cs 453 computer networks lecture 15 medium access control sublayer

CS 453CS 453Computer NetworksComputer Networks

Lecture 15Lecture 15

Medium Access Control Medium Access Control

SublayerSublayer

MAC sublayerMAC sublayer

Gigabit EthernetGigabit Ethernet History has shown us that like Peanut butter History has shown us that like Peanut butter

cookies, you can’t have enough data rate cookies, you can’t have enough data rate capacitycapacity

As data rates have grown applications have As data rates have grown applications have quickly swelled to fill the capacityquickly swelled to fill the capacity

About the time that Fast Ethernet was hitting About the time that Fast Ethernet was hitting the streets, the IEEE 802 committee was the streets, the IEEE 802 committee was working on what became known as Gigabit working on what became known as Gigabit EthernetEthernet


Gigabit EthernetGigabit Ethernet IEEE defined Gigabit Ethernet in 1998 and IEEE defined Gigabit Ethernet in 1998 and

labeled it 802.3zlabeled it 802.3zThey apparently thought was the final frontier in They apparently thought was the final frontier in terms of ethernetterms of ethernet

They were wrong on that pointThey were wrong on that point 802.3z goal to be802.3z goal to be

10x faster than 802.3u (Fast Ethernet)10x faster than 802.3u (Fast Ethernet)

Backward compatible with other Ethernet Backward compatible with other Ethernet standardsstandards


Gigabit EthernetGigabit Ethernet GoalsGoals

Same Ethernet Frame formatSame Ethernet Frame format

Same min/max frame sizesSame min/max frame sizes

Same 48 bit Ethernet addressing schemeSame 48 bit Ethernet addressing scheme

Offer unacknowledged datagram serviceOffer unacknowledged datagram service

Unicast & multicastUnicast & multicast

MAC sublayerMAC sublayerGigabit EthernetGigabit Ethernet All connections are point-to-pointAll connections are point-to-point

No multidrop like 803.2 and 802.5No multidrop like 803.2 and 802.5

Any GigE cable – only two devicesAny GigE cable – only two devices

One device can be switch or hubOne device can be switch or hub Modes – full-duplexModes – full-duplex

NormalNormal

Connected to a switchConnected to a switch

Send/Receive at same timeSend/Receive at same time

What about cable length and collision alarm delay???What about cable length and collision alarm delay???

Remember the issue of cable length and the delay time for Remember the issue of cable length and the delay time for collision alarm to propagate from the station detecting the collision alarm to propagate from the station detecting the collision to the station transmitting?collision to the station transmitting?

MAC sublayerMAC sublayerGigabit EthernetGigabit Ethernet In standard Ethernet (802.3) with a multidrop medium…In standard Ethernet (802.3) with a multidrop medium…

……the minimum packet size (minus the preamble and SOF flag) is the minimum packet size (minus the preamble and SOF flag) is 64 bytes….64 bytes….

……so that a collision alarm could, worst case, reach from one end so that a collision alarm could, worst case, reach from one end of the medium to a transmitting station at the other end of the of the medium to a transmitting station at the other end of the medium while is it still transmitting…medium while is it still transmitting…

..but this can only be true for a maximum length cable…..but this can only be true for a maximum length cable…

For 802.3 that is 2500 metersFor 802.3 that is 2500 meters For full-duplex GigE, this is not an issueFor full-duplex GigE, this is not an issue

Each cable is “private” for two devicesEach cable is “private” for two devices

Collision are not possibleCollision are not possible

No CSMA/CDNo CSMA/CD

Only cable length issue is signal strength lossOnly cable length issue is signal strength loss

MAC sublayerMAC sublayerGigabit EthernetGigabit Ethernet Half-duplex Half-duplex

All connections are point-to-point, butAll connections are point-to-point, but

Connected to a common hubConnected to a common hub

… … functions like a bus-in-a-boxfunctions like a bus-in-a-box

So, collisions are possibleSo, collisions are possible

Uses CSMA/CDUses CSMA/CD

Then, in theory, due to collision alarm propagation delayThen, in theory, due to collision alarm propagation delay

Max cable length = [10base2MaxCableLength]/100 = Max cable length = [10base2MaxCableLength]/100 =

2500/100 = 25 meters2500/100 = 25 meters

That won’t do!That won’t do!

MAC sublayerMAC sublayerGigabit EthernetGigabit Ethernet Half-duplex - CollisionsHalf-duplex - Collisions

Carrier ExtensionCarrier Extension Pad the frame to 512 bytesPad the frame to 512 bytes Done padded and unpadded by hardware, no changes to softwareDone padded and unpadded by hardware, no changes to software Poor bandwidth efficiency for small payloadsPoor bandwidth efficiency for small payloads

Frame BurstingFrame Bursting ““Bunch up” several frames and transmit at one timeBunch up” several frames and transmit at one time If grouped frame is still less than 512 bytes pad to 512If grouped frame is still less than 512 bytes pad to 512 Efficient if there a good flow of frames to transmitEfficient if there a good flow of frames to transmit Allows cable lengths to 200 metersAllows cable lengths to 200 meters

MAC sublayerMAC sublayerGigabit EthernetGigabit Ethernet 1000BaseT Encoding1000BaseT Encoding

Uses 4 pair of Cat5/Cat6 cableUses 4 pair of Cat5/Cat6 cable

Five symbols using 5 voltage levelsFive symbols using 5 voltage levels

00, 01,10,11 and a control symbol00, 01,10,11 and a control symbol

So 2 bits per symbolSo 2 bits per symbol

Each symbol over one twisted pair Each symbol over one twisted pair

So, 2 bit per symbol * 4 pairs = 8 bits transmitted at same timeSo, 2 bit per symbol * 4 pairs = 8 bits transmitted at same time

125 Mhz * 8 bits = 1 Gbps125 Mhz * 8 bits = 1 Gbps

MAC sublayerMAC sublayerGigabit EthernetGigabit Ethernet Flow controlFlow control 1 msec delay in processing arriving data = up to 1953 1 msec delay in processing arriving data = up to 1953

frames lost in 1 msecframes lost in 1 msec GigE uses a flow control frameGigE uses a flow control frame

If busy host send PAUSE frame == type field = 0x8808 …If busy host send PAUSE frame == type field = 0x8808 …

First 2 bytes of payload field controls the flow control commandFirst 2 bytes of payload field controls the flow control command

Next bytes contain pause time in 512 nsec incrementsNext bytes contain pause time in 512 nsec increments

802.11 Wireless LANs

WiFi

Very popular local area networking

Operates in two modes Using a Base station /Access Point

(Infrastructure mode) Without an Access Point – station to station

(adhoc mode)


802.11 Physical layer/Data Link Layer Originally (1997) three transmission media

Infrared

FHSS

DSSS


802.11 Physical layer/Data Link LayerInfrared

Diffused Infrared light at 0.85 or 0.95 microns 1 or 2 Mbps Uses Gray code encoding

For 2 bits creates 4 bit codeword with never more than one 1 bit

Cannot penetrate walls – so good cell isolation Low bandwidth Inference from sunlight Not very popular


802.11 Physical layer/Data Link Layer FHSS (remember Hedy Lammar) 79 1 Mhz channels At 2.4 GHz ISM band All stations generate pseudorandom

sequence of channels to hop to If stations use same PRN seed and stay

synchronized… Will hop to the next channel in sequence

simultaneously


802.11 Physical layer/Data Link Layer FHSS (remember Hedy Lammar) Dwell time adjustable Dwell time must be < 400 msec Pretty secure from eavesdropping

Sniffer does not know hop sequence or

dwell time Uses same band as garage door openers,

microwave ovens and cordless phones


802.11 Physical layer/Data Link Layer DSSS (Direct Sequence Spread Spectrum)

Data bits combined with higher data rate bit Pseudo noise sequence called “chipping code”…

Then divides data according to spreading ratio

Chipping code is a redundant bit pattern of data bits

Bit errors can be corrected

Difficult to intercept

Difficult to jam


802.11 Physical layer/Data Link Layer 802.11a Orthogonal Frequency Division

Multiplexing (OFDM) 54 Mbps 5 GHz ISM band 52 frequency channels – 48 data,

synchronization Phase shift modulation up to 18 Mbps QAM from 18 Mbps to 54 Mbps


802.11 Physical layer/Data Link Layer 802.11b - High Rate Direct Sequence Spread

Spectrum 11 million chipping codes per second ~ 11

Mbps in 2.4 GHz band Actual preceded 802.11a 1,2, 5.5 and 11 Mbps Slower than 802.11a but range much greater


802.11 Physical layer/Data Link Layer 802.11g OFDM Enhancement to 802.11g Approved in 2001 Operates in the 2.4 GHz ISM band Up to 54 Mbps


Station B tries to communicate with A…

C cannot hear B’s communication and tries to communicate

AB C


D transmits to CA wants to transmit to B, butHears noise, delays transmission, unnecessarily

AB C

D


802.11 can’t use CSMA/CD

Can’t use “dead air” to indicate that it is ok to transmit

Need protocol to coordinate medium access DCF – Distributed Coordination Function PCF – Point Coordination Function


DCF – Distributed Coordination Function CSMA/CA - Collision Avoidance Physical Channel Sensing Virtual Channel Sensing


Virtual Channel Sensing A transmits a Request To Send (RTS) to B B responds with Clear To Send (CTS) to A C can hear RTS so self imposes Network Allocation Vector

(NAV), can’t transmit until hears ACK from B D does not hear RTS but hears CTS from B, self imposes NAV

until is hears ACK

Diagram from Tanenbaum (2003), pg. 297


DCF – Distributed Coordination Function Another problem – Wireless medium is

inherently noisy and unreliable Probability of large frame getting through

without error is relatively small… And will need retransmitted Solution: Frame fragmenting

802.11 Wireless LANsDCF – Distributed Coordination Function

Fragmented framesBreak large frames up into small frames

After sending RTS and receiving CTS, …

Sender sends a burst of frame fragments



PCF – Point Coordination Function Media Access is control from a Point (Access

Point) AP polls stations and asks if they have a

frame to send Transmission order is determined by AP


PCF – Point Coordination Function AP broadcasts beacon frame every 0.01 to

0.1 seconds Beacon frame contains hopping sequences,

dwell times, synchronization clock, etc. Invites stations to “log on” to polling service Once on polling service AP guarantees

designated fraction of bandwidthTherefore, can make QOS guarantee


Can use DCF and PCF at the same time

Requires wait periods or Interframe Spacing SIFS – Short Interframe Spacing PIFS – PCF Interframe Spacing DIFS – DCF Interframe Spacing EIFS – Extended Interframe Spacing

802.11 Wireless LANs802.11 Interframe Spacing

AfterSIFS – control frames or next fragmentsPIFS – PCF frames may be sentDIFS – DCF frames may be sentEIFS – bad frame recovery can be started



802.11 Frame Structure

From: ANSI/IEEE Std. 802.11 , 1999 edition (R2003)

Frame Control

802.11 Wireless LANs802.11 Frame Structure

Frame ControlProtocol version – allow multiple versions of protocol

Type – Data, Management, Control

Subtype – RTS, CTS

To DS/From DS – going to/coming from distribution system (i.e. ethernet)

MF – more fragments coming

Retry – frame is a retry of a previous frame

Pwr – controls power of receiving station

More – more frames to come

W – Frame encrypted with WEP algorithm

O – frames must be processed in order


802.11 Wireless LANs802.11 Frame Structure Data frame

Duration – how long the frame and ACK will use channel

Address 1/Address 2 – Source Address/Destination Address

Address 3/Address 4 – Base station (source/destination) addresses for intercell traffic

Sequence – fragment sequence number

Data – payload max length 2312 bytes

Checksum -


802.11 Wireless LANs802.11 Services Distributions Services

Assocation – allows stations to connect to access point

Disassociate – breaks relationship between station and access point (leave network)

Reassociation – handoff station to another access point

Distribution – routing local air or wired network

Integration – bridging/conversion to other addressing/framing formats

802.11 Wireless LANs802.11 Services Station Services

Authentication – authenicates station to access point

Deauthentication - - logs station out of network cell

Privacy – encryption/decryption

Data delivery – move data from station to station

cs 453 computer networks lecture 15 medium access control sublayer

Documents

gigabit ethernet slide

standard ethernet

ethernet standards

terms of ethernet

ethernet frame format

u fast ethernet

issue of cable length

cable length issue