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Network Technologies
802.3 Ethernet 802.11 WiFi 802.16 WiMax DSL Cable modem Cellular (3G, 2.5G, …)
SONET, STS, OC …
LAN: local area network
MAN: metropolitan area network
WAN: wide area network
Backbone network Access network The “last mile” aka “first mile” aka “local loop”
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Link Layer Functionalities
Framing Error detection or correction Media access control
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Framing
Message transmitted over link as string of 0’s and 1’s
Sender and receiver has to agree where is the beginning and end of a message framing
A frame is a link layer message unit - The prelude and/or postlude are special characters or character sequences that help establish the beginning and end of the frame
- The header contains control information that is used by the network (e.g., network address; error detection)
- The payload contains data that is meaningful only for the sender and receiver
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Dealing with Errors
Data can be corrupted during transmission- Bits lost- Bit values changed
Frame includes additional information to help detect or correct errors- Set by sender; checked by receiver
Statistical guarantee
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Error Detection
Message M H = f(M) Transmitted T = M || H
Received R = M’ || H’
If H’ = f(M’) then no error
M
H
f
M H
M’ H’
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Error Detection Schemes
Parity- Send an additional parity bit (H) per character
- Even parity: if # of 1’s in character is odd, H = 1; else H = 0
- Odd parity: if # of 1’s in character is odd, H = 0; else H = 1
- Cannot detect even numbers of bit errors Checksum
- Treat data as sequence of integers- Compute and send arithmetic sum (H)- Handles multiple bit errors, but not all errors
Cyclic Redundancy Check (CRC)
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Checksum Examples
Checksum computed over data Checksum appended to frame
2nd bit reversed in each item, but checksum is the same
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Cyclic Redundancy Check
CRC is a mathematical function of data, computed as the remainder from a division of two binary numbers, one representing the message M, and the other a fixed divisor P.
Example: CRC-CCITT:- P(X) = x16 + x12 + x5 + 1; or- P = 10001000000100001
CRC-CCITT can detect:- all single/double bit errors- All odd-numbered bit errors- 100% of burst errors <= 16 bits- 99.997% of burst errors = 17 bits- 99.998% of burst errors >= 18 bits
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CRC Algorithm CRC can be computed and verified using binary long division
Numerical example- Use CRC scheme with P(x) = x5 + x4 + x + 1- P: 110011 (6 bits)- Message M: 11100011 (8 bits)- Divide M by P; use the remainder as the CRC (what we call ‘H’ on slide 6) [Note: remainder one bit shorter than P]
- Transmitted message T = M || H- Receiver performs CRC verification on received message T’: divide T’ by P will produce zero remainder if no error
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Numerical Example 10110110 110011 ) 1110001100000 110011 101111 110011 111000 110011 101100 110011 111110 110011 11010 = HT = M || H = 1110001111010
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Verification 10110110 110011 ) 1110001111010 110011 101111 110011 111001 110011 101010 110011 110011 110011 00000No error since remainder is 0
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Media Access Control
How do multiple, independent computers coordinate access to a shared communication medium?
Ethernet
Wi-Fi
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Local Area Networks
LAN characteristics:- High throughput- Relatively low cost
- Distance limitations
- Often rely on shared media
Different topologies
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Ethernet
Most widely deployed LAN technology
IEEE 802.3 standard Several generations
- Same frame format- Different data rates (10Mbps, 100Mbps, 1Gbps, 10Gbps)
- Different media (coax, twisted pair, fiber)
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Shared Medium
Shared medium used for all transmissions
Only one station transmit at any time
Stations take turns using medium
Media access control (MAC) policy ensures fairness
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Data Transmission in Ethernet
Only one station transmit at any time Signal propagates across entire cable All stations receive transmission CSMA/CD media access scheme
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CSMA/CD
Multiple access (MA)- Multiple computers attach to shared media
- Each uses same access algorithm Carrier Sense (CS)
- Wait until medium is idle- Begin to transmit frame
Simultaneous transmission possible
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CSMA/CD (2)
Two simultaneous transmissions- Interfere with one another- Called a collision
CSMA plus collision detection (CD)- Listen to medium during transmission- Detect whether another station’s signal interferes
- Back off from interference and try again
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Example
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Back-off after Collision
When collision occurs- Wait random time t1, 0 <= t1 <= d- Use CSMA and try again
If second collision occurs- Wait random time t2, 0 <= t2 <= 2d
Double range for each successive collision
Called exponential backoff
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Wireless Ethernet (Wi-Fi)
Uses unlicensed spectrum (ISM band)
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WLAN Media Access
Limited range- Not all stations receive all transmissions- Cannot use CSMA/CD
Example- Maximum transmission distance is d- Stations 1 and 3 do not receive each other’s transmissions
Known as the “hidden terminal” problem
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CSMA/CA
Collision avoidance (CA)- upon sensing idle channel, waits for a random backoff duration before attempting to transmit
RTS/CTS Mechanism- Handshake before data transmission- Request to Send (RTS): “X is about to send to Y”
- Clear to Send (CTS): “Y is about to receive from X”
- Data frame sent from X to Y- Collisions of control messages possible; but control messages are much shorter than data frames
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Addressing in Shared Medium
All stations on shared media receive all transmissions
Each frame contains address of intended recipient
Stations discard any frame addressed to another station
Shared media provide no confidentiality- Network analyzers can run in promiscuous mode- Designed for testing/debugging- Allows network interface to accept all packets
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Ethernet Addressing & Frame Format
Each station assigned unique 48-bit (6 byte) address- Known as Ethernet address, MAC address, or Physical address
Address assigned when network interface card (NIC) manufactured
Ethernet frame format:
/Size
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Ethernet Evolution
Original Ethernet: bus topology
Modern Ethernet: star topology
Ethernet hub: - Propagates each incoming
signal to all connections- noise and collisions also
propagated Ethernet switch:
- Operates on frames- Does not forward noise or
collisions- Understands addresses- Only forwards when necessary- Allows independent
transmission on different segments
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Layer 2 is not limited to LANs
Backbone operators (e.g., AT&T) deploy and operate long-haul copper-based or fiber-based digital circuits
SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) standards support framing, multiplexing, synchronization.
OC-192 9.953Gbps
OC-768 39.813Gbps
