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The Medium Access ControlSublayer

Chapter 4

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MAC Layer

LAN

WAN

router

Manch

MAC PPP

QAM

IP

Manch

MAC

IP QAM

PPP

IP

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The Channel Allocation ProblemStatic Allocation Dynamic Allocation

Tc = Ts/(1 – )

Tn = (Ts/N)/(1 – )

Tn

Ts/N

N

Tc

Ts

Example T1 carrier: N = 24, 6400 bits/frame. Ts = 6400/64000 = 0.1 s. For = 0.8 Tc = 0.5 s, Tn = 0.5/24 s for the same traffic.

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Dynamic Channel Allocation in LANs

Station Model: N independent stations sending messages according to Poisson distribution.

Single Channel: A single channel is available to all stations to transmit to and receive from.

Carrier Sense: Stations sense if the channel is in use and wait vs. stations cannot tell if the channel is in use.

Collision Assumption: If stations transmit at the same time, frames will collide and garbled. All stations can detect collision and retransmit frame later.

Frame beginning: Pure Aloha (frame can start transmission at any time) vs. slotted Aloha (frame can start transmission only at a given time instance).

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Multiple Access Protocols

• ALOHA• Carrier Sense Multiple Access Protocols• Collision-Free Protocols• Wavelength Division Multiple Access Protocols

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Pure ALOHA (1970)

In pure ALOHA, frames are transmitted at completely arbitrary times.

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Collision conditions in pure Aloha

period is 2t

If another frame startshere we will have collision

• All frames are of the same size.

• Frame transmission can start at any time instant.

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Throughput versus offered traffic for ALOHA systems.

Max throughputs: 18% at G = 0.5 for pure 37% at G = 1 for slotted.

(thr

ough

put

per

pack

et t

ime)

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(1-p) carr

->random back off

CSMA: Persistent and Non-persistent

no carr

-> transmit

carr

-> check afterrandom time

nonpersistent

no carr

-> transmitwith prob p.

p-persistent is slotted

carr

->check next slot

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CSMA with Collision Detection

CSMA/CD can be in one of three states: contention, transmission, or idle. Minimum contention slot is 2 where ( 5 s/km) is the propagation delay between the two most remote stations.

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Reservation protocols: basic bit-map protocol

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The binary countdown protocol

Wavelength Division Multiple Access

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Station A Station B

tunable control transmitfixed data transmit

tunable data receivefixed control receive

A wants to send data to B1. A tunes to listen B status S.S tells B control slots #3 is free.2. A tunes transmit control and sends to B “data for you in my slot 4”in B control slot #3.3. B tunes to A output and receives data from slot 4.

x x x x

m time slots for control

A

station

n time slots for data plus 1 for status

S S

x x x x

Fiber LAN implementation

Channel allocations per station

x x x x x

BS S

x x x x x

x x x x

CS S

x x x x

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Ethernet• Ethernet Cabling• Manchester Encoding• The Ethernet MAC Sublayer Protocol• The Binary Exponential Backoff Algorithm• Ethernet Performance• Switched Ethernet• Fast Ethernet• Gigabit Ethernet• IEEE 802.2: Logical Link Control• Retrospective on Ethernet

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Ethernet 802.3

10 MHzSegment length in hundredths meters

Baseband

Vampire taps

T conn.

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Ethernet Cabling

Three kinds of Ethernet cabling.

(a) 10Base5, (b) 10Base2, (c) 10Base-T.

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Ethernet coding (+/- 0.85 V)

(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

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Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX (DEC, Intel Xerox) (b) IEEE 802.3.

• Address bit 46 determines local or global address.• Min frame is 64 bytes from dest. address to checksum.

Ethernet 802.3

• Preamble (PRE). The PRE is an alternating pattern of ones and zeros.• Start-of-frame delimiter (SOF). The SOF is an alternating pattern of

ones and zeros, ending with two consecutive 1-bits. • Destination address (DA). Source addresses (SA). • Length/Type indicates either the number of MAC-client data bytes

that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format.

• Data n bytes (46=< n =<1500) of any value. (The total frame minimum is 64bytes.)

• Frame check sequence (FCS) 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

FCDataLength/typeSADASOFPre

446-150026617

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Collision detection can take as long as 2

This is to sense a collision before end of the frame reach far end.In 10 Mbps LAN 1 bit is 100 nsec, and max segment 2500 m roundtrip delay is 2 = 50 mksec = 500 bits = 64 bytes.

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Binary exponential backoff

Simplified version with persistance p:

If k stations contend for a channel the probability that any of k gets a channel is:

A = kp(1 – p)k-1 p = 1/k gives Amax -> 1/e for k -> inf.

Average number of contention slots =

jA(1 – A)j-1 = 1/A.Therefore,

channel efficiency = P/(P + 2/A)

Frame Frame

first collision

second collision

for i-th collision station randomly chooses between 0 and 2i - 1 contention slots until 1023.

P sec

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Efficiency of Ethernet at 10 Mbps with 512-bit slot times

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Switched Ethernet

Plug-in card handles collision domain either usual way or w/out collision. Frames destined outside plug-in domain are switched over backplane to the destined plug-in card.

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Gigabit Ethernet

(a) A two-station Ethernet. (b) A multistation Ethernet.

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Gigabit Ethernet cabling

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IEEE 802.2: Logical Link Control

(a) Position of LLC. (b) Protocol formats.

When reliable service is required the LLC (Logical Link Control) layer is

added on the top of MAC layer. LLC is HDLC based (frame sequencing etc.).

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Wireless LANs (WiFi)

• The 802.11 Protocol Stack• The 802.11 Physical Layer• The 802.11 MAC Sublayer Protocol• The 802.11 Frame Structure• Services

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Part of the 802.11 protocol stack

FHSS - Frequency Hopping Spread SpectrumDSSS - Direct Sequence Spread SpectrumOFDM - Orthogonal Frequency Division MultiplexingHR-DSSS - High Rate DSSS

MAC: PCF - Point Coordin. Funct.: Base Station Polls mobiles within its cell. DCF - Distributed Coordination Function further discussed.

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Wireless LAN (bandwidth is 11 to 54 MHz)

(a) A transmitting: C is out of range and cannot hear A.

(b) B transmitting: C hears B and falsely concludes that it cannot transmit to D.

Before transmitting the transmitting station wants to know whether is

any activity around receiver. CSMA tells only activity around transmitter.

Solution is that sender stimulate receiver to transmit short frame.

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DCF: wireless MACA (Multiple Access with Collision Avoidance)

• C hears RTS A to B (30 bytes) frame with the length of the frame to follow.

• D hears B responding to A with a CTS (copying the length of the next frame).

• A starts transmitting.

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The use of virtual channel sensing using CSMA/CA

NAV - Network Allocation Vector

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A fragment burst

Due to the noisy wireless channels data packets are split into short fragments.

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Interframe spacing in 802.11

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The 802.11 Data Frame Structure

Frames must process in order

Encrypted using WEP

More frames to follow

Toggle bit to put receiver to sleep

Retransmitted frame

Data, Control, Mngmnt.

RTS, CTS

Frame out of Cell

More fragments to follow

For NAV For Base Station

12 bit frame, 4 bits fragment

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802.11 Distribution Services

• Association (Attach)• Disassociation (Detach)• Reassociation (Handoff)• Distribution (Frame routing)• Integration (Protocol conversion)

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802.11 Intracell Services

• Authentication (of mobile)• Deauthentication (on leave)• Privacy (encryption)• Data Delivery (data exchange)

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Broadband Wireless (WiMax)

• Comparison of 802.11 and 802.16• The 802.16 Protocol Stack• The 802.16 Physical Layer• The 802.16 MAC Sublayer Protocol• The 802.16 Frame Structure

802.11 vs. 802.16

802.11 802.16environment indoor outdoormobility mobile fixedaccess point-to-multipoint point-to-pointdistance meters kmspectrum ISM (900 MHz-5.7 GHz) 10 - 66 GHz (mm waves) QoS internet multimedia (con orient) errors detection correction (Hamming) security optional mandatorybit rate 10 Mbps 100 Mbps

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The 802.16 Protocol Stack

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The 802.16 Physical Layer

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The 802.16 Physical Layer (2)

Frames and time slots for time division duplexing.

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The 802.16 MAC Sublayer Protocol

Service Classes

• Constant bit rate service (for T1)

• Real-time variable bit rate service (for multimedia)

• Non-real-time variable bit rate service (file transfer)

• Best efforts service (for contended upstream traffic)

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The 802.16 Frame Structure

(a) A generic frame. (b) A bandwidth request frame.