building.
Cable Typical Bandwidths Distances
Cat-5 twisted pairThin-net coaxThick-net coaxMultimode fiberSingle-mode fiber
10-100 Mbps10-100 Mbps10-100 Mbps100 Mbps100-2400 Mbps
100 m200 m500 m2 km40 km
3
Leased LinesLeased Lines
• Used to connect site to site. Service Bandwidth
DS1 (or T1)DS3 STS-1 (or OC1)STS-3STS-12STS-48STS-192
1.544 Mbps44.736 Mbps51.840 Mbps155.250 Mbps622.080 Mbps2.488320 Gbps9.953280 Gbps
4
Last-Mile LinksLast-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 LinksWireless Links
• Global links – PCS, GSM• Wireless LAN – IEEE 802.11a, b, g• Wireless MAN – IEEE 802.16, WiBro• Piconet – Bluetooth (upto 1 Mbps)
6
Encoding - NRZEncoding - NRZ
Signalling component
Signal
Bits
Node NodeAdaptor Adaptor
Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
• Problems of NRZ– Consecutive 0s – cannot distinguish with dead links– Consecutive 1s - Baseline wander– Clock recovery
7
NRZI and Manchester CodingNRZI 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.
Bits
NRZ
Clock
Manchester
NRZI
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
*baud rate: the rate at which the signal changes
8
4B/5B Coding4B/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 data 5 bit code
0000000100100011010001010110011110001001101010111100110111101111
11110010011010010101010100101101110011111001010011101101011111010110111110011101
9
FramingFraming
Frames
BitsAdaptor Adaptor Node BNode A
10
Byte-Oriented ProtocolsByte-Oriented Protocols
• Sentinel approach
• Byte-counting approach
SY
N Header Body
8 8 8 8 168S
YN
SO
H
ST
X
ET
X CRC
SY
N
Header Body
8 8 4214 168
SY
N
Cla
ss CRCCount
11
Bit-Oriented ProtocolsBit-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.
Header Body
8 16 16 8
CRCBeginningsequence
Endingsequence
12
Error-Detection: Naïve Error-Detection: Naïve approachapproach• 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 DetectionError 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 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)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)).
Sender Receiver
Frame
ACK
Tim
eout
Tim
e
Sender Receiver
Frame
ACK
Tim
eout
Frame
ACKTim
eout
Sender Receiver
Frame
ACKTim
eout
Frame
ACKTim
eout
Sender Receiver
Frame
Tim
eout
Frame
ACKTim
eout
(a) (c)
(b) (d)
16
Stop and Wait (2/2)Stop and Wait (2/2)
• Use 1 bit sequence number to distinguish packets.
• The main shortcoming is low utilization.
Frame 0
ACK 0
Sender Receiver
Frame 1
ACK 1
Frame 0
ACK 0
17
Sliding WindowSliding 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
Sender Receiver
……
18
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.
19
Physical Properties of Physical Properties of EthernetEthernet• 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 FormatEthernet 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.
Destaddr
64 48 32
CRCPreamble Srcaddr
Type Body
1648
21
Ethernet AddressEthernet 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 Ethernet Transmitter AlgorithmAlgorithm• 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 WLANIEEE 802.11 WLAN
Physical Layer
Date Link Layer
Network Layer
Transport Layer
Application Layer
IEEE 802.11
24
IEEE 802.11 WLANIEEE 802.11 WLAN
Data Link Layer
Logical Link Layer (802.2)Interface with upper layer,
framing, error control
MAC Layer (802.11)CSMA/CA
802.11FHSS
802.11DSSS
802.11aOFDM
802.11bHR/DDSS
Physical Layer
25
Technical IssuesTechnical Issues
• Unstable channel• Hidden terminal problem• Exposed terminal problem
26
802.11 MAC802.11 MAC
Point Coordination Function (PCF)
Distributed Coordination Function (DCF)
Contention-freeDelivery Contention-based
Delivery
27
Access ModeAccess 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 ControlDCF 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
Busy Transmission
DIFS
SIFSPIFS
Contention Window
29
Backoff TimerBackoff 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