computer communication and distributed algorithms 202-2-1131 data link layer © תקשורת...

Computer Communication and

Distributed Algorithms

202-2-1131

Data link layer

תקשורת מחשבים ואלגוריתמים © (2011-2012מבוזרים )חורף

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Data link layer

Application

Physical

Link

Network

Transport

Session

Presentation

The 7-layer OSI Model

תקשורת מחשבים ואלגוריתמים מבוזרים 2(2011-2012)חורף

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Data link layer

Motivation: The basics of the

data link service: Error detection and

correction. Multiplexing –

Multiple access to the channel

Accessing network resources using the link layer.

Representation of different Link layer technologies

Overview: Link layer services Error detection and

correction. Multiplexing protocols and

local networks (LANs). Link layer technologies:

Ethernet – Local area network protocol.


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Data Link Layer services

תקשורת מחשבים ואלגוריתמים מבוזרים (2011-2012)חורף

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Data link layer – Communication framework

PDU (protocol data unit) – Data is transferred as units. May include control data, addresses, or content.


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Physicly connecting two devices. host-router, router-router, host-host

Data unit: A frame.

applicationtransportnetwork

linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card

Data link layer- Communication framework


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Data link layer services Discrete messages, channel access:

Framing the data, adding a header and a trailer A physical address to every header to identify source

and destination. The physical address is not the IP address

A reliable delivery between two connected physical devices: Low throughput with low noise is rarely used. Usually a wireless channel with errors is used.


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Data link layer services

Flow control: Controlling the data transmission between two

computers (Sender and receiver). Error detection:

Occur as a result of weakening the signal along the cables, and noise.

The receiver detects the presence of errors. Sender retransmits. The receiver solves the problem by himself.

Error correction: The receiver identifies the bit errors without

retransmission Named FEC – Forward error correction.


Implementation Adapter – An extension card connected to the

motherboard. E.g. Ethernet card. Usually includes RAM, Link interface, host bus.

adapter card

applicationtransportnetwork

linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol


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) Frames (מסגרות


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Motivation

In stead of sending the data in one block, we break the data to a series of frames:

Allows a smaller buffer size. When the sending time is longer the probability for an error is

bigger, and retransmission is needed. Better use of shared resources. (e.g. Multiplexed transmission)

11תקשורת מחשבים ואלגוריתמים

(2011-2012מבוזרים )חורף

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Identifying frames Counting bytes (A basic data unit of 8

bits). This method is used to identify a frame.

Byte stuffing – Sending special characters to identify the beginning or the end of the frame.

Bit stuffing – Sending a binary flag in the beginning and the end of the frame. The two methods are used.


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Frames

Starting and ending the frame with a special binary code. (Byte/Character stuffing)

Starting and ending with a binary flag (bit stuffing).


Character Count

5 1 2 3 4 5 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3

character count

frame 15 characters

frame 25 characters

frame 38 characters

frame 48 characters

5 1 2 3 4 7 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3

frame 2(wrong)

frame 15 characters

one-bit error

Now a character count

Even if the bit checksum can identify an error, and the destination knows that the frame is faulty, it cannot say when the next frame starts.

© תקשורת מחשבים ואלגוריתמים מבוזרים 14(2011-2012)חורף

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Byte Stuffing Also referred as Character Stuffing. ASCII characters are used as delimiters of the

frame. E.g. DLE STX and DLE ETX.

Problem: What if DLE is part of the data? Solution: The sender adds additional DLE to the

data stream before every original DLE. (Stuffing) The receiver removes the DLE before sending the

data to the communication layers above. (Destuffing)

DLE STX user data DLE ETX

beginningof frame

end offrame

DLE(0x10): Data Link EscapeSTX(0x02): Start of TeXtETX(0x03): End of TeXt

15תקשורת מחשבים ואלגוריתמים

( 2011-2012מבוזרים )חורף

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Byte Staffing

user data 0x10

DLE stuffed at the sender

DLE destuffed at the receiver


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Bit Stuffing Every frame begins and ends in a special pattern called

flag byte [01111110].

Problem: The flag pattern may appear in the data stream.

Solution: Every time the link layer finds five continuous values 1 in

the data stream ([11111]) it will automatically add the value 0 to the outgoing data stream.

When the receiver find five values of 1, it will automatically remove the next 0.

01111110 user data 01111110

beginningof frame

end offrame

flag

תקשורת מחשבים ואלגוריתמים 17( 2011-2012מבוזרים )חורף

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Bit Stuffing


Example: PPP protocol


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Dealing witherrors


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Errors

Information may become faulty during the transmission.

Example: 1 bit error.

Example: a burst of errors.


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Dealing with errors

Frames include additional data:Discover errors or validation.Additional data by the sender.Testing by the receiver.Trying to validate data by statistics (Not absolute).


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Error detectionEDC - Error Detection and Correction bits.D represent a data protected by error detection and may include a header•Error detection is not 100% proof! Why?-The protocol may miss some of the errors, but that rarely happens.-Longer EDC produces better error detection.


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Error detection 1: Parity check

Parity bit is a bit used as a check digit where its value signals if the number of bits with the value of 1. Every word is added a bit to create an even number of ‘1’. Two types of parity bits:

Even parity bit equals 0 if the number of ‘1’ values is even, and equals 1 for uneven.

Odd parity bit equals 0 when the number of ‘1’ vales is odd, and equals 1 for even.

Adding a parity bit in a bit stream allows error detection but do not allow correction.


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Parity Checking - בדיקת זוגיות The parity bit is added to each data unit, where the

numbe of ‘1’ values is even. (or odd in odd parity bit).

תקשורת מחשבים ואלגוריתמים 25(2011-2012מבוזרים )חורף

Parity Checking - בדיקת זוגיות

One dimensional parity check: Adding one bit to every byte (7 bits) complimenting to an even number of ‘1’ values.

© תקשורת מחשבים ואלגוריתמים 26(2011-2012מבוזרים )חורף

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2D parity check

1 0 1 1 0 1 1 11 1 0 1 0 1 1 10 0 1 1 1 0 1 01 1 1 1 0 0 0 01 0 0 0 1 0 1 10 1 0 1 1 1 1 10 1 1 1 1 1 1 0

VRC

LRC

A two dimensional parity check: done by a longitudinal row check )LRC( and a vertical row:


Checksum Checksum is an error detection code that allow error

detection. It is a type of redundancy check. The Checksum adds an additional part to the message which

is a result of a known function applied on the message. It is possible to apply the function on the message and

validate that the result is identical to the result added to the message.

The efficiency of the redundancy check depends on the function chosen. The most simple redundancy function is the identity function. Given a message M the output will be the message itself.

Different Checksum methods include: Parity check, Modular sum, CRC.


Checksum This function allow an efficient error detection. It do not allow

error correction because we cannot know if the error was in the message or in the checksum.

Another simple function is given a message M the output will be MM and when data is sent using the checksum there are 3 versions and in a case of an error we can compare and fix by majority.

Checksum is the redundancy of the sum of all the bytes in the data. Example: assume 4 bytes, 0x25, 0x62, 0x3F,0x52. The sum is 0x118. We remove the carrier bit and get 0x18. Calculating the Two’s complement gives 0xE8. That is the “Modular sum” checksum.


Calculating One’s complement

Calculating One’s complement:

Turn “1” to “0” and “0” to “1”0 1 1 0 1 1 0 0 1 0 0 1 0 0 1 1

Representing negative numbers in a simple way where we can subtract by adding the negative of the number.

We would like that the addition would give zero.

תקשורת מחשבים ואלגוריתמים מבוזרים ©(2011-2012)חורף

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Calculating two’s complement

Calculating Two’s complement

1. Turn “1” to “0” and “0” to “1”

2. Add 1. 0 1 1 0 1 1 0 0

1 0 0 1 0 0 1 1 + 1

1 0 0 1 0 1 0 0

Solution: We calculate one’s complement and add 1.

© תקשורת מחשבים ואלגוריתמים מבוזרים

(2011-2012)חורף 31

1חיסור בעזרת משלים ל –

סיביות.n+1 בעזרת 1 סיביות, מיוצגים ע"י משלים ל –n מספרים בעלי "גודל" של X, Y קלט:..s comp’1 והשאר התוצאה ב – X + Y חשב:

.X + Yא. חבר ביצוע:(נשא מעגלי)ב. אם יש נשא סופי חבר אותו אל התוצאה

:n=3דוגמא ע"י

X3X2X1X0

Y3Y2Y1Y0

Z3Z2Z1Z0

c0c1c2c3

C

נכונות:ע"י חלוקה

למקרים בדומה ל –

2’s comp.נשא סופי

1 0 1 1 1 1 0 10 1 0 0 2 - 4

-0100

שלילי -0010 = -2

1 0 0 0

0 0 1 11 0 1 0 5 - 3

-00111

+

0 0 1 032תקשורת מחשבים ואלגוריתמים מבוזרים

(2011-2012)חורף ©

1011

01011

: 1חיסור בעזרת המשלים ל- .10010 הוא 01101 של 1המשלים ל-

10110

1

00110

01001

01011

)יש נשא(

דוגמא לתרגיל: חיסור

000001

01011

: המשלים 1חיסור בעזרת המשלים ל- .011111 הוא 100000 של 1ל-

100111

111110

010110

של התוצאה 1אין נשא. שלילת המשלים ל- -000110היא:

1דוגמאות לחיסור בעזרת משלים ל–

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דוגמא לתרגיל: חיסור

:2חיסור בעזרת משלים ל –

)n+1( ספרות וספרת סימן n מספרים בינאריים בעלי X,Y קלט:s Complement’2 מיוצגים ע"י

X,Yקלט:

X+Yחבר:

overflowבדוק גלישה

Z=X+Y

Xn-1Xn-2…X1 X0

Yn-1Yn-2…Y1Y0

Zn-1Zn-2…Z0

נשא

נשא סופי התעלם

מנשא סופי

.. s Comp’2התוצאה הנה חיובית / שלילית בייצוג

Xn-1Xn-2…X0

Yn-1Yn-2…Y0

Xn-1Xn-2…X0

+Yn-1Yn-2…Y0


34 ©

5 = 01012 – 3 = 0 01 1 2

1 1 0 1 2 0 1 0 0

1 0 1 1

1 0 1 0

10

) ביט סימן.1 סיביות (+4 נזדקק ל - 3-5 ) 3 + (5-, חשב n=3דוגמא:

1“1”

נשא סופי "התעלם".

3 = 00112 – 5 = 0 1 0 1 2

1 0 1 1 22 0 1 0 0 -

0 1 1 1

1 1 0 11 1 0 0

11

שלילי-2

2’s Comp.

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1 0 1 0 2

+ 1

1 1 0 0 2

+ 1


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:3-5

:5-3

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Error detection 2: Modular sum calculation

Sender does the following: Data is divided to k parts. Each

part is n bits long. Add all parts and use Two’s

complement. This is the checksum. The checksum is delivered with

the data.


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Modular sum calculation

The receiver performs the following: Data unit received is divided to k

parts. Each part is n bits long. Add all parts. Add the checksum. If the result is zero the data is

accepted. Otherwise it is rejected.


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Modular sum example

Assume a 16 bit block sent and using 8 bit Modular sum checksum.

10101001 00111001 We add the numbers using the Two’s complement.

10101001 00111001------------

Sum 11100010 סיכוםOne’s complement 00011101 המשלים

1ל-Two’s complement 00011110The sent pattern: 10101001 00111001 00011110


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Modular sum example

When the reciever receive the pattern and there are no errors,10101001 00111001 00011101

When the receiver sums all the three parts he is supposed to get a series of zeros. That means there are no errors in transmission.

10101001 0011100100011110

Sum 00000000 means that the pattern is OK.


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Modular sum exampleAssuming there is a burst of 5 error bits which affect 4 bits.

10101111 11111001 00011101When the receiver sums the three parts:

1010111111111001 00011110

Partial Sum 1 11000110

the pattern is corrupted.


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Modular sum

Advantages: Easy to calculate. Small size.

Disadvantages: Do not discover all errors

Example: 2nd bit is flipped in every part.


Cyclic redundancy check (CRC) Cyclic redundancy check (CRC) is a error detection code or a hash

function used to find errors in data transfer. Before data transfer CRC is calculated and added to the transferred data.

After the data transfer the receiver verifies using CRC that the data transmitted with no errors. The CRC is common because of the ease of binary hardware implementation, the ease of mathematic computation and the efficiency in discovering common errors of noisy channels.

אופן הפעולהוקטור באורך CRC כפולינום שמקדמיו הם קואורדינטות הווקטור. nשמסתכלים על כל

מדרגה יוצר בפולינום המוגדר בפולינום קוד r.משתמש של שונים סוגים CRC משתמשים בפולינומים יוצרים שונים.

מדרגה יוצר פולינום הודעה r בהינתן את Mובהינתן לבצע עלינו לקדד, שברצוננו הפעולות הבאות:

p נוסיףr .אפסים מימין להודעה p ( 2נחלק בפולינום )תוך שימוש בחילוק של השדה מודולוp-נחסר את השארית תוך שימוש בxor .במקום בחיסור רגיל

.נצרף את התוצאה שקיבלנו מימין להודעה המקורית ונשלחכמו בכל קידוד, Checksum יבצע את שלבים ו-1הצד המקבל ויוודא ש-2 r

הביטים האחרונים שנשלחו זהים לתוצאה שהתקבלה. ©


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Cyclic Redundancy Check (CRC) A popular error detection technique. A binary data string length K represents a polynomial of

degree K. the K bits are the coefficients for the polynomial G(x) with K powers of x, xk-1 to x0. 110001 represents the polynomial G(x)=x5+x4+x0.

Possible generating polynomials: G(x) = x16 + x15 + x2 + 1 CRC-16 ( 16 סיביות בדיקה ) G(x) = x16 + x12 + x5 + 1 CRC-16 ITU ( 16 סיביות בדיקה ) G(x) = x32 +x26 +x23 +x22 +x16 +x12 +x11 +x10 +x8 +x7 +x5 +x4

+x2 +x+1 CRC-32 ( 32 סיביות בדיקה ) This allows detection of one bit error, two bit error, up to 16

bit errors. 99.997% probability of detection 17 bit errors, 99.998 probability of detecting 18 bit errors and above.


Cyclic Redundancy Check (CRC)

Every binary vector with n elements may represent a polynomial where the elements are the coefficients. CRC uses a r ranked generating polynomial. Different CRC methodes use different generating polynomials.

Given a polynomial ranked r and a message M:1. Add r zeros to the end of the message.2. Divide by the polynomial.3. Subtract the residue using XOR.

Add the result to the original message


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Sender has d data bits => D, as a binary number. Sender and receiver choose an r+1 generating

pattern => G. Choose r bits => R, such that:

<D,R> is divided by G Receiver knows G, devides <D,R> by G. If the result is not

zero: Error detected. Used in ATM, HDCL.

טבלתXOR:

CRC Algorithm

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תקשורת מחשבים 46ואלגוריתמים מבוזרים

(2011-2012)חורף

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CRC ExampleD=101110, d=6, G=1001, r=3

>=< D,R> = 101110000

Want:

D.2r XOR R = nGequivalently:

D.2r = nG XOR R

equivalently:

if we divide D.2r by G, want reminder R

R = remainder[ ]D.2r

G

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בחילוק עושים XORבין הביטים


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Multiple AccessProtocols

תקשורת מחשבים ואלגוריתמים ( 2011-2012מבוזרים )חורף

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3 types: Channel Partitioning

The channel is divided to timeslots. Allocating timeslots to an edge for an exclusive use.

Random Access Allow collision. Recovery after collision.

Taking turns Monitoring the channel tightly to avoid collisions.

Advantages: Efficiency, equality, simplicity, Distribution.

MAC (Multiple Access Control)


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TDMA: time division multiple access Access to the channel is given by a queue. Stations

get a limited time to transmit when it gets the entire bandwidth. The stations broadcast one after another.

Time slots lengths are constant. Each cycle all stations broadcast one time slot.

Unused timeslots are wasted. Example: LAN with 6 stations. Stations 1,3,4

broadcast while the timeslots of stations 2,5,6 are unused.

TDMA MAC Protocol


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FDMA: frequency division multiple access The entire bandwidth is divided to equal frequency sub channels. Between

two sub channels a safety distance is defined – unused bandwidth. Every channel get a sub channel with a constant frequency range. Unused broadcast time is lost. Example: LAN with 6 stations. Stations 1,3,4 broadcast while the frequencies of stations

2,5,6 are unused.

frequ

ency

bands

time

FDMA MAC Protocol


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CDMA(Code Division Multiple Access)

In CDMA broadcasts from all stations are divided over the time-frequency by coding. CDMA is based on Spectrum Spread, that is distributing the original data over a bigger bandwidth.

The source uses a chipping sequence to code the data over the entire bandwidth. The receivers use their own chipping sequence to decode and transmit the data to the target. Mainly used in satellite and cellular communication.

Encoded signal = Original data X Chipping sequence. Decoding = Encoded signal X Chipping sequence.


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CDMA Encode/Decode


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CDMA: two-sender interference


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Random Access protocols

When an edge has a packet to be sent The station will send the package over the entire bandwidth of

the channel. Rate R. No priority for one station over the other.

Two or more stations broadcast together => Collision.

MAC random access protocols detail: How to discover a collision How to recover from a collision (e.g. delay and retransmission).

MAC protocols ALOHA ALOHA slotted CSMA and CSMA/CD


ALOHA protocol Background – during the 70s the Hawaii university were developing a protocol for

channel access. Two main versions for the protocol: Pure Aloha and Slotted Aloha. The difference between them is in the time division. The Pure Aloha protocol do not use discrete timeslots while the Slotted Aloha uses them.

Pure Aloha – The basic idea is simple, users transmit when they have something to transmit. When a collision occurs framed are destroyed. The broadcasting station identifies collisions by listening to the channel. In case of a collision a random delay is in order, followed by retransmission. If the time is not random than a collision will happen again. Systems with shared channels in this fashion are called Contention systems.

Aloha transmission scheme:

Users transmit frames in a predefined length. When two frames are in the channel at the same time there is a collision and both frames are lost. Notice that even if only the first bit of the new frame collides with the last bit of the frame, the two frames are lost and will be retransmitted.


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Pure ALOHA Pure Aloha: Simpler and uses no synchronization. A frame is sent:

Sent with no wait to the time slot.

Collisions will grow: A frame sent at time t0 will collide other frames sent between

times [t0-1, t0+1]


Pure ALOHA Efficiency The analysis of the efficiency of a channel using this protocol. What

it the percentage of frames that will arrive to the destination with no collisions.

Frame time – the time needed to transmit a frame.S - The average frame creation frequency. The transmission rate, is

assumed to have Poison distribution. If S>1 the rate is higher then the channel transmission rate. In this case almost every frame would suffer from collisions. We must define 0<S<1 to have a reasonable transmission.

G – Broadcast attempt rate. Assumed to have Poison distribution as well. Of course, S<=G since there are more broadcast attempts than actual data transfers. When the rate is low G~S and when the rate is high S~P0G.

P0 – The probability of having a successful transmission.

We assume that the next frame will not be created until the current frame is transmitted.


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Pure ALOHA Efficiencyt – Time needed to send a frame.t0 – The transmission start time.If a user sends another frame between t0 to t0+t, it will collide with the grey frame.Also, any frame starting between t0+t to t0+2t would collide with the frame.


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Pure ALOHA Efficiency The quiet interval is 2t. The time delay needed in order to avoid a collision. The probability of creating K frames in one time frame is given by the Poison

distribution:

The probability of having no frames is:

During 2t interval the broadcast attempts are 2G and the probability of no frames is:

The rate of frame creation is:

We can see that the maximum throughput is when G=0.5, and:

The maximum throughput is very low. We can try to optimize by manipulating G but if we enlarge the broadcast frequency

too much the probability of no collisions is lower. Tradeoff!


© 61

!K

eGKP

GK

GeP 0

GeP 2]0[ GeGPGS 2]0[

%4.18184.02

1

eS

Throughput vs. offered traffic


Slotted AlohaIn 1972 the Slotted Aloha method was published, aimed to double

the throughput. The method suggested divide the time to intervals where one frame

would fit a time slot. This attitude needs a synchronization between the users. May be done by

beeping by one of the stations. Method:

Time is divided to slots which fit a frame. The edge waits with a frame and will transmit at the next slot time. If a collision is detected: Retransmit at the next time slot with the probability P. Repeat until

successful transmission.

Success )S(, Collision )C(, Empty )E( slots


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Slotted Aloha Efficiency בשיטה זו, התחנה אינה רשאית לשלוח מידע כל הזמן. התחנה נאלצת לחכות לתחילת

בלבד.t והוא כעת 2מקטע הזמן הבא. אינטרוול השקט קטן פי . - ההסתברות לחוסר התנגשות כעת היא ולכן ,כפי שניתן לראותSlotted Aloha מגיע לשיא כאשר , בתפוקה מקסימאלית

.pure Aloha של , פעמיים מהתפוקה המקסימאלית של למקטעים ריקים 37%במקרה האופטימאלי נוכל להגיע בפרוטוקול זה להסתברות של

הצלחות.37%)אי-שידור(, הסתברות של -עבודה בG גבוה יותר תקטין את מספר המקטעים הריקים, אך תגדיל את מספר

ההתנגשויות. הסתברות לכך ששידור ידרוש ניסיונות )כלומר, התנגשויות, הצלחה אחת(:

:)מספר ניסיונות ממוצע לשידור )תוחלת -מספר השידורים הממוצע תלוי בצורה מעריכית בG ולכן עלייה קטנה בעומס,תגרום ,

עלייה אקספוננציאלית במספר ניסיונות השידור ובכך תוריד את הביצועים בצורה חדה. ©

GeP ]0[GeS G

1G368.0

1

e

K1K1)1( KGG

K eeP

חוסר התנגשות פעם אחת

1kהתנגשות פעמים

1

1)1(k

GkGGk eeekkPE


Slotted Aloha Efficiency This method to not allow starting of transmission at any given time. The

station has to wait to the next interval. The quiet interval is now halved to only t.

The probability of no collision is now:Resulting:

As shown slotted aloha has its maximum efficiency when G=1 and is equal:

That is twice the throughput of Pure Aloha. Using the optimal case we may get a probability of 37% of empty slots

and 37% of successful transmission. The probability that a transmission will take K transmission attempts,

that is k-1 attempts and one success is:

The mean transmission attempts (Average): Average transmission attempts depends

exponentially on G

©

65

GeP ]0[

GeS G

368.01

e

1)1( KGGK eeP

1

1)1(k

GkGGk eeekkPE

Carrier Sense multiple Access – CSMA Slotted Aloha allows a maximal throughput of 1/e. This is very low since

there is no ability to identify if the channel is free or occupied. In many Local Area Networks the stations can identify what the other

stations are doing and change their behavior accordingly. These networks may perform much better than Slotted Aloha.

Protocols that listen to the broadcast and change behavior are called Carrier sense protocols.


Carrier Sense multiple Access – CSMAPersistent and Non-persistent CSMA 1 Persistent CSMA When a station has some data to transmit, it first listens to the channel and checks if

someone else is already transmitting. When a station recognizes that the channel is free it transmits a frame. If there is a collision the station waits a random time and starts again.

This protocol is called 1 Persistent since the station is transmitting in probability 1 in case that the channel is free.

Propagating time has a major effect on the performance of this protocol. There is a chance that another station started sending a packet, both stations check the channel. If their signals didn’t reach the other station, the other station will sense a free channel and start to transmit as well. Collision will occure.

If the propagating time is zero, there are still going to be collisions because if two stations would like to transmit at the same time as a third station they both are going to wait until the end of transmission to start their own transmission at the same time. Still, this protocol shows a significant improvement from Pure Aloha since the stations wait until the end of the third transmission.

Intuitively there is an improvement from both Aloha protocols.


© 67

CSMA p persistent CSMA

This protocol is used in slotted channels. When a station is ready to transmit it checks the channel, if it is free it will transmit in probability p, or wait until the next segment in probability q=1-p. If the station waits it will go over the protocol again and transmit in probability p or wait in probability q and so on... This happens until a frame is transmitted or until another station started to transmit.

The graph shows the throughput as a function of transmission attempts. The delays are not referred.

For a very high transmission rate queues start to build and the buffer needs to grow. Transmission that was once bursting is turning into a constant data flow, and we can switch to TDM system. There is a transmission at every timeslot and the throughput is 100% when we disregard the delay. 68 ©

CSMA/CD - CSMA with Collision Detection Another performance increase will be achieved when the stations will stop their

transmission in case of a collision detection. If two stations sense a free channel and start to transmit at the same time, they

both will detect the collision almost immediately. The stations will stop their transmission immediately in stead of finishing the transmission of the frames.

A fast transmission stop saves time and bandwidth. CSMA/CD is used on LAN networks over the MAC layer. CSMA/CD uses the following model:

At a certain time a station stopped transmitting. Any station may transmit now. If two or more stations decide to transmit together there is going to be a collision, a station may identify a collision by watching the power or the width of the transmitter pulses.

When a station identifies a collision, transmission is stopped, a random time is waited, and retransmission will be attempted. That is why the CSMA/CD model alternates between contention slots and data frames. Quiet times appear where no station is transmitting.


©

©

CSMA/CD CSMA/CD:

Collisions are detected shortly after they occur. A collided transmission is stopped, and lowers

transmission loss. Retransmission may be forced.

Collision detection: Easy with cabled LAN: Measuring signal channel,

comparing transmitted signal to the received signal. Difficult in wireless: Since the receivers may miss the

collision. In practice the network has a “jamming time” to make

sure everyone received the collision signal.


“Taking Turns” MAC protocols

©

Polling: The master is inviting the

slaves to transmit according to a queue.

Typically silent devices are used.

Issues: Overhead- wasted time Reaction time- the time

between the invitation and the beginning of the transmission.

One failure vulnerability- The master may have problems.

master

slaves

poll

data

data


“Taking Turns” MAC

©

Token passing: Controlled token pass between one

edge to another. Token message. Issues:

-Token overhead – wasted time-Reaction time- Between receiving the token and transmitting the data.-One failure vulnerability – Token issues.

T

data

(nothingto send)

T


©

MAC protocols overview

How to use shared media? Channel divided by time, frequency, or code. Random, dynamic division.

ALOHA, S-ALOHA, CSMA, CSMA/CD Carrier sensing easy to implement over wired

technology, difficult over wireless. CSMA/CD is used over Ethernet.

Taking Turns Choosing the transmitting edge by a central

processor or a token. Bluetooth, FDDI (Fiber Distributed Data Inteface),

IBM Token ring.


©

MAC protocols overviewChannel partitioning MAC protocols

High efficiency in high loads Not efficient on low loads: Access delay, 1/N bandwidth

is assigned even if only one computer is active.Random access MAC protocols

Efficient in low load: One edge can utilize the entire channel.

Not efficient on high load, multiple collisions.

Taking turns Most efficient for high or low load. Sensitive failure points.


© 75

LAN Technologies


©

LAN technologiesUntil now we covered Data Link layer issues :

Services, error detection/correction, multiple access.Next: LAN technologies

Addressing – accessing network resources. Ethernet – LAN protocol. Network devices that find the route of datapackets to

their destination: Hubs, bridges, switches. IEEE - 802.XX netork standards. Among them is 802.11. PPP – point to point protocol ATM – High throughput protocol.


© 5-79

MAC Addresses and ARP 32-bit IP address:

network-layer address used to get datagram to destination IP subnet

MAC (or LAN or physical or Ethernet) address: function: get frame from one interface to

another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software

settableתקשורת מחשבים ואלגוריתמים מבוזרים

(2011-2012)חורף

© 5-80

LAN Addresses and ARPEach adapter on LAN has unique LAN address

Broadcast address= FF-FF-FF-FF-FF-FF

=adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or

wireless)


© 5-81

LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space

(to assure uniqueness) analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address ➜ portability

can move LAN card from one LAN to another IP hierarchical address NOT portable

address depends on IP subnet to which node is attached


© 5-82

ARP: Address Resolution Protocol

Each IP node (host, router) on LAN has

ARP tableARP table: IP/MAC

address mappings for some LAN nodes

<IP address; MAC address; TTL>

TTL (Time To Live): time after which

address mapping will be forgotten (typically

20 min)

Question: how to determineMAC address of B

knowing B’s IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137.196.7.23

137.196.7.78

137.196.7.14

137.196.7.88


© 5-83

ARP protocol: Same LAN (network)

A wants to send datagram to B, and B’s MAC address

not in A’s ARP table.A broadcasts ARP query

packet, containing B's IP address

dest MAC address = FF-FF-FF-FF-FF-FF

all machines on LAN receive ARP query

B receives ARP packet, replies to A with its (B's)

MAC addressframe sent to A’s MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information

becomes old (times out) soft state: information

that times out (goes away) unless refreshed

ARP is “plug-and-play:”nodes create their ARP

tables without intervention from net

administrator


© 5-84

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when joining network

support for mobile users joining networkhost holds address only while connected and “on” (allowing

address reuse)renew address already in use

DHCP overview:1 .host broadcasts “DHCP discover” msg2 .DHCP server responds with “DHCP offer” msg3 .host requests IP address: “DHCP request” msg4 .DHCP server sends address: “DHCP ack” msg


© 5-85

DHCP client-server scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

DHCP server

arriving DHCP client needs

address in this(223.1.2/24 )network


© 5-86

DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67

yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67


DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68



© 5-87

Addressing: routing to another LAN

R

1A-23-F9-CD-06-9B

222.222.222.220111.111.111.110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

A74-29-9C-E8-FF-55

222.222.222.221

88-B2-2F-54-1A-0F

B222.222.222.222

49-BD-D2-C7-56-2A

walkthrough: send datagram from A to B via R assume A knows B’s IP address

two ARP tables in router R, one for each IP network (LAN)


© 5-88

A creates IP datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110A creates link-layer frame with R's MAC address as dest,

frame contains A-to-B IP datagramA’s NIC sends frame R’s NIC receives frame R removes IP datagram from Ethernet frame, sees its

destined to BR uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

A74-29-9C-E8-FF-55

222.222.222.221

88-B2-2F-54-1A-0F

B222.222.222.222

49-BD-D2-C7-56-2A

This is a really importantexample – make sure you

understand!


©

LAN is a dominant technology: Inexpensive – about 20$ for a 100Mbps device. Common over the LAN. Less expensive and simpler than token technologies and ATM. Bitrates: 10, 100, 1000 Mbps

Metcalfe’s Etheret sketch

Ethernet


©

The framing includes the following: Framing, the ability to identify the beginning and the end

of a message. Addressing: Fields that include source and destination

addresses. Error detection: Redundancy codes designed to find

transmission errors.

Preamble - Sets of bits101010 used to declare the frame existance.

Ethernet frames


©

Ethernet frames

Addressing: 6 Bytes, accepted by all the LAN devices, unused if the address do not match.

Type: Defines the data type that is going to be on the data field. Allows us to work with several higher protocols, IP, DECNET, or IPX.

Data: The size of the data field is between 46 to 1500.

CRC: The receiver check for errors. If there are errors the frame is not used.


Ethernet implementation The Ethernet is based on CSMA/CD - Carrier Sense Multiple

Access/Collision Detection. Network access in 4 steps:

1. Sensing – The station checks if there is a carrier, that is if someone is transmitting.

2. The station waits 9.6 microseconds, and if there is no broadcast at the time the station is transmitting but keeps monitoring.

3. The frame transmitted is distributed all over the network. It is possible that another station have been waiting and starts to transmit. In this case both stations detect collision and stops the transmission.

4. Both stations would try to transmit again later. The probability that they would try to transmit at the same time again is lowered by using a special algorithm, BACK OFF algorithm, implemented over the NIC.


©

CSMA/CD Algorithm

A: sense channel, if idle then {

transmit and monitor the channel; If detect another transmission then { abort and send jam signal;

update # collisions; delay as required by exponential backoff algorithm; goto A}

else {done with the frame; set collisions to zero}}

else {wait until ongoing transmission is over and goto A}


©

Ethernet’s CSMA/CDJam Signal: Make sure that all others are aware of the collision.

48 bitsExponential Backoff: Make sure that the transmission is getting adjusted

to the current load. Heavy load: longer wait.

First collision: choose K out of {0,1}; the delay is K X send time of 512 bits. (on a 10 Mbps it is 51.2 microseconds)

N-th collision choose K out of {0,1,…, 2n-1} . After 10 collisions or more choose K out of

{0,1,2,3,4,…,1023}. תקשורת מחשבים ואלגוריתמים מבוזרים 95(2011-2012)חורף

©

Ethernet: 10Base2 10:10Mbps 2: Maximum cable length 200m. Thin coaxial cable in bus technology.

Amplifiers are used to connect many segments.


Ethernet 10BaseT and 100BaseT Frequency of 10/100 Mbps is called fast Ethernet. T: Twisted Pair. Uses Hub to connect computers with twisted pairs, that’s why

this technology is called Star topology. CSMA/CD is applied by the hub.

Maximum distance between the computer and the hub is 100m. Hub may disconnect noisy adapters. Hub may statistically monitor the throughput for the Admin.

twisted pair

hub

©

Ethernet 1Gbit Uses the standard Ethernet frame. Allows point to point and broadcast to all the LAN. Uses CSMA/CD with short distances for high

efficiency. Buffered Distributors in stead of hubs. Full Duplex needed for 1Gbps point to point.

Half duplex – data can flow only in one direction at a certain time.

Full duplex – data may flow both directions at the same time.


Token Ring Since CSMA is based on contention, it may not be efficient. Token

Ring systems use a different approach. Network access is given to the station that holds the token. Than

the token is passed to other stations until reaching a station waiting to submit a message.

Two topologies: Token passing rings and token passing busses. Token passing ring the ring topology defines the logical topology- the order of message passing. The token passing bus is more flexible, the order of message passing is determined by tables stored at each station. If there is a station that do not initiate messages (such as a printer), it may only end the cycle, and will not be included in the tables. If one station needs high priority it may appear several times in the tables.


Token passing rings and token passing busses

Token passing busses: Stations are physically connected on a bus but organized as a ring by the software.

Token passing rings: The ring topology defines the logical topology, the order of message passing.


©

LAN extension

Motivation: Distance limitation. Lower performance with more users.

Devices: Hub Repeater Bridge Switch


©

Hub - מרכזיה

HUB Hardware Creates one LAN segment

The Hub is a central point to the entire network. A differen cable connect each computer to the Hub.


Each Ethernet frame is sent from one port to all the other ports. Logically distributes the signals. Every signal goes to all the ports.

Single collision segment

Repeaters מגבר משחזר -

Repeater – connects two segments of the same network and allows extension of the distance between the two ends of the network. The repeater is acting when a transmission is received in one of its ends, cleans the signals, amplifies the signal and transmits it on the other side. A repeater do not confine the broadcast segments or collision segments.

Each communication media uses its own repeater. different devices exist for different media, for example, fiber optics and wireless.

It is possible to extend the network with a repeater and renew weak signals. Hardware. Connects two LAN segments. Copies a signal from one segment to another.

Amplifies the signal. Adds noise and collisions.

Full duplex - Works on both ways at the same time.

R

RepeaterDirect connection

Ethernet segmentEthernet segment


Bridges Bridge connects two LAN networks Bridge, as opposed to a HUB, differentiate the collision segments of each sub

network. It learns the network and knows how to route packets to the right port according to the MAC address and a bridging table.

The bridge learns the topology dynamically and the bridge is transparent to all the clients. When a packet is received the bridge keeps the source address and the port received if not already exists and the current time.

If the destination address does not exist in the table, the bridge will send a copy of the packet to all other ports according to CSMA/CD. If the address exists, packets will be directed to the port.

The bridge will delete records when their aging time is high over a predefined time.


©

©

Bridges Link layer hardware. Connects two LAN segments. Frame delivery

Stores and sends Ethernet frames. Examines the frame header and selectively sends the frame according

to the destination’s MAC. When the frame is sent to the LAN segment, CSMA/CD is used in order

to access the segment. Do not extend noise or collisions. Learns the addresses and filters. Transparent

Computers are not aware of the bridge existance. Plug-and-play, Self-learning.

Bridge does not need to be configured.


©

Bridges differentiate the load

Bridge breaks the local network to smaller segments.

The bridge filters packets: Frames of the same segments are not sent to other

segments. The small LAN segments are different collision

segments.

bridge collision domain

collision domain

= hub

= host

LAN segment LAN segment תקשורת מחשבים ואלגוריתמים מבוזרים 106(2011-2012)חורף

©

Bridge learning algorithm

Listens in mixed mode, all frames are copied and analyzed.

Looks at the source addresses of incoming frames.

Prepares a list of the computers on each segment.

Retransmits only if needed. Always retransmits on

broadcast/multicast.


©

Bridge learning algorithm

A bridge has a bridge table. Writing to the bridge table:

LAN addresses of the edges, the ports and the current time.

Old records are deleted (60 min. lifetime). The bridge learns which computers could

be accessed through each port. When a frame is received, the bridge learns

the sender’s position, LAN segment. A record of the tuple sender-position on the

bridge table.תקשורת מחשבים ואלגוריתמים מבוזרים 108

(2011-2012)חורף

©

Bridge exampleAssume that C sends a frame to D and D returns another frame to C.

The bridge receive a frame from C.C do not appear on the bridging table.Since D do not appear on the bridging table the bridge sends a frame to ports 2 and 3.

Frame is received by D 109תקשורת מחשבים ואלגוריתמים מבוזרים

(2011-2012)חורף

©

Bridge example D creates a frame for C and sends it. The bridge receives the frame.

Notice that now D is on port 2 in the bridging table. The bridge knows that C is on port 1 and selectively

transmits the frame on port 1.


©

אלגוריתם של עץ פורשSpanning Tree Algorithm A bridges network is a graph.

Usually the graph is hierarchal. If the bridge is at the end of the hierarchy it fails – LAN could be

disconnected. If the same data is duplicated too many times over the LAN – May

even cause a network failure. Spanning tree may find a sub graph that spans

all the edges with no loop. Spanning – All LAN segments are included. Tree – one topology without loops.

Distributed protocol: Determines which bridge is the root. Every bridge directs ports that are not part of the tree

to other ports.


©

Spanning Tree example

B3

B5

B7B2

B1

B6 B4

The protocol:1. Choose a root.2. For each LAN choose

the closest bridge to the root.

3. All LAN bridges send packets towards the bridge closest to the root.

B8


©

Switch High performance, doubles the bridge

interfaces. Physically similar to HUB. Logically similar to bridge.

Works on the packets. Understands addresses. Forwards a message only when needed. Enhanced forwarding algorithms

Allows full duplex.


computer communication and distributed algorithms 202-2-1131 data link layer © תקשורת...

Documents

data link service

data link layermotivation

control data

data transmission

data stream

basic data unit

link interface

byte staffinguser data