Data Link Layer

Chong-kwon Kim

SNU SCONE lab.

Data Link Layer

Data link layer is concerned with direct link networks Two nodes directly connected by a single point-to-

point link

A broadcast link segment with multiple attachments

Different names at different layersFrame: L2Packet, Datagram: L3Segment: L4Message: L7

SNU SCONE lab.

Node Configuration Network adapter connects to I/O bus & network

link– Device driver manages a network adapter– Manage message passes between main memory and

network adapter buffer

CPU

Memory

CacheNetworkAdapter

I/O bus

(To network)

Example OpenWRT: An embedded kernel for WLAN APath9K: Device driver for Atheros WiFi chipsets

SNU SCONE lab.

Data Link Layer

Functions– Framing– Error control– Flow control– Medium access control (MAC) – LAN

Standard protocols– HDLC (High-level Data Link Control) - ISO– LAPB (Link Access Protocol Balanced) - ITU– LLC (Logical Link Control) - IEEE

• Type 1, 2, 3

Error Control

Chong-kwon Kim

SNU SCONE lab. 6

Error Control

First, detect error occurrences and then ensure correct transmission

Error types & detection methods– Bit error

• Error detection code such as CRC

– Lost or duplicated frames• Use sequence numbers (SN) that uniquely identify frames

Principle– A sender retransmits a frame repeatedly until it assures that

correct one is transferred

Two methods– Positive Acknowledgement (ACK) + Automatic retransmission

based on Timeout (TO)– Retransmission based on Negative Acknowledgement (NAK)

SNU SCONE lab. 7

ACK + Automatic Retransmission Basic action

Automatic Retransmission based on TO

What happens if packet / ACKis erred / lost ?

ACK

Sender

Receiver

1 2

Sender

Receiver

TO11

ｘ

SNU SCONE lab. 8

ACK + Automatic Retransmission

Sender

Receiver

TO11

The ACK is lost and the receiver receives the same frameWhat would you do?1. Do nothing The sender will retransmit the same frame infinitely

2. Resend the ACK

ｘ

9

ACK + Automatic Retransmission

Sender

Receiver

TO11 2

The frame #2 is lost, but two ACKs are received Should distinguish ACKs

Use Request Number (RN)RN specifies the next packet expecting instead of the last correctly received packet

Sender

Receiver

TO11 2

22

ｘ

ｘ

SNU SCONE lab. 10

NAK Based Method

When to send NAKs?– Receive corrupted frames

• How do you know that a corrupted frame is the frame?– Receive out of order frames

• Error control is triggered by following frames• What if there is a long gap between frames?• What if there is no next frame?• Lost NAK

NAK is used only as a supplementary mechanism

Sender

Receiver

21 1

1

ｘ

SNU SCONE lab. 11

ARQ (Automatic Repeat reQuest) Stop-and-wait ARQ Continuous ARQ

– Go-back-N ARQ– Selective-repeat ARQ

Stop & Wait ARQ

Chong-Kwon Kim

13

Stop & Wait ARQ - 1Stop and Wait

Send a single packet(frame) and stop & wait for an Ack.

Stop & Wait ARQ - 2

Sender Algorithm

Step 0: Set SN = 0Step 1: Transmit a frame, Set the timer,

Wait for ACK with right RNStep 2: One of two events occurs

A: If receive right ACK before TO, Set SN (SN+1) mod 2, Goto Step 1

B: If TO, Goto Step 1 to retransmit

Receiver Algorithm

Step 0: Set RN = 0Step 1: Wait for a frameStep 2: Packet arrives

A: If errorless & SN = RN, Accept the frame, RN (RN+1) mod 2, Send ACK

B: Discard the frame, Retransmit ACK

Stop & Wait ARQ - Example

State– SN represents the sender’s state– RN represents the receiver’s state– Combined state = (SN, RN)

State transitions– (0, 0) -> (0, 1) -> (1, 1) -> (1, 0) -> (0, 0)– (0, 0) -> (0, 1), (1, 1) -> (1, 0) : Receiver knows the combined state– (0, 1) -> (1, 1), (1, 0) -> (0, 0): Sender knows the combined state– Sender & Receiver do not knows the combined state simultaneously

SN

RN

0 0 1 0

0 1 0 1

TO0 1 0 1

SNU SCONE lab.

Three Army Problem

Two Generals’ Problem (Byzantine Generals Problem )

17

– Propagation time : time taken for signal to travel from S to R. Thus first bit transmitted at t=0 arrives at R at t=Tp=d/V

– Transmission time : time taken to emit all bits of frame at sender=Tt=L/B

Utilization (Efficiency) of Stop & Wait

SNU SCONE lab. 18

19

– With stop & wait scheme, for high channel utilization, we need a low α (since U=1/(1+2 α ))

– In practice, it is not desirable to increase length L indefinitely• Error probability increases with L• Starvation/high avg. delay with multipoint lines• Realtime applications – short packet assembly time

– A more efficient scheme is called for, especially with HSN/WAN/satellite communication

Derive Utilizationusing the Little’s Theorem

Go-Back-N ARQ

Chong-Kwon Kim

SNU SCONE lab.

Go-Back-N Send multiple packets without receiving ACKs

– N: Window size• Maximum number of unacked packets that can be sent

– Stop and Wait is a special case of Go-back-N– Window: Packets that have been sent (but unacked) +

that can be sent• Window size = N• MinSN: First packet that are not ACKed yet• MaxSN: Highest packet that can be sent

MaxSN = MinSN+N-1• SN: Next packet to be sent

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Example: N=8, MinSN = 3

Sent but not Acked yet

Go-Back-N Algorithm Sender algorithm

Receiver algorithm

Step 0: Set MinSN = SN = 0, MaxSN = MinSN+N-1,Send a frame, Set the timer

Step 1: Do one of three events foreverA: If SN < MaxSN, Set SN SN+1,

Transmit the next frame, (Set the timer)B: If ACK with RN > MinSN arrives, Set MinSN RN,

MaxSN MinSN+N-1C: If TO, Retransmit (Which frames ?)

Step 0: Set RN = 0Step 1: Receive frames forever

A: If the frame is error-free and RN = SN, Accept the frame, RN RN+1, Transmit ACK

B: O.W., Retransmit ACK

SNU SCONE lab.

Go-Back-N Example (N=4 case)

Properties of Go-Back-N– Properly designed, Go-Back-N achieves 100% utilization

– Right size of N ?– In case of a single packet loss/error

• Retransmit packets [MinSN, SN]– How about an ACK loss/error ?– How about out-of-sequence packets?

0 1 2 3 4 1 2 3 4

RN 0 1 2 3

SN[0, 3] [1, 4] [2, 5]

TO

SNU SCONE lab.

ACK Semantics Cumulative ACK

– ACK (RN) acknowledges packets up to (RN-1) Lost ACKs may be recovered

0 1 2 3 4 5 2 3 5

RN 0 1 2 3 4 5

SN[0, 3] [2, 5] [4, 7] [5, 8]

TO

Compare to Point ACK

SNU SCONE lab. 25

– Utilization : U is a function of a and N• Case 1 : N > 1 + 2a

– Frame 1 ACK reaches to the sender before transmission of Nth

frame continuous transmission possible

Link Utilization of Go-Back-N

SNU SCONE lab. 26

– Case 2 : N < 1 + 2a : U = N / (1+2a)• Wasted time between N and 1 + 2a

YA ARQ: Selective Repeat

Survey the Internet and learn Selective Repeat ARQ

SNU SCONE lab. 27

Medium Access ControlBasic & ALOHA

Chong-kwon Kim

SNU SCONE lab. 29

Taxonomy of Medium Access Methods

MAC protocols can be considered as distributed ATDM

ContentionBased

SNU SCONE lab. 30

Contention Based Method Every station is a peer

– No master-slave relation

Fully distributed, non-coordination method– Judge & behave based on locally available information

Stations use care not to interfere other– But should be aggressive to earn the fair share

Collisions still can occur

Aloha

CSMA/CA

CSMA/CD

CSMA

SNU SCONE lab. 31

ALOHA - 1 Proposed by Abramson in 1970 Implemented and used at the Univ. of Hawaii U. Hawaii campuses

SNU SCONE lab. 32

ALOHA - 2 Transmit a frame and check if the frame is transmitted correctly

– ACK– Retransmit in case of no ACK

Example

SNU SCONE lab. 33

ALOHA – Performance Analysis

A is successful if– No attempt in (t-1, t) &&

(t, t+1)

n stations each with p attempt rate A frame sent at time t is successful if only one attempt in (t, t+1)

and no attempt in (t-1, t)

Binomial distribution, Bin(x; n, p) = nCx ∙𝑝 ∙ 1 𝑝

Total attempt rate = n∙p

P(success) = b(1; n, p)∙b(0; n, p) = nC1 𝑝 1 𝑝 · nC0 1 𝑝

= 𝒏 · 𝒑 · 1 𝑝 𝟐 𝟏

Tfr = 1(unit time)

ALOHA – Performance Analysis Note b(x; n, p) ≈ Po(x; G) where G = n∙p if n is large and p is small

SNU SCONE lab. 34

The throughput for pure ALOHA is S = G × e −2G .The maximum throughputSmax = 0.184 when G = (1/2)

What happens if G=n∙p is too small OR too large?

SNU SCONE lab. 35

ALOHA - 3 Procedure

Slotted ALOHA & CSMA

Chong-kwon Kim

Problem of ALOHA Problem with Pure (Unslotted) Aloha

– An attempt may be interfered by later attempts– Vulnerable time is long

SNU SCONE lab. 37

SNU SCONE lab. 38

Slotted Aloha - 1 Solution

– Synchronize the transmissions – How to synchronize stations?

(know the beginning of slots)

SNU SCONE lab. 39

Slotted Aloha - 2 A station that has a packet to send, waits until the next

slot

A slot is either Idle, Success or Collision– Idle: No arrival during the previous slot time– Success: Exactly one arrival– Collision: More than one arrival

Time

Success Idle Idle Collision

SNU SCONE lab. 40

Slotted Aloha – Performance Analysis A slot is successful if

– There is exactly one attempt during the previous slot

Pr(1 arrival in a unit time) = G×e-G

The throughput for slotted ALOHA is S = G × e−G .

The maximum throughput Smax = 0.368 when G = 1

Slotted Aloha - Problem

Slotted Aloha is unrealistic– Synchronization is a big problem– One solution is to have a control node and the node transmits

beacons that announce the beginning of slots– Or adopt NTP (Network Time Protocol)

A centralized controller brings out other problems

SNU SCONE lab. 41

SNU SCONE lab. 42

CSMA (Carrier Sense Multiple Access) - 1

Two problems of slotted Aloha– Long Idle slot– Long Collision slot

How to shorten the Idle slot? Pure Aloha blindly accesses the medium even though there are on-

going transmissions On the other hand, slotted Aloha blindly postpones transmission

even though the medium is idle Check the medium and transmit frames immediately if the medium is idle, not waiting until the next slot Carrier sensing

TimeSuccess Idle Idle Collision

SNU SCONE lab. 43

CSMA - 2 How to determine if the medium is idle or not?

– Measuring the signal strength, voltage level

CSMA– Listen-before-talk

TimeSuccess Idle Idle Collision

Time

CCA: Clear Channel AssessmentRSSI: Received Signal Strength Ind.

Performance

SNU SCONE lab. 44

P-Persistent CSMA

CSMA/CD

Chong-kwon Kim

46

CSMA Collision CSMA still suffers from

collisions

SNU SCONE lab. 47

CSMA/CD (Collision Detection) Recall the second problem of Slotted Aloha

– Long Collision Slot

How to shorten the length of collision slots? Method

– While transmitting a frame, listen if there is a collision– If collision, terminate the transmission immediately

How to detect collisions?– In the case of a single transmission, the voltage level confined within

a certain level– Two or more concurrent transmissions cause higher than the normal

voltage level

How about Wireless link?

SNU SCONE lab. 48

CSMA/CD – CD Duration How long should we listen for collision?

– Two times of the maximum propagation delay

SNU SCONE lab. 49

CSMA/CD Procedure

Failed transmission

Further (personal) investigations Various persistence methods

– Non/1/p-persistent

The speed of Ethernet increased from 10 Mbps 100 Mbps 1 Gbps 10 Gbps 100 Gbps. Problem of high speed Ethrrnets. Solutions.

Modern layer 2 switches no longer use CSMA/CD. They use switch fabrics that enable multi-point to multi-point dedicated full-duplex connections.

SNU SCONE lab. 50

Wireless LAN

Chong-kwon Kim

SNU SCONE lab. 52

Wireless Characteristics Performance of wireless links is

much lower than that of wired links

Root cause:

Attenuation of wireless medium is much higher than that of wired medium

WHY?

Attenuation

SNU SCONE lab. 53

IEEE 802.11 WLAN Standard IEEE 802.11 a/b/g

.11g OFDM

6~54 Mbps

2.4 GHz

DS

1 & 2 Mbps1 & 2 Mbps

.11a OFDM

.11b CCK

5 GHz

5.5 & 11 Mbps

6~54 Mbps

802.11(1997)

1999 (up to 11 channels @ 20MHz)

FH IR

2.4 GHz

2003(Both 2.4 GHz & 5 GHz)

PMD

PLCP

MAC

LLC

TCPIP

SNU SCONE lab. 54

802.11 PHY - 1 802.11 - 1997

– Three physical layer specifications operating at 1 and 2 Mbps

Two additional parts in 1999 - 802.11a and 802.11b

IEEE 802.11b– 2.4-GHz band at 5.5 and 11 Mbps– Complementary code keying (CCK) modulation– Input data treated in blocks of 8 bits at 1.375 MHz

• 8 bits/symbol 1.375 MHz = 11 Mbps

802.11 PHY - 2 802.11a

– 5-GHz band up to 54 Mbps– Uses orthogonal frequency division multiplexing (OFDM)– Multiple carrier signals at different frequencies– Data rates 6, 9, 12, 18, 24, 36, 48, and 54 Mbps

IEEE 802.11g (2002)– Extends IEEE 802.11b to

higher data rates– Combines 802.11a and b

IEEE 802.11n (2006)– 100 Mbps

55

SNU SCONE lab. 56

Transmission RatesAMC: Adaptive Modulation & Coding

Learn: Performance Anomaly Problem

SNU SCONE lab. 57

Topology Infrastructure Mode

– All packets pass through the Access Point (AP)– No direct communications between Mobile Stations (MS, or

client/host)

Ad-hoc mode– Peer-to-peer (station-to-station) direct communications

AP APAP

SNU SCONE lab. 58

IEEE 802.11 Architecture

SNU SCONE lab. 59

IEEE 802.11 – BSS / ESS Smallest building block is Basic Service Set (BSS)

– One AP and multiple stations– Same MAC protocol– Competing for access to same shared wireless medium– BSSID (= MAC addr. of the AP)

ESS (Extended Service Set)– Two or more BSSes interconnected by DS– Appears as a single logical LAN to LLC

AP– Logic within station that provides access to DS– Provides DS services in addition to acting as station

Portal– Integrate IEEE 802.11 architecture with wired LAN

Wireless LAN- Signal & Interference

Chong-kwon Kim

Data

A

SNU SCONE lab. 61

Basic Operation In CSMA/CD, we can be almost sure that a frame is

delivered successfully in case of no collision No longer true in wireless networks

– Collision Detection is impossible

1. How do you know a frame is successfully transmitted? Use ACK

2. No ACK Retransmission ACK should be transmitted before any other frames

How about CSIn wireless links?

SNU SCONE lab. 62

Wireless Characteristics - CS Carrier sense

– Detect EM signal strength

In wired networks, Sender’s CS ≈ Receiver’s CS

In wireless networks, CS at sender ≠ CS at receiver

How do you know the receiver’s channel status?

O R

O R T

T

SNU SCONE lab. 63

Hidden / Exposed Terminal Hidden Terminal

Problem– C cannot sense BA

transmission and attempts to send a frame

Exposed Terminal Problem– C can send to D without

disturbing AB transmission

– But, C thinks the medium is busy

SNU SCONE lab. 64

RTS/CTS & Virtual CS Request To Send (RTS) & Clear To Send (CTS)

– Short control frames that specifies the intention to transmit (RTS) and willingness to receive (CTS)

Specifies the remaining time of the transaction in RTS/CTS frames Network Allocation Vector(NAV)

NAV

NAV

SNU SCONE lab. 65

RTS/CTS & Hidden Terminal RTS/CTS can mitigate the hidden terminal problem

Suppose you hear an RTS but not a CTS, what will you do?

Suppose you hear a CTS without RTS, what will you do?

SIFS

How long should you wait beforesending your frame?

SNU SCONE lab. 66

RTS/CTS & Exposed Terminal

RTS/CTS exchange cannot solve the exposed terminal problem

Wireless LAN- Protocol

Chong-kwon Kim

SNU SCONE lab. 68

Inter- Frame Space (IFS) An ACK should be immediately delivered

IFS: Minimum time that the medium should be idle to transmit a frame

SIFS (short IFS): – For all immediate response actions

• RTS-CTS, DATA-ACK, Poll response

PIFS (point coordination function IFS): – Used by the centralized controller in PCF scheme when issuing polls

DIFS (distributed coordination function IFS):– Used as minimum delay for asynchronous frames contending for access

If SIFS < DIFS, then data frames cannot intervene DATA-ACK transaction

SNU SCONE lab. 69

Media Access Control Distributed wireless foundation MAC (DWFMAC)

– Distributed access control mechanism

Lower sublayer is distributed coordination function (DCF)– Contention algorithm to provide access to all traffic

Point coordination function (PCF)– Centralized MAC algorithm– Contention free

DCF Basics

The DATA frame should wait at least DIFS to protect ACK, CTS and etc

A frame that arrives while the medium is busy should wait extra in addition to DIFS– The extra waiting time should

be randomized

In the case of collision, should reduce attempt rate

SNU SCONE lab. 70

DCF Basic

SNU SCONE lab. 71

DIFS

New packetMedium = Idle

Idle for DIFS

DATA

DIFS

New packetMedium = Busy

Idle for DIFS

DATADATA

Slot, Δ

Random Backoff

After DIFS, decrease backoffcounter by one if the medium isidle for ΔIf medium is busy, freeze countdown

Backoff counter = Rand[0, CW]

Backoff counter = 0

Collision?

SNU SCONE lab. 72

BEB (Binary Exp. Backoff) Collision = Overloaded Contention Window (CW) Backoff delay = Random (0 .. CW) At the beginning, CW = CWmin

For each unsuccessful transmission, double CW up to CWmax

BC=1

BC=2

BC=5

Example

SNU SCONE lab. 73

Data

A

Data

A DataSTA A

STA B

STA C

STA D

SIFS

DIFSDIFS DIFS

SNU SCONE lab. 74

DCF – CSMA/CA

BC=0 & !TxPend

TxSuccess TxFail

Access Control

Back-off

Access Control Tx Idle

Access Control

Tx & Wait Ack

TxPend & CCA>DIFS

CCA>DIFS & BC=0 & TxPend

TxPend & CCA.busy

BC=BC-1

Slot & CCA>DIFS

GetBC

GetBC

CW=CWmin DoubleCW

Wait DIFS

CCA>DIFS

!(CCA>DIFS)

CA (Collision Avoidance)RTS/CTS exchange is optional

Frame Format PLCP Header - DSSS

Sync 128 (56)

SFD 16

Length16

Service8

Signal8

CRC16

MPDU

192 us1 Mbps 1/2/5.5/11 MbpsRate Indication

No. of Bytes

Preamble (srt/long) PLCP header

MAC Frame Fields (1)

Frame Control:– Frame type/sub-type

• Control, management, or data– To DS/From DS– More fragment– Retry– Power Mgmt– More data– WEP– Order

Duration/Connection ID:– Indicates time (in s) for successful transmission of MAC frame– May contain AID (Association ID)

MAC Frame Fields (2) Sequence Control:

– 4-bit fragment number subfield• For fragmentation and reassembly

– 12-bit sequence number – Number frames between given transmitter and receiver

Frame Body:– MSDU (or a fragment of)

• LLC PDU or MAC control information

Frame Check Sequence:– 32-bit CRC(Cyclic Redundancy Check)

Address FieldsTo DS From DS Address 1 Address 2 Address 3 Address 4

0 0 DA SA BSSID -

0 1 DA BSSID SA -

1 0 BSSID SA DA -

1 1 RA TA DA SA

Address 1: All stations filter on this address Address 2: Transmitter address (TA)

- Identify transmitter to address the ACK frame to Address 3: Dependent on To and From DS bits Address 4 : Only needed to identify the original source ofWDS(Wireless DS) frames

Wireless LAN- Other Features

Chong-kwon Kim

AP Discovery To use AP

1. Discovery 2. Authentication3. Association

Scan– Discover APs in transmission range

Active scan, Passive scan Passive scan

– Client listen “Beacon” frames that AP transmits periodically

Active scan– Client probe the existence of APs by sending “Probe request”

frame– AP replies with “Probe response” frame– “Probe response” frame is similar to Beacon frameSNU SCONE lab. 80

Authentication

Open system and Shared key system

SNU SCONE lab. 81

Authentication Auth. Req

Challenge

Answer

WEP Key

Association

SNU SCONE lab. 82

AssociationResponse

State 1UnauthenticatedUnassociated

Class 1Frames

State 2AuthenticatedUnassociated

Class 1 & 2Frames

State 3AuthenticatedAssociated

Class 1,2 &3Frames

DeauthenticationNotification

DeassociationNotification

Authentication

Association

AssociationRequest

Allocate Association ID

Power Saving Energy is very scarce resource in mobile devices Network interfaces consume sizable amount of energy

SNU SCONE lab. 83

Turn off network interfaces when not in use PSM (Power Saving Mode)

– Sleep state– Listen

To sleep– Signals to the AP not to send frames

How to resume communication (Wakeup)?– Transmission (Station AP)– Reception (AP Station)

How to Conserve Energy?

Packet Reception - PSM

Beacon(TIM) Interval DTIM Interval

DTIMDTIM

AP transmits beacons every beacon interval (~100msec)- Defer if channel is busy- PS nodes wake up for beacon reception

TIM(Traffic Indication Map)- Indicate the existence of backlogged unicast traffic to PS nodes- PS node may request packet transmission

Broadcast packets are announced by a Delivery TIM(DTIM) and are sent immediately afterwards

- DTIM interval is a multiple of TIM interval

AP

Stationwakeups

Broadcast

Delivery in PS Mode If TIM indicates frames buffered

– STA sends PS-Poll to AP and stays awake to receive data

Learn– U-APSD (Unscheduled Automatic Power Saving Delivery)

SNU SCONE lab. 85

PS-Poll

DATAAP

Station

802.11e QoS Priority and Parameterized QoS

Priority scheme– EDCA (Enhanced DCF Channel Access)– Four access classes– Different CW and AIFS

Parameterized QoS scheme– HCCA(Hybrid Coordination Function Channel Access)– PCF based resource allocation

SNU SCONE lab. 86

Access Classes

SNU SCONE lab. 87

AC Description CW Min. AIFS

0 Background 15 7

1 Best Effort 15 3

2 Video 7 2

3 Voice 3 2

2 0~3 slots

2 0~7 slots

3 0~15 slots

7 0~15 slots

Random BCAIFSSIFS

AC3

AC2

AC1

AC0