Transcript PPT

Direct Link Networks
• Encoding and Framing
• Error Detection/Correction
• Reliable Transmission will be covered later
(with TCP)
• Media Access Control
• (Wired) LAN Technologies
• Readings
– Chapter 2 except Section 2.5, which is delayed to the
discusson on Transport layer.
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1
Encoding
• Signals propagate over a physical medium
– modulate electromagnetic waves (on amplitude,
frequency, and phase): the process of encoding source
data onto a carrier signal with frequency f_c
– e.g., vary voltage (amplitude)
• Encode binary data onto signals
– e.g., 0 as low signal and 1 as high signal
– known as Non-Return to zero (NRZ)
Bits
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
NRZ
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Problems: Consecutive 1s or 0s
• Low signal (0) may be interpreted as no
signal
• High signal (1) leads to baseline wander
– Receiver keeps an average of the signal it has seen so far
and uses this average to distinguish between low and high
signals
• Unable to recover clock
– Clocks at the sender and receiver use signal transition to
synchronize each other
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Alternative Encodings
• Non-return to Zero Inverted (NRZI)
– make a transition from current signal to encode a one; stay
at current signal to encode a zero
– solves the problem of consecutive ones
• Manchester
– transmit XOR of the NRZ encoded data and the clock
– only 50% efficient.
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Encodings (cont)
• 4B/5B
– every 4 bits of data encoded in a 5-bit code
– 5-bit codes selected to have no more than one leading 0
and no more than two trailing 0s
– thus, never get more than three consecutive 0s
– resulting 5-bit codes are transmitted using NRZI
– achieves 80% efficiency
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Encodings (cont)
Bits
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
NRZ
Clock
Manchester
NRZI
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Framing
• Break sequence of bits into a frame – why?
determine where a frame starts and ends
• Typically implemented by network adaptor
Node A
Adaptor
Bits
Adaptor
Node B
Frames
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Approaches
• Sentinel-based
– delineate frame with special pattern: 01111110
– e.g., HDLC, SDLC, PPP
8
Beginning
sequence
16
Header
16
Body
8
CRC Ending
sequence
– problem: special pattern appears in the payload
– solution: bit stuffing
• sender: insert 0 after five consecutive 1s
• receiver: delete 0 that follows five consecutive 1s
• What if the receiver sees a 1?
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Approaches (cont)
• Couter-based
8
8
8
14
42
SYN
SYN
Class
– include payload length in header
– e.g., DDCMP
Count
Header
16
Body
CRC
– problem: count field corrupted
– solution: catch when CRC fails
– How many frames are affected when counter or sentinel
is corrupted?
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Approaches (cont)
• Clock-based
– each frame is 125us long
– e.g., SONET: Synchronous Optical Network
Overhead
Special bit pattern
Payload
9 row s
90 columns
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Handling Errors
• Data can be corrupted during transmission
– Bit values changed
• Frame includes additional information
– Set by sender
– Checked by receiver
• Error-detection vs error-correction
– Both need redundant information
– Detection: error exists or not.
– Correction: repair if there was an error
• Statistical guarantee
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Error Detecting and Correcting
Codes
• How many check/redundancy bits?
– To detect single-bit error
• 1 bit: even/odd parity
– To correct a single-bit error in m-bit message
• Need a minimum of r bits such that
• (m + r + 1)  2r
• Example: 3-bit message needs 3-bit redundancy
• Correction or detection+retransmission?
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Error Detection Techniques
• Checksum
–
–
–
–
Treat data as sequence of integers
Compute and send arithmetic sum
Detects some multiple bit errors not all
Internet Checksum
• Cyclic Redundancy Check
– Mathematical function of data
– More complex to compute
– Handles more errors
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Cyclic Redundancy Check
• Add k bits of redundant data to an n-bit
message
– want k << n
– e.g., k = 32 and n = 12,000 (1500 bytes)
• Represent n-bit message as n-1 degree
polynomial
– e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1
• Let k be the degree of some divisor
polynomial
– e.g., C(x) = x3 + x2 + 1
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CRC (cont)
• Transmit polynomial P(x) that is evenly
divisible by C(x)
– shift left k bits, i.e., M(x)xk
– subtract remainder of M(x)xk / C(x) from M(x)xk
• Receiver polynomial P(x) + E(x)
– E(x) = 0 implies no errors
• Divide (P(x) + E(x)) by C(x); remainder zero
if:
– E(x) was zero (no error), or
– E(x) is exactly divisible by C(x)
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CRC Example
Generator
11111001
1101 10011010000
1101
Message
1001
1101
1000
1101
1011
1101
1100
1101
1000
1101
101
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Remainder
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Choice of C(x)
• Want to ensure C(x) doesn’t divide E(x)
• We can detect
– All single-bit errors if
• C(x) has at least 2 terms
– All double-bit errors if
• C(x) doesn’t divide xj + 1
– X15 + x14 + 1 doesn’t divide xj + 1 for any j below 32768
– Any odd number of errors if
• C(x) contains the factor x+1
– Any “burst” error of length less than or equal to k bits
– Most burst errors of length greater than k bits
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Error Detection Summary
• To detect data corruption
– Sender adds additional information to packet
– Receiver checks
• Techniques
– Parity bit
– Checksum
– Cyclic Redundancy Check
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Hamming Error Correcting Code
• Achieve the theoretical lower limit of the number of
redundant bits
• Correcting single-bit errors
• Method:
– Put information bits at positions that are not equal to powers of two
– Compute the check bits at positions that are power of two
• A check bit at position 2^j, where j=0, 1, …, checks an information bit at
position i if and only if the binary representation of i contains a 1 at position j.
• The value of an “even” check bit equals the xor of all checked information bits
• At the receiver side, all checked bits and the check bit are xored and a zero
indicates error free; otherwise, the sum of the positions of the check bits with
a non-zero value indicates the bit that is in error
• Example
– Information bits: 1001000
– Coded bits: 00110010000
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Error Recovery
• Reliable delivery over unreliable channel
– How to recover from corrupted/lost packets
• Error detection and retransmission
– With acknowledgements and timeouts
– Also called Automatic Repeat Request (ARQ)
– Retransmission incurs round trip delay
• Error correcting codes
– Also called Forward Error Correction (FEC)
– No sender retransmission required
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Summary
• Encoding
• Framing
• Error correction/detection codes
– Parity/Checksum/CRC
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Shared Media and
Local Area Networks
• Shared Media Access Problem
– Media access control (MAC) and LAN
– MAC addresses and network adaptors (NICs)
– MAC protocols: random access vs. controlled access
• Ethernet
• Token Ring and FDDI (read yourself)
• 802.11 Wireless LAN (WiFi and WiMAX)
(next lecture)
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Multiple Access Links and LANs
Two types of “links”:
• point-to-point, e.g.,
– PPP for dial-up access, or over optical fibers
• broadcast (shared wire or medium), e.g.
– traditional Ethernet
– 802.11 wireless LAN
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Typical LAN Structure
• Transmission
Medium
• Network Interface
Card (NIC)
• Unique MAC
“physical” address
Ethernet
Processor
RAM
ROM
RAM
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Adaptors Communicating
sending
node
network packet
link layer protocol
frame
frame
adapter
• link layer implemented in
“adaptor” (aka NIC), with
“transceiver” in it
adapter
• receiving side
– Ethernet card, dial-up modem,
802.11 wireless card
• sending side:
– encapsulates packet in frame •
– adds error checking bits, flow
control, reliable data
transmission, etc.
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rcving
node
– looks for errors, flow
control, reliable data
transmission, etc
– extracts packet, passes to
receiving node
data link & physical layers are
closely coupled!
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MAC (Physical) Addresses
• Addressing needed in shared media
– MAC (media access control) or physical addresses
– To identify source and destination interfaces and get
frames delivered from one interface to another
physically-connected interface (i.e., on same physical
local area network!)
• 48 bit MAC address (for most LANs)
– fixed for each adaptor, burned in the adapter ROM
– MAC address allocation administered by IEEE
• 1st bit: 0 unicast, 1 multicast.
• all 1’s : broadcast
• MAC flat address -> portability
– can move LAN card from one LAN to another
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MAC (Physical, or LAN) Addresses
MAC addressing operations on a LAN:
•
•
each adaptor on the LAN “sees” all frames
accept a frame only if dest. (unicast) MAC address
matches its own MAC address
• accept all broadcast (MAC= all 1’s) frames
• accept all frames if set in “promiscuous” mode
• can configure to accept certain multicast addresses
(first bit = 1)
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MAC Sub-layer
OSI
IEEE 802
Network layer
Network layer
802.2 Logical link control
LLC
Data link
layer
MAC
802.11
802.3
802.5
CSMA-CD Token Ring Wireless
LAN
Physical
layer
Other
LANs
Various physical layers
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Physical
layer
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Broadcast Links: Multiple Access
Single shared communication channel
• Only one can send successfully at a time
• Two or more simultaneous transmissions
– interference!
• How to share a broadcast channel
– media access control uses same shared media
• Humans use multi-access protocols all the time
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Random Access
• Stations contend for channels
• Overlapping transmissions (collisions) can
occur
– Carrier sensing?
– Collision detection?
• Protocols
– Aloha
– Slotted Aloha
– Carrier Sense Multiple Access: Ethernet
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Controlled Access
• Stations reserve or are allocated channel
– No collisions
– Allocation: static or dynamic
• Protocols
– Static channel allocation
• Time division multiple access
– Demand adaptive channel allocation
• Reservation protocols
• Token passing (token bus, token ring)
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Taxonomy of MAC Protocols
WiFi (802.11)
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Pure (unslotted) Aloha
• Simpler, no synchronization
• Just send: no waiting for beginning of slot
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Slotted Aloha
• Time is divided into equal size slots
• Nodes transmit at the beginning of a slot
• If collision, retransmit later
Success (S), Collision (C), Empty (E) slots
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Performance of Aloha Protocols
0.4
0.3
Slotted Aloha
0.2
0.1
Pure Aloha
0.5
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1.0
1.5
2.0
G = offered load = Np
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Carrier Sense Multiple Access
• Aloha is inefficient (and rude)
– Doesn’t listen before talking
• CSMA: Listen before transmit
– If channel idle, transmit entire packet
– If busy, defer transmission
• How long should we wait?
– Human analogy: don’t interrupt others
• Can carrier sense avoid collisions completely?
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Persistent and Non-persistent CSMA
•
Non-persistent
–
–
–
•
If idle, transmit
If busy, wait random amount of time
If collision, wait random amount of time
p-persistent
–
–
–
If idle, transmit with probability p
If busy, wait till it becomes idle
If collision, wait random amount of time
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CSMA/CD
CSMA with collision detection (CD)
• Listen while talking
• Stop transmitting when collision detected
– Compare transmitted and received signals
– Save time and bandwidth
– Improvement over persistent and nonpersistent
protocols
• Human analogy
– Polite conversationalist
• Worst case time to detect a collision?
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Collisions
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A
B
A
B
A
B
A
B
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Worst Case Collision Detection Time
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CSMA/CD: Illustration
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Ethernet Overview
• History
– developed by Xerox PARC in mid-1970s
– roots in Aloha packet-radio network
– standardized by Xerox, DEC, and Intel in 1978
– similar to IEEE 802.3 standard
• CSMA/CD
– carrier sense
– multiple access
– collision detection
• Frame Format
64
48
Preamble
Dest
addr
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Src
addr
16
Type
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Body
CRC
42
Ethernet
“Dominant” LAN technology:
• cheap $20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token ring LANs and ATM
• Kept up with speed race: 10, 100, 1000 Mbps
Metcalfe’s Ethernet
sketch
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Ethernet Frame Format
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
DIX frame format
8 bytes
Preamble
6
Dest
addr
6
Src
addr
2
Type
0-1500
Data
4
CRC
IEEE 802.3 format
8 bytes
Preamble
6
Dest
addr
6
Src
addr
2
0-1500
Length
Data
4
CRC
• Ethernet has a maximum frame size: data portion <=1500 bytes
• It imposes a minimum frame size: 64 bytes (excluding preamble)
If data portion <46 bytes, pad with “junk” to make it 46 bytes
Q: Why minimum frame size in Ethernet?
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Fields in Ethernet Frame Format
• Preamble:
– 7 bytes with pattern 10101010 followed by one byte with
pattern 10101011 (SoF: start-of-frame)
– used to synchronize receiver, sender clock rates, and identify
beginning of a frame
• Addresses: 6 bytes
– if adapter receives frame with matching destination address, or
with broadcast address (eg ARP packet), it passes data in
frame to network layer protocol (specified by TYPE field)
– otherwise, adapter discards frame
• Type: indicates the higher layer protocol, mostly IP but
others may be supported such as Novell IPX and AppleTalk
– 802.3: Length gives data size; “protocol type” included in data
• CRC: checked at receiver, if error is detected, the
frame is simply dropped
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IEEE 802.3 MAC: Ethernet
MAC Protocol:
• CSMA/CD
• Truncated binary exponential backoff
– for retransmission n: 0 < r < 2k * ASlotTime, where
k=min(n,10)
– give up after 16 retransmissions
• Slot Time is the critical system parameter
– upper bound on time to detect collision
– upper bound on time to acquire channel
– upper bound on length of frame segment generated by
collision
– quantum for retransmission scheduling
– max{round-trip propagation, MAC jam time}
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IEEE 802.3 Parameters
• 1 bit time = time to transmit one bit
– 10 Mbps  1 bit time = 0.1 s
• Maximum network diameter  2.5km
– Maximum 4 repeaters
• “Collision Domain”
– Distance within which collision can occur and be detected
– IEEE 802.3 specifies:
worst case collision detection time: 51.2 s
• Slot time
– 51.2 s = 512 bits = 64 bytes
• Why minimum frame size?
– 51.2 s => minimum # of bits can be transited at 10Mpbs
is 512 bits => 64 bytes is required for collision detection
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Ethernet MAC Protocol: Basic Ideas
1-persistent CSMA/CD
• Carrier sense: station listens to channel first
– Listen before talking
• If idle, station may initiate transmission
– Talk if quiet
• Collision detection: continuously monitor channel
– Listen while talking
• If collision, stop transmission
– One talker at a time
• Exponential binary back-off algorithm
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Ethernet CSMA/CD Alg. Flow Chart
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Ethernet CSMA/CD Algorithm
1. Adaptor gets datagram and
creates frame
2. If adapter senses channel idle,
it starts to transmit frame. If
it senses channel busy, waits
until channel idle and then
transmits
3. If adapter transmits entire
frame without detecting
another transmission, the
adapter is done with frame !
Signal to network layer
“transmit OK”
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4. If adapter detects another
transmission while transmitting,
aborts and sends jam signal
5. After aborting, adapter enters
exponential backoff: after the
mth collision, adapter chooses a
K at random from
{0,1,2,…,2m-1}. Adapter waits
K*512 bit times and returns to
Step 2
6. Quit after 16 attempts, signal
to network layer “transmit
error”
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Ethernet’s CSMA/CD (more)
Jam Signal: make sure all
other transmitters are
aware of collision; 32 bits;
Bit time: .1 microsec for 10
Mbps Ethernet ;
for K=1023, wait time is
about 50 msec
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Exponential Backoff:
• Goal: adapt retransmission
attempts to estimated
current load
– heavy load: random wait
will be longer
• first collision: choose K
from {0,1}; delay is K x 512
bit transmission times
• after second collision:
choose K from {0,1,2,3}…
• after ten collisions, choose
K from {0,1,2,3,4,…,1023}
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CSMA/CD Efficiency
Relevant parameters
– cable length, signal speed, frame size, bandwidth
• tprop = max prop between 2 nodes in LAN
• ttrans = time to transmit max-size frame
efficiency 
1
1  5t prop / ttrans
• Efficiency goes to 1 as tprop goes to 0
• Goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
simple, and cheap
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IEEE 802.3 Physical Layer
IEEE 802.3 10 Mbps medium alternatives
Medium
Max. Segment Length
Topology
10base5
10base2
10baseT
10baseFX
Thick coax
Thin coax
Twisted pair
Optical fiber
500 m
200 m
100 m
2 km
Bus
Bus
Star
Point-topoint link
(b)
(a)
Hubs & Switches!
transceivers
Thick Coax: Stiff, hard to work with
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Ethernet Technologies: 10Base2
• 10: 10Mbps; 2: under 200 meters max cable length
• thin coaxial cable in a bus topology
• repeaters used to connect up to multiple segments
• repeater repeats bits it hears on one interface to
its other interfaces: physical layer device only!
• has become a legacy technology
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10BaseT
• 10 Mbps rate
• T stands for Twisted Pair
• Nodes connect to a hub: “star topology”; 100 m
max distance between nodes and hub
nodes
hub
• Hubs are essentially physical-layer repeaters:
–
–
–
–
bits coming in one link go out all other links
no frame buffering
no CSMA/CD at hub: adapters detect collisions
provides net management functionality
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Ethernet Hubs & Switches
Single collision domain
(a)
High-Speed backplane
or interconnection fabric
(b)
     

Twisted Pair Cheap
Easy to work with
Reliable
Star-topology CSMA-CD
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


Twisted Pair Cheap
Bridging increases scalability
Separate collision domains
Full duplex operation
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Evolution of Ethernet
From early 80’s 10Base Ethernet to 90’s 100Base
(Fast) Ethernet
to today’s Gigabit Ethernet to 10 Gigabit Ethernet,
……
IEEE 802.3 Original Parameters
•
•
•
transmission Rate: 10 Mbps
Min Frame: 512 bits = 64 bytes
slot time: 512 bits/10 Mbps = 51.2 sec
•
max Length: 2500 meters + 4 repeaters
– 51.2 sec x 2x105 km/sec =10.24 km
– 5.12 km round trip distance
• For compatibility, desire to maintain same frame format!
– Each x10 increase in bit rate, must be accompanied by x10
decrease in distance ?!
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100Base T (Fast) Ethernet: Issues
• 1 bit time = time to transmit one bit
– 100 Mbps  1 bit time = 0.01 s
• If we keep the same “collision domain”, i.e.,
worst case collision detection time kept at 51.2 s
Q: What will be the minimum frame size?
– 51.2 s => minimum # of bits can be transited at
100Mpbs is 5120 bits => 640 bytes is required for
collision detection
– This requires change of frame format and protocol!
• Or we can keep the same minimum frame size, but
reduce “collision domain” or network diameter!
• slot time from 51.2 s to 5.12 s!
• maximum network diameter  100 m!
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Fast (100Mbps) Ethernet
IEEE 802.3 100 Mbps Ethernet medium alternatives
100baseT4
100baseT
100baseFX
Twisted pair category 3
UTP 4 pairs
Twisted pair category 5
UTP two pairs
Optical fiber multimode
Two strands
Max. Segment
Length
100 m
100 m
2 km
Topology
Star
Star
Star
Medium
To preserve compatibility with 10 Mbps Ethernet:
• Same frame format, same interfaces, same protocols
• Hub topology only with twisted pair & fiber
• Bus topology & coaxial cable abandoned
• Category 3 twisted pair (ordinary telephone grade) requires 4 pairs
• Category 5 twisted pair requires 2 pairs (most popular)
• Most prevalent LAN today
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Gigabit Ethernet
Gigabit Ethernet Physical Layer Specification
(IEEE 802.3 1 Gigabit Ethernet medium alternatives)
Medium
Max. Segment
Length
Topology
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1000baseSX
1000baseLX
1000baseCX
1000baseT
Optical fiber
multimode
Two strands
Optical fiber
single mode
Two strands
Shielded
copper cable
Twisted pair
category 5
UTP
550 m
5 km
25 m
100 m
Star
Star
Star
Star
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Gigabit Ethernet
• use standard Ethernet frame format
• allows for point-to-point links and shared broadcast
channels
• in shared (half-duplex hub) mode, CSMA/CD is used
- slot time increases to 512 bytes
- small frames need to be extended to 512 B
- carrier extension:
– frame bursting: allow stations to transmit burst of short
frames
• Commonly used today: Gigabit switches!
– Full-Duplex at 1 Gbps for point-to-point links
– Frame structure preserved but CSMA-CD essentially
abandoned
– Extensive deployment in backbone of enterprise data
networks and in server farms
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Carrier Extension
Frame
RRRRRRRRRRRRR
Carrier Extension
512 bytes
• For 10BaseT : 2.5 km max; slot time = 64 bytes
• For 1000BaseT: 200 m max; slot time = 512 bytes
• Carrier Extension : continue transmitting control
characters [R] to fill collision interval.
• This permits minimum 64-byte frame to be handled.
• Control characters discarded at destination.
• For small frames net throughput is only slightly better
than Fast Ethernet.
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Frame Bursting
Frame Extension
Frame
Frame
Frame
512 bytes
Frame burst
• Source sends out burst of frames without relinquishing
control of the network.
• Uses Ethernet Interframe gap filled with extension
bits (96 bits)
• Maximum frame burst is 8192 bytes
• Three times more throughput for small frames.
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10 Gigabit Ethernet
IEEE 802.3 10 Gbps Ethernet medium alternatives
10GbaseSR
Medium
10GBaseLR
10GbaseEW
10GbaseLX4
Two optical
fibers
Multimode at
850 nm
Two optical fibers
Two optical fibers
Single-mode at
1310 nm
64B66B code
64B66B
Single-mode at
1550 nm
SONET
compatibility
Two optical fibers
multimode/singlemode with four
wavelengths at
1310 nm band
8B10B code
Max. Segment
Length
•
•
•
•
•
300 m
10 km
40 km
300 m – 10 km
Frame structure preserved
CSMA-CD protocol officially abandoned
LAN PHY for local network applications
WAN PHY for wide area interconnection using SONET OC-192c
Extensive deployment in metro networks anticipated
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Typical Ethernet Deployment
Server farm
Server
Server
Server
Gigabit Ethernet links
Switch/router
Server
Ethernet
switch
100 Mbps links
Hub
10 Mbps links
Department A
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Gigabit Ethernet links
Ethernet
switch
100 Mbps links
Server
Hub
10 Mbps links
Department B
Direct Link networks
Switch/router
Ethernet
switch
100 Mbps links
Server
Hub
10 Mbps links
Department C
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Ethernet Summary
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1-persistent CSMA/CD
51.2 s to seize the channel
Collision not possible after 51.2 s
Minimum frame size of 64 bytes
Binary exponential backoff
Works better under light load
Delivery time non-deterministic
Csci 183/183W/232
Direct Link networks
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Summary (continued)
• Address
– Unique number assigned to station
– Put in frame header
– Recognized by hardware
• Address forms
– Unicast
– Broadcast
– Multicast
Csci 183/183W/232
Direct Link networks
67
Summary (continued)
• Type information
– Describes data in frame
– Set by sender
– Examined by receiver
• Frame format
– Header contains address and type information
– Payload contains data being sent
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Direct Link networks
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Summary (continued)
• LAN technologies
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Ethernet (bus)
Token Ring
FDDI (ring)
Wireless 802.11
• Wiring and topology
– Logical topology and Physical topology (wiring)
– Hub allows
• Star-shaped bus
• Star-shaped ring
Csci 183/183W/232
Direct Link networks
69