Transcript lecture13

EEC-484/584
Computer Networks
Lecture 13
Wenbing Zhao
[email protected]
Outline
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No class this Wednesday (Thanksgiving Eve)
Medium Access Control
Ethernet
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Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
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Wenbing Zhao
Medium Access Control Sublayer
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Broadcast channels often used on data link layer
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Broadcast channels often referred to as multiaccess or
random access channels
The channel allocation problem: Who gets to use
the channel?
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Static Channel Allocation
Dynamic Channel Allocation
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Static Channel Allocation
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FDM – Frequency Division Multiplexing
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TDM – Time Division Multiplexing
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Frequency spectrum divided into logical channel
Each user has exclusive use of own frequency band
Time divided into slots each user has time slot
Users take turns in round robin fashion
Problem: wasted bandwidth if user does not use
his/her frequency band or timeslot
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Frequency Division Multiplexing
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Time Division Multiplexing
T1 Carrier (1.544 Mbps)
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Model for
Dynamic Channel Allocation
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N independent stations (also called terminals)
Once a frame has been generated, the station is
blocked until the frame is transmitted successfully
Single Channel shared by all stations
Collision – event when two frames transmitted
simultaneously and the resulting signal is garbled
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All stations can detect collisions
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Model for
Dynamic Channel Allocation
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Frame transmission time
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Continuous Time – can begin at any instant
Slotted Time – always begin at the start of a slot
Carrier sense or not
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Carrier sense – stations can tell if the channel is
busy. Do not send if channel is busy
No carrier sense – just go ahead and send
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Multiple Access Protocols
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ALOHA
Carrier Sense Multiple Access Protocols
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Pure ALOHA
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Let users transmit whenever they have data to
send
If frame destroyed
(due to collision),
sender waits
random amount
of time, sends again
User does not listen before transmitting
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Pure ALOHA: Vulnerable Period
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Vulnerable period for a frame: A collision will happen if
another frame is sent during this period
2 frame time
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Slotted ALOHA
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Idea: divide time into intervals, each interval
corresponds to one frame
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Station is permitted to send only at the beginning
of next slot
Vulnerable period is halved (1 frame time)
Probability of no collision in time slot = e-G
 Throughput S = G e-G
 Max occurs when G = 1, S = 2*0.184
(G: # of attempts per frame time)
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Throughput for ALOHA Systems
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Carrier Sense Multiple Access
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When station has data to send, listens to channel to
see if anyone else is transmitting
If channel is idle, station transmits a frame
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Else station waits for it to become idle
If collisions occurs, station waits random amount of
time, tries again
Also called 1-persistent CSMA
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With probability 1 station will transmit if channel is idle
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Carrier Sense Multiple Access:
Collision Still Possible
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After a station starts sending, it takes a while before
2nd station receives 1st station’s signal
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2nd station might start sending before it knows that another
station has already been transmitting
If two stations become ready while third station
transmitting
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Both wait until transmission ends and start transmitting,
collision results
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p-persistent CSMA:
Reduce the Probability of Collision
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Sense continuously, but does not always send when
channel is idle
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Applicable for slotted channels
When ready to send, station senses the channel
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If channel idle, station transmits with probability p, defers to
next slot with probability q = 1-p
Else (if channel is busy) station waits until next slot tries
again
If next slot idle, station transmits with probability p, defers
with probability q = 1-p
…
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Non-Persistent CSMA
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Does not sense continuously, send if it senses the
channel is idle
Before sending, station senses the channel
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If channel is idle, station begins sending
Else station does not continuously sense, waits random
amount of time, tries again
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Persistent and Nonpersistent CSMA
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Improves over ALOHA because they ensure no
station to transmit when it senses channel is busy
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CSMA with Collision Detection
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If two stations start transmitting simultaneously, both
detect collision and stop transmitting
Minimum time to detect collision?
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Minimum Time to Detect Collision
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To ensure the sender can detect collision
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All frames must take more than 2t to send so that
transmission is still taking place when the noise burst gets
back to the sender
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Manchester Encoding
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Binary encoding
 Hard to distinguish 0 bit (0-volt) from idle (0-volt)
 Requires clocks of all stations synchronized
Manchester encoding
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Ethernet Frame Structure
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Preamble: for clock synchronization
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First 7 bytes with pattern 10101010, last byte with pattern
10101011
The two consecutive 1’s indicate the start of a frame
How can the receiver tell the end of the frame?
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No current on the wire (interesting discussion at
http://www.tomshardware.com/forum/19951-42-detecting-length-ethernet-frame)
Not considered
as part of the
header!
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>= 64 bytes
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Ethernet Frame Structure
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Destination address: 6 bytes (48 bits)
Highest order bit: 0 individual, 1 multicast;
all 1’s broadcast
 Frames received with non-matching destination address is
discarded
Type/Length: type of network layer protocol (or length of payload)
Pad – used to produce valid frame >= 64 bytes
Checksum – 32-bit cyclic redundancy check
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CSMA with Collision Detection
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If two stations start transmitting simultaneously, both
detect collision and stop transmitting
Monitor collision while sending
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Minimum time to detect collision => minimum frame length
Time divided into slots
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Length of slot = 2t = worst-case round-trip propagation time
To accommodate longest path, slot time = 512 bit times = 51.2
msec (10Mbps Ethernet)
=> min frame length: 51.2 msec X 10 Mbps = 512 b = 64 byte
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Ethernet MAC Sublayer Protocol
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Connectionless: No handshaking between sending and
receiving NICs
 Ethernet resides in the Network Interface Card (NIC)
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Unreliable: receiving NIC doesn’t send acks or nacks to
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sending NIC
 stream of datagrams passed to network layer can have gaps
(missing datagrams)
 gaps will be filled if app is using TCP
 otherwise, app will see gaps
Ethernet’s MAC protocol: CSMA/CD
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Ethernet CSMA/CD algorithm
1. NIC receives datagram from
4. If NIC detects another
network layer, creates frame
transmission while
transmitting, aborts and sends
2. If NIC senses channel idle,
jam signal
starts frame transmission If
NIC senses channel busy,
5. After aborting, NIC enters
waits until channel idle, then
exponential backoff: after
transmits
mth collision, NIC chooses K at
random from
3. If NIC transmits entire frame
{0,1,2,…,2m-1}. NIC waits K·512
without detecting another
bit times, returns to Step 2
transmission, NIC is done with
frame !
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Randomization and
Binary Exponential Backoff
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After 1st collision, station picks 0 or 1 at random, waits that
number of slots and tries again
After 2nd collision, station picks 0,1,2,3 at random, waits that
number of slots and tries again
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After i-th collision, station picks 0,1,…,2i-1 at random, …
If 10 <= i < 16, station picks 0,1,…,210-1 at random
If i=16, controller reports failure to computer
Why randomization is needed?
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Ethernet Performance
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Binary exponential backoff results in
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Low delay when few stations collide
Reasonable delay for collision resolution when
many stations collide
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Exercise
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An IP packet to be transmitted by Ethernet is
60 bytes long. Is padding needed in the
Ethernet frame, and if so, how many bytes?
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Exercise
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Consider building a CSMA/CD network
running at 1 Gbps over a 1-km cable. The
signal speed in the cable is 200,000 km/sec.
What is the minimum frame size?
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Wenbing Zhao