March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 11
CPET 355 CPET 355
4. The Medium Access Control 4. The Medium Access Control SublayerSublayer
Paul I-Hai Lin, Professor Paul I-Hai Lin, Professor Electrical and Computer Engineering TechnologyElectrical and Computer Engineering Technology
Purdue University, Fort Wayne CampusPurdue University, Fort Wayne Campus
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 22
MAC SublayersMAC Sublayers
The Channel Allocation ProblemThe Channel Allocation Problem Multiple Access ProtocolsMultiple Access Protocols EthernetEthernet Wireless LANsWireless LANs Broadband WirelessBroadband Wireless BluetoothBluetooth Data Link Layer SwitchingData Link Layer Switching
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 33
The Channel Allocation ProblemThe Channel Allocation Problem
Static Channel AllocationStatic Channel Allocation• Frequency Division Multiplexing (FDM) – poor Frequency Division Multiplexing (FDM) – poor
utilizationutilization• Time Division Multiplexing (TDM) – poor utilizationTime Division Multiplexing (TDM) – poor utilization
Dynamic Channel AllocationDynamic Channel Allocation• Station ModelStation Model• Single Channel AssumptionSingle Channel Assumption• Collision AssumptionCollision Assumption• Continuous Time, Slotted TimeContinuous Time, Slotted Time• Carrier Sense, No Carrier SenseCarrier Sense, No Carrier Sense
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 44
Multiple Access ProtocolsMultiple Access Protocols
ALOHA – 1970, University of HawaiiALOHA – 1970, University of Hawaii• Pure ALOHA, Slotted ALOHAPure ALOHA, Slotted ALOHA
CSMA (Carrier Sense Multiple Access) – CSMA (Carrier Sense Multiple Access) – 1975, contention-based1975, contention-based• Persistent and Non-persistent CSMAPersistent and Non-persistent CSMA• CSMA/CD (Collision Detection) CSMA/CD (Collision Detection)
Collision-Free ProtocolCollision-Free Protocol• Bit-MappedBit-Mapped• Binary CountdownBinary Countdown
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 55
CSMA/CD ProtocolCSMA/CD Protocol
A multiple access network: a set of nodes A multiple access network: a set of nodes or stations send and receive frames over a or stations send and receive frames over a shared linkshared link
Time-division multiplexed busTime-division multiplexed bus Carrier Sense - means that all the nodes Carrier Sense - means that all the nodes
can distinguish between an idle and a can distinguish between an idle and a busy linkbusy link
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 66
CSMA/CD Access RuleCSMA/CD Access Rule
Step 1. All stations listen to the transmission Step 1. All stations listen to the transmission medium. medium.
1a. If the medium is idle, transmit; 1a. If the medium is idle, transmit; otherwise, go to step 2 (listen)otherwise, go to step 2 (listen)
1b. If a collision is detected during 1b. If a collision is detected during transmission, transmit a brief jamming transmission, transmit a brief jamming signal to ensure that all stations know that signal to ensure that all stations know that there has been a collision and then cease there has been a collision and then cease transmissiontransmission
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 77
CSMA/CD Access RuleCSMA/CD Access Rule
1c. After transmitting the jamming signal, 1c. After transmitting the jamming signal, wait a random amount of time, then wait a random amount of time, then attempt to transmit againattempt to transmit again
Step 2. If the medium is busy, continue to Step 2. If the medium is busy, continue to listen until the channel is idle, then listen until the channel is idle, then transmit immediatelytransmit immediately
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 88
CSMA/CD Protocol CSMA/CD Protocol (cont.)(cont.)
Collision Detection - means that a node Collision Detection - means that a node listens as it transmits, and can therefore listens as it transmits, and can therefore detect a frame when it is transmitting has detect a frame when it is transmitting has interfered (collided) with a frame interfered (collided) with a frame transmitted by another nodetransmitted by another node
When a station is in transmission, When a station is in transmission, according to the CSMA/CD scheme no according to the CSMA/CD scheme no other station may transmit.other station may transmit.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 99
Collision DetectionCollision Detection
If two stations transmit at once, their If two stations transmit at once, their signals interfere with each other signals interfere with each other
Since each sender listens (sense the Since each sender listens (sense the carrier) before the transmission, they know carrier) before the transmission, they know that there has been a collision that there has been a collision
Both of the stations stop and wait a Both of the stations stop and wait a random amount time before next attempt random amount time before next attempt to transmitto transmit
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1010
CSMA/CD State DiagramCSMA/CD State Diagram
MediumIdle?
Transmit
Channel isBusy
YesNo
Transmit abrief
jammingSignal
Wait for arandom
amount oftime
Tryagain
Tryagain
Channel Busy
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1111
Multiple Access ProtocolsMultiple Access Protocols
Limited-Content ProtocolsLimited-Content Protocols• The Adaptive Tree Walk ProtocolThe Adaptive Tree Walk Protocol
Wavelength Division Multiple Access Protocol Wavelength Division Multiple Access Protocol (WDMA)(WDMA)
Wireless LAN ProtocolsWireless LAN Protocols• Multiple Access with Collision Avoidance (MACA, Multiple Access with Collision Avoidance (MACA,
Karn 1990)Karn 1990)• Multiple Access with Collision Avoidance for Wireless Multiple Access with Collision Avoidance for Wireless
(MACA for Wirelss, Bharghavan, et. Al, 1994)(MACA for Wirelss, Bharghavan, et. Al, 1994)
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1212
EthernetEthernet
IEEE StandardsIEEE Standards• IEEE 802.3 (Ethernet)IEEE 802.3 (Ethernet)• IEEE 802.11 (Wireless LAN)IEEE 802.11 (Wireless LAN)• IEEE 802.15 (Bluetooth)IEEE 802.15 (Bluetooth)• IEEE 802.16 (Wireless MAN)IEEE 802.16 (Wireless MAN)
IEEE 802.3 IEEE 802.3 • Access Methodology – CSMA/CDAccess Methodology – CSMA/CD• Logical Topology – BroadcastLogical Topology – Broadcast• Physical Topology – StarPhysical Topology – Star
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1313
LAN TechnologyLAN Technology
MAC Lower Sublayer MAC Lower Sublayer • Medium Access Control - Different options for Medium Access Control - Different options for
IEEE 802.x IEEE 802.x • 802.3 (Ethernet, CSMA/CD or Carrier Sensed 802.3 (Ethernet, CSMA/CD or Carrier Sensed
Multiple Access/Collision Detection)Multiple Access/Collision Detection)• 802.4 (Token Bus) - obsolete802.4 (Token Bus) - obsolete• 802.5 (Token Ring)802.5 (Token Ring)
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1414
Layers of TCP/IP over LANLayers of TCP/IP over LAN
Application Layer (http, ftp, telnet, etc)Application Layer (http, ftp, telnet, etc) TCP LayerTCP Layer IP LayerIP Layer LLC Layer (Logical Link Control)LLC Layer (Logical Link Control) MAC Layer (Medium Access Control)MAC Layer (Medium Access Control) Physical LayerPhysical Layer
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1515
LAN SwitchLAN Switch
Server
Printer
Ethernet Card
Hub
10BASE-T wall plate
10BASE-T wall plate
10BASE-T wall plate
Ethernet Card
Print Server
10BaseT Ethernet Hub
Station A
Station BEthernet Card
Station C
Transmit
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1616
IEEE 802.3 - EthernetIEEE 802.3 - Ethernet
IEEE 802.3 CSMA/CDIEEE 802.3 CSMA/CD• Carrier Sensed Multiple Access/Collision Carrier Sensed Multiple Access/Collision
DetectionDetection• Access Medium Access Medium
Base band coaxial cableBase band coaxial cable Unshield twisted pair (UTP)Unshield twisted pair (UTP) Shield twisted pairShield twisted pair Broadband coaxial cablesBroadband coaxial cables Optical fiberOptical fiber
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1717
Cable TopologyCable Topology
Linear Bus (tap)Linear Bus (tap) Spine (backbone)Spine (backbone) StarStar TreeTree MashMash Segmented (repeater, bridge)Segmented (repeater, bridge)
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1818
EthernetEthernet
Ethernet Cabling (10 M bits/second)Ethernet Cabling (10 M bits/second)• 10Base5, Thick coax, 500m, 100 nodes/seg, 10Base5, Thick coax, 500m, 100 nodes/seg,
obsoleteobsolete• 10Base2, Thin coax, 185 m, 30 nodes/seg, no 10Base2, Thin coax, 185 m, 30 nodes/seg, no
hub neededhub needed• 10BaseT, Twisted pair, 100 m, 1024 10BaseT, Twisted pair, 100 m, 1024
nodes/seg, cheapestnodes/seg, cheapest• 10BaseF, Fiber optics, 2000 m, 1024 10BaseF, Fiber optics, 2000 m, 1024
nodes/seg, between buildingsnodes/seg, between buildings• 100BaseT100BaseT
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 1919
Other Devices/ConnectionsOther Devices/Connections
Transceiver TapTransceiver Tap• A connecting mechanism that allows the A connecting mechanism that allows the
transceiver to tap into the cable line at any transceiver to tap into the cable line at any pointpoint
BNC-T connectorBNC-T connector• A T-shaped device with three ports: use for A T-shaped device with three ports: use for
the NIC and one each for the input and output the NIC and one each for the input and output ends of cables.ends of cables.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2020
High Speed EthernetHigh Speed Ethernet
100BaseT Ethernet Cabling (CSMA/CD, 100BaseT Ethernet Cabling (CSMA/CD, IEEE 802.3u, 100 M bits/second, base IEEE 802.3u, 100 M bits/second, base band, up to 2 repeaters/hubs, 210 m band, up to 2 repeaters/hubs, 210 m diameter)diameter)• 100BaseTX - Twisted pair, two-pair Cat 5 100BaseTX - Twisted pair, two-pair Cat 5
UTPUTP• 100BaseT4 – 4-pair, Cat 3,4 or 5 UTP100BaseT4 – 4-pair, Cat 3,4 or 5 UTP• 100BaseFX – Duplex multimode fiber-optics 100BaseFX – Duplex multimode fiber-optics
cablecable
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2121
Giga-Bit EthernetGiga-Bit Ethernet
1000BaseT Ethernet Cabling (CSMA/CD, 1000BaseT Ethernet Cabling (CSMA/CD, IEEE 802.3z, 1000M bits/second)IEEE 802.3z, 1000M bits/second)• 1000BaseSX – 850nm short wave length 1000BaseSX – 850nm short wave length
laser fiber-media, laying on floorlaser fiber-media, laying on floor• 1000BaseLX – 1300 nm long wave length 1000BaseLX – 1300 nm long wave length
laser fiber media, backbonelaser fiber media, backbone• 1000BaseCX – Copper twinaxial cable, 25m 1000BaseCX – Copper twinaxial cable, 25m
distance, data centerdistance, data center• 1000BaseTX – UTP Cat 5, 100 m1000BaseTX – UTP Cat 5, 100 m
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2222
Signal EncodingSignal Encoding
Binary EncodingBinary Encoding• 1 = + V, 0 = -V;1 = + V, 0 = -V;
1 0 1 1 1 0
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2323
Signal EncodingSignal Encoding
Manchester Encoding (Ethernet)Manchester Encoding (Ethernet)• High +0.85 V, Low -0.85 VHigh +0.85 V, Low -0.85 V• One bit has two equal intervals, non-return to One bit has two equal intervals, non-return to
zerozero• 0 – Low to high, 1 – High to low0 – Low to high, 1 – High to low
1 0 1 1 1 0
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2424
IEEE 802.3 Frame Format IEEE 802.3 Frame Format (64 to 1518 Octets)(64 to 1518 Octets)
Preamble – 7 octets or 56 bit(synch. )Preamble – 7 octets or 56 bit(synch. ) Start Frame Delimiter – 1 octet or 8-bitStart Frame Delimiter – 1 octet or 8-bit Destination Address – 2 to 6 octetsDestination Address – 2 to 6 octets Source Address – 2 to 6 octetsSource Address – 2 to 6 octets Length of Protocol Data Unit (PDU) – 2 octetsLength of Protocol Data Unit (PDU) – 2 octets LLC or 802.2 Data Frame – 46 to 1500 bytesLLC or 802.2 Data Frame – 46 to 1500 bytes CRC or Frame Check Sequence – 4 OctetsCRC or Frame Check Sequence – 4 Octets
Pre. SFD D.Addr S.Assr Len Data Pad CkSum
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2525
IEEE 802.3 Frame FormatIEEE 802.3 Frame Format
Minimum frame size – 64 octetsMinimum frame size – 64 octets• Overhead 18 octetsOverhead 18 octets• 46-byte min data field46-byte min data field
PreamblePreamble• 7-bytes (56 bits) pattern of alternating 1s and 7-bytes (56 bits) pattern of alternating 1s and
0’s (1010101..) 0’s (1010101..) • Alert the receiving station to the incoming Alert the receiving station to the incoming
frame, and enable it to synchronize its input frame, and enable it to synchronize its input timing.timing.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2626
IEEE 802.3 Frame FormatIEEE 802.3 Frame Format
Start Frame Delimiter (SFD) Start Frame Delimiter (SFD) • The second field (one byte 10101011) of the The second field (one byte 10101011) of the
frame indicates the beginning of the frameframe indicates the beginning of the frame• It tells the receiver that the next thing coming It tells the receiver that the next thing coming
is the destination address of the receiveris the destination address of the receiver
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2727
IEEE 802.3 Frame FormatIEEE 802.3 Frame Format
Destination Address (DA) Destination Address (DA) • When the packet reaches the target network, the DA When the packet reaches the target network, the DA
field contains the physical address of the destination field contains the physical address of the destination stationstation
• DA filed is either 2- or 6-byte for holding a unique DA filed is either 2- or 6-byte for holding a unique physical address, a group address, or a global physical address, a group address, or a global address of the packet’s next destination address of the packet’s next destination
• DA address may hold the router connection address DA address may hold the router connection address at the beginning if the packet must cross from one at the beginning if the packet must cross from one LAN to another LAN to another
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2828
IEEE 802.3 Frame FormatIEEE 802.3 Frame Format
Source Address (SA)Source Address (SA)• SA field is either 2- or 6-bytes for holding the SA field is either 2- or 6-bytes for holding the
frame sender’s addressframe sender’s address Length of PDU (Protocol Data Unit)Length of PDU (Protocol Data Unit)
• two bytes length field for indicating the two bytes length field for indicating the number of bytes of the data field.number of bytes of the data field.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 2929
IEEE 802.3 Frame FormatIEEE 802.3 Frame Format
LLC or 802.2 Data frameLLC or 802.2 Data frame• Entire data frame from 46 to 1500 bytes long Entire data frame from 46 to 1500 bytes long • DSAP – Destination Service Access PointDSAP – Destination Service Access Point• SSAP – Source Service Access PointSSAP – Source Service Access Point• Control Control • InformationInformation
CRC or FCSCRC or FCS• 4-byte field that holds a CRC-32 (Cyclic Redundancy 4-byte field that holds a CRC-32 (Cyclic Redundancy
Check) for checking the frame integrity.Check) for checking the frame integrity.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3030
IEEE 802.3 Frame FormatIEEE 802.3 Frame Format
RFC 1042 RFC 1042 • A Standard of Transmission IP Datagrams A Standard of Transmission IP Datagrams
over Ethernetover Ethernet• 802.2/802.3 Encapsulation802.2/802.3 Encapsulation
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3131
Ethernet AddressEthernet Address
RFC 826 – Ethernet AddressingRFC 826 – Ethernet Addressing NIC Address NIC Address
• 48-bit address in Hexadecimal Notation48-bit address in Hexadecimal Notation• An example: 06-01-02-01-21-48An example: 06-01-02-01-21-48
NIC Address Checking Commands:NIC Address Checking Commands:• Windows 98 - winipcfgWindows 98 - winipcfg• Windows NT/2000/XP command - ipconfig/allWindows NT/2000/XP command - ipconfig/all
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3232
Ethernet AddressEthernet Address
Linux Commands:Linux Commands:
$ dmesg | grep eth$ dmesg | grep eth
divert: allocating divert_blk for eth0divert: allocating divert_blk for eth0
eth0: 3c5x9 at 0x220, 10baseT port, eth0: 3c5x9 at 0x220, 10baseT port, address 00 60 08 14 4b da, IRQ 5.address 00 60 08 14 4b da, IRQ 5.
eth0: Setting 3c5x9/3c5x9B half-duplex eth0: Setting 3c5x9/3c5x9B half-duplex mode if_port: 0, sw_info: 1321mode if_port: 0, sw_info: 1321
eth0: Setting Rx mode to 1 addresses.eth0: Setting Rx mode to 1 addresses.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3333
Ethernet AddressEthernet Address
Source AddressSource Address• Unicast - a source address is always a Unicast - a source address is always a
unicast addressunicast address• Bit 0 of byte 1 Bit 0 of byte 1
Destination AddressDestination Address• Multicast – received by a group of stationsMulticast – received by a group of stations• Broadcast (1111 …1111) – destination Broadcast (1111 …1111) – destination
address is 48-bit of all 1’s, received by all address is 48-bit of all 1’s, received by all stationsstations
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3434
Network Analyzer ProtocolNetwork Analyzer Protocol
Ethereal (Open Source)Ethereal (Open Source)• www.ethereal.comwww.ethereal.com• Data frame capture Data frame capture • Protocol AnalyzingProtocol Analyzing
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3535
Example 1: Time needed for Example 1: Time needed for collision detectioncollision detection
Question: Question: Two stations begin transmitting at exactly Two stations begin transmitting at exactly
the same time. the same time. How long will it take to realize that there How long will it take to realize that there
has been a collision? has been a collision? Contention period, delay, and throughput.Contention period, delay, and throughput.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3636
Example 1: Time needed for Example 1: Time needed for collision detectioncollision detection
Answer: Answer: The minimum time for detecting the The minimum time for detecting the
collision is the time that it takes to collision is the time that it takes to propagate from one station to the other.propagate from one station to the other.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3737
Example 2: Elapses time and Example 2: Elapses time and throughputthroughput
Consider the transfer of a file containing Consider the transfer of a file containing one million characters from one station one million characters from one station to another. to another.
What is the total elapsed time and What is the total elapsed time and effective throughput for the 10BaseT effective throughput for the 10BaseT star topology with a data rate of 10 star topology with a data rate of 10 Mbps? Mbps?
Assume that the network setup time is Assume that the network setup time is negligible.negligible.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3838
Example 2: Elapses time and Example 2: Elapses time and throughputthroughput
Solution: 0.8 sec.Solution: 0.8 sec.
8 bit/char x 1 M char 8 bit/char x 1 M char 10 Mbits/sec = 10 Mbits/sec = 0.8 seconds.0.8 seconds.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 3939
Example 3: Carrier Detection Example 3: Carrier Detection CircuitCircuit
Collision detection process:Collision detection process: We imagine that the station’s hardware must We imagine that the station’s hardware must
listen to the cable while it is transmitting. listen to the cable while it is transmitting. If what it reads back is different from it is putting If what it reads back is different from it is putting
out, it knows a collision is occurring. out, it knows a collision is occurring. A station wants to acquire the medium it must A station wants to acquire the medium it must
first output a 1 bit, if the channel is busy the first output a 1 bit, if the channel is busy the return bit is 0, otherwise the bit is 1.return bit is 0, otherwise the bit is 1.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4040
Collision Detection CircuitCollision Detection Circuit
XOR
Ethernet CardTransmit=1
Busy = 1Idle = 0
Channel Idle = 1Channel Busy = 0
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4141
Example 4: Slot TimeExample 4: Slot Time
What is the most amount of time needed What is the most amount of time needed for a station to know that if it be granted for a station to know that if it be granted the access to the channel? the access to the channel?
This time is normally called slot time.This time is normally called slot time.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4242
Example 4: Slot Time Example 4: Slot Time (Calculation)(Calculation)
Let Tau be the time for a signal to propagate Let Tau be the time for a signal to propagate between the two farthest stations.between the two farthest stations.
At time t0, station A begins transmitting.At time t0, station A begins transmitting. At time t0 - tAt time t0 - t, before the signal arrive at station , before the signal arrive at station
B which is the farthest station, the station B also B which is the farthest station, the station B also begins transmission. The station B detects the begins transmission. The station B detects the collision instantaneously.collision instantaneously.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4343
Example 4: Slot Time Example 4: Slot Time (Calculation)(Calculation)
But the jamming signal did not get back to the But the jamming signal did not get back to the station A until (2Tau - tstation A until (2Tau - t).).
So that the most amount of the time needed to So that the most amount of the time needed to be sure that it has seized the channel is it has be sure that it has seized the channel is it has transmitted for 2Tau time without hearing a transmitted for 2Tau time without hearing a collision.collision.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4444
Example 5: Effective Data Rate Example 5: Effective Data Rate CalculationCalculation
What is the effective data rate, excluding What is the effective data rate, excluding overhead? overhead?
Assuming that there are no collisions for a Assuming that there are no collisions for a 10 Mbps Ethernet with the following 10 Mbps Ethernet with the following specification:specification:
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4545
Example 5: Effective data rate Example 5: Effective data rate calculationcalculation
1 km long cable that has a propagation 1 km long cable that has a propagation speed of 200 m/speed of 200 m/sec. Data packets are sec. Data packets are 1024 bits long, including 32-bit header, 1024 bits long, including 32-bit header, CRC check sum and other overhead.CRC check sum and other overhead.
The first bit slot after a successful The first bit slot after a successful transmission is reserved for the receiver to transmission is reserved for the receiver to capture the channel to send a 32-bit capture the channel to send a 32-bit acknowledgement packet.acknowledgement packet.
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4646
Example 5: Effective data rate Example 5: Effective data rate calculationcalculation
Solution:Solution: The round trip propagation time of the The round trip propagation time of the
cable is cable is
(200 m/(200 m/sec sec 1 km) 1 km) 2 = 10 2 = 10 ss And we know that a complete transmission And we know that a complete transmission
has four phases:has four phases: Sender seizes cable (10 Sender seizes cable (10 sec)sec)
March 22, 2004March 22, 2004 Prof. Paul LinProf. Paul Lin 4747
Example 5: Effective data rate Example 5: Effective data rate calculationcalculation
Solution:Solution: Transmit data ( 1024 bits Transmit data ( 1024 bits 10 Mbps = 10 Mbps =
102.4 102.4 sec)sec) Receiver seizes cable (10 Receiver seizes cable (10 sec)sec) Ack. Sent (32 bits Ack. Sent (32 bits 10 Mbps = 3.2 10 Mbps = 3.2 sec)sec) The sum of these time is 125.6 The sum of these time is 125.6 sec.sec. In this period (1024 - 32 = 992) data bits In this period (1024 - 32 = 992) data bits
are sent, for a rate of 7.9 Mbpsare sent, for a rate of 7.9 Mbps
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