lecture10-wlan and lan briges.pdf
TRANSCRIPT
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B i gi ng M ng Vi n thng
Tr n Xun Nam Khoa V tuy n i n t
H c vi n K thu t Qun s
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B i9 C c Giao th c i u khi n
Truy nh p Mi trngv M ng C c b
Part II: M ng C c b T ng quan v LANsEthernet
Token Ring and FDDI802.11 Wireless LAN
LAN Bridges
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B i9 C c Giao th c i u khi n
Truy nh p Mi trngv M ng C c b
Tng quan v L A N s
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Chapter 6 Medium Access Control
Protocols and Local AreaNetworks
802.11 Wireless L A N
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Wireless Data Communications
Cc yu c u i v i wireless communicationsD tri n khai, chi ph thpMobility & roaming: truy nh p thng tin b t k ni u H tr cc thi t b c nhn
PDAs, laptops, data-cell-phones
H tr cc thi t b lin lcCameras, thi t b nh v , nh n d ng v tuyn
Kh khn C ng t n hi u bi n i theo khng gian & thi gian
T n hi u c th b thu trmPh t n b gi i h n v th ng b h n ch
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C c ch lin l c WLAN
Ch AdhocCc tr m l m vi c tr c ti p v i nhau khng qua tr mi u khi n trung tm (Access Point)
Ch c c u trc (Infrastructure)Cc tr m l m vi c thng qua m t tr m trung tm(Access Point)
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B D
C A
Ad Hoc Communications
Gn t m th i m t nh m cc trmTrong ph m vi lin l c c a gi a cc trmC n trao i thng tin VD: Tr nh chi u trong h i ngh , hay game n i mng
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A2 B2
B1 A1
AP1
AP2
Distribution SystemServer Gateway tothe InternetPortal
Portal
BSS A BSS B
Infrastructure Network
Cc Access Points c nh cung c p truy nh p ti Internet
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A transmits data frame
(a)
Data Frame Data Frame
A
B C
C transmits data frame& collides with A at B
(b)
C senses medium,
station A is hidden from C
Data Frame
B
C A
Hidden Terminal Problem
New MAC: CSMA with Collision Avoidance
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RTS
A requests to send
B
C
(a)
CTS CTS
A
B
C
B announces A ok to send
(b)
Data Frame
A sends
B
C remains quiet
(c)
CSMA with Collision Avoidance
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IEEE 802.11 Wireless LAN
S d ng bng t n khng c p ph p ( unlicensedspectrum )
U.S. Industrial, Scientific, Medical (ISM) bands902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz
T c mong mu n ban u 20 Mbps Thi t k giao th c MAC cho WLAN t c cao M ng Ad Hoc & C u trc (Infrastructure) S d ng cc l p v t l khc nhau
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802.11 Definitions
Basic Service Set (BSS)Nh m cc tr m i u phi truy nh p s d ng m tphin b n c a MAC
t t i mt Basic Service Area (BSA)Cc tr m trong BSS c th lin l c vi nhauC th t n t i cc BSS trong c ng m t vng
Extended Service Set (ESS)Cc BSSs k t n i v i nhau qua Distribution System(DS)M i BSS nh m t cell v cc tr m trong BSS lin l cv i m t Access Point (AP)Cc portals g n v i DS cung c p truy nh p tiInternet
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A2 B2
B1 A1
AP1 AP2
Distribution SystemServer
Gateway tothe InternetPortal
Portal
BSS A BSS B
Infrastructure Network
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C c d ch v phn ph i
Cc tr m trong BSS c th lin l c th ng vinhauDS cung c p cc d ch v phn phi (distributionservices ):
Chuy n MAC SDUs gi a cc APs trong ESSChuy n MSDUs gi a cc portals & BSSs trong ESSChuy n MSDUs gi a cc tr m trong cng 1 BSS
Multicast, broadcast, theo t y ch n c a tr m
ESS gi ng nh m t BSS i v i lp con LLC
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C c D ch v C u trc
Ch n AP v thi t lp gn m ng (association ) v i APSau c th pht/thu cc frame qua AP & DS
Gn m ng l i (Reassociation service ) chuy n
t m t AP t i m t AP khcT ch m ng (Dissociation service) ng t g nm ngNh n thc (Authentication service) thit l pidentity c a cc tr m khcB o m t (Privacy service) gi b o m t n idung
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IEEE 802.11 MAC
Nhi m v ca MAC sublayerTruy nh p knh nh a ch PDU, nh d ng frame, ki m tra liPhn o n (fragmentation) & ti h p (reassembly) ccMAC SDUs
T y ch n d ch v an ton MAC Authentication & privacy
D ch v qu n lMACChuy n gi a cc AP (roaming) trong ph m vi ESSQu n l cng su t
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C c d ch v MACD ch v c nh tranh: Best effort (khng tin cy)
D ch v khng c nh tranh (contention-free): truy n gi i h n theo th igianMAC c th thay i gia Contention Periods (CPs) & Contention-Free Periods (CFPs)
Physical
Distribution coordination function(CSMA-CA)
Point coordinationfunction
Contention-free service
Contentionservice
MAC
MSDUs MSDUs
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Distributed Coordination Function (DCF)
DCF cung c p d ch v truy nh p c bnTruy n d li u best-effort khng ng b T t c cc tr m c nh tranh truy nh p mi trng
CSMA-CATr m s n s ng pht i khi k t th c truynT t c cc tr m ph i i m t khong Interframe Space (IFS)
DIFS
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access Wait forreattempt time
Time
Busy medium
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G n u tin qua Interframe Spacing
Cc frame u tin cao i kho ng th i gian ng n ShortIFS (SIFS)
Th ng k t th c trao i trao i d li u hi n ti ACKs, CTS, data frames c a cc MSDU phn o n etc .
PCF IFS (PIFS) kh i t o Contention-Free PeriodsDCF IFS (DIFS) pht d li u v& MPDUs
DIFS
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access Wait forreattempt time
Time
Busy medium
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Contention & Backoff Behavior
N u knh r i sau DIFS period, cc tr m s n s ng c thtruy n m t MPDU ban uN u knh b n tr c DIFS, tr m c n nh tr nh l i thigian backoff th l i
Backoff period l m t s nguyn l n cc timeslot c nh tranh riTr m ang ch pht gim st mi tr ng v gi m th i gian chm i khi c m t khe c nh tranh r i xy raTr m c th c nh tranh khi th i gian backoff k t thc
M t tr m v a k t th c truy n m t frame khng ctruy n ngay
Ph i th c hi n th tc backoff
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RTS
CTS CTS
Data Frame
A requests to send
B
C
A
A sends
B
B
C
C remains quiet
B announces A ok to send
(a)
(b)
(c)
ACK B(d)
ACK
B sends ACK
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Carrier Sensing in 802.11
C m nh n S ng mang V t l (Physical Carrier Sensing) Phn t ch t t c cc frame tch cGim st c ng t n hi u tng i t cc tr m khc
C m nh n S ng mang o (Virtual Carrier Sensing) l pMAC
Cc tr m ngu n thng bo cho cc tr m khc v th i giantruy n ( msec) cho m t MPDUThng tin v th i gian truy n c mang trngDuration c a RTS & CTSCc tr m i u ch nh Network Allocation Vector (tham s m ng c) bi t khi n o knh ri
Knh b n khi m t trong hai c m nh n cho bi t bn
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DataDIFS
SIFS
Defer AccessWait for
Reattempt Time
ACK
DIFS
NAV
Source
Destination
Other
Truy n MPDU khng s d ng RTS/CTS
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Data
SIFS
Defer access
Ack
DIFSNAV (RTS)
Source
Destination
Other
RTSDIFS
SIFS
CTS
SIFS
NAV (CTS)
NAV (Data)
Truy n MPDU c s d ng RTS/CTS
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Va ch m, M t khung & L i
Collision AvoidanceKhi tr m c m nh n knh b n, n i n t n khi knh r i chom t chu k DIFS & sau b t u t th i gian backoff ng unhin (theo n v slot ri)Tr m truy n frame khi th i gian backoff k t thcN u c va ch m x y ra, t nh l i backoff qua kho ng th i gian d ig p i
Cc tr m thu cc frame khng c l i truyn ACKTr m pht coi tr ng h p ACK khng n nh l mt khungTi n h nh backoff v sau truy n liCc tr m thu s d ng s th t xc nh cc frame thu p
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Point Coordination Function
PCF cung c p d ch v nh h ng k t n i,khng c nh tranh thng qua polling Point coordinator (PC) AP th c hi n PCFPolling table up to implementor
Chu k l p li CFP (Contention-Free Period)Xc nh t n xu t CFP xy raKh i t o b i beacon frame pht t PC APCh a CFP v CPTrong chu k n y cc tr m CFP ch c th truy n png l i m t poll t PC hay truyn ACK
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CFEnd
NAV
PIFS
B D1 +Poll
SIFS
U 1 +ACK
D2+Ack+Poll
SIFS SIFS
U 2 +ACK
SIFS SIFS
Contention-free repetition interval
Contention period
CF_Max_duration
Reset NAV
D1, D2 = frame sent by point coordinatorU1, U2 = frame sent by polled stationTBTT = target beacon transmission timeB = beacon frame
TBTT
PCF Frame Transfer
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C c ki u Frame
Management FrameStation association & disassociation with APTiming & synchronization
Authentication & deauthentication
Control framesHandshaking
ACKs during data transfer
Data framesData transfer
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Address2
FrameControl
Duration/ID
Address1
Address3
Sequencecontrol
Address4
Framebody CRC
2 2 6 6 6 2 6 0-2312 4MAC header (bytes)
Frame Structure
MAC Header: 30 bytesFrame Body: 0-2312 bytesCRC: CCITT-32 4 bytes CRC over MAC header
& frame body
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Address2
FrameControl
Duration/ID
Address1
Address3
Sequencecontrol
Address4
Framebody CRC
Protocolversion Type Subtype
ToDS
FromDS
Morefrag Retry
Pwrmgt
Moredata WEP Rsvd
2 2 6 6 6 2 6 0-2312 4
2 2
MAC header (bytes)
4 1 1 1 1 1 1 1 1
Frame Control (1)
Protocol version = 0
Type: Management (00), Control (01), Data (10)Subtype within frame typeType=00, subtype=association; Type=01, subtype=ACKMoreFrag=1 if another fragment of MSDU to follow
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ToDS
FromDS
Address1
Address2
Address3
Address4
0 0 DestinationaddressSourceaddress BSSID N/A
0 1 Destinationaddress BSSID Sourceaddress N/A
1 0 BSSID SourceaddressDestination
address N/A
1 1 ReceiveraddressTransmitter
addressDestination
addressSourceaddress
Meaning
Data frame from station tostation within a BSS
Data frame exiting the DS
Data frame destined for theDS
WDS frame being distributedfrom AP to AP
Address2
FrameControl
Duration/ID
Address1
Address3
Sequencecontrol
Address4
Framebody CRC
Protocolversion Type Subtype
ToDS
FromDS
Morefrag Retry
Pwrmgt
Moredata WEP Rsvd
2 2 6 6 6 2 6 0-2312 4
2 2 4 1 1 1 1 1 1 1 1
To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS
Frame Control (2)
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Address2
FrameControl
Duration/ID
Address1
Address3
Sequencecontrol
Address4
Framebody CRC
Protocolversion Type Subtype
ToDS
FromDS
Morefrag Retry
Pwrmgt
Moredata WEP Rsvd
2 2 6 6 6 2 6 0-2312 4
2 2
MAC header (bytes)
4 1 1 1 1 1 1 1 1
Frame Control (3)
Retry=1 if mgmt/control frame is a retransmissionPower Management used to put station in/out of sleepmodeMore Data =1 to tell station in power-save mode moredata buffered for it at APWEP=1 if frame body encrypted
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Physicallayer
LLC
Physical layerconvergence
procedure
Physical mediumdependent
MAClayer
PLCPpreamble
LLC PDU
MAC SDUMACheader CRC
PLCPheader
PLCP PDU
Physical Layers
802.11 designed toSupport LLCOperate over many physical layers
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IEEE 802.11b Channels
Channels khng gy nhi u
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IEEE 802.11 Physical Layer Options
FrequencyBand Bit Rate Modulation Scheme
802.11 2.4 GHz 1-2 Mbps Frequency-Hopping SpreadSpectrum, Direct SequenceSpread Spectrum
802.11b 2.4 GHz 11 Mbps Complementary Code Keying& QPSK
802.11g 2.4 GHz 54 Mbps Orthogonal Frequency DivisionMultiplexing
& CCK for backwardcompatibility with 802.11b
802.11a 5-6 GHz 54 Mbps Orthogonal Frequency DivisionMultiplexing
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Chapter 6 Medium Access Control
Protocols and Local AreaNetworks
LA N Br idges
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Hub
Station Station Station
Two TwistedPairs
Hubs, Bridges & Routers
Hub: Active central element in a star topologyTwisted Pair: inexpensive, easy to insallSimple repeater in Ethernet LANsIntelligent hub: fault isolation, net configuration, statistics Requirements that arise:
Hub
Station Station Station
Two TwistedPairs
User community grows, need to interconnect hubs
?
Hubs are for different types of LANs
Hub
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Hub
Station Station Station
Two TwistedPairs
Hubs, Bridges & RoutersInterconnecting Hubs
Repeater: Signal regeneration All traffic appears in both LANs
Bridge: MAC address filteringLocal traffic stays in own LAN
Routers: Internet routing
All traffic stays in own LAN
Hub
Station Station Station
Two TwistedPairs
?
HigherScalability
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Operation at data link level implies capability to workwith multiple network layersHowever, must deal with
Difference in MAC formatsDifference in data rates; buffering; timersDifference in maximum frame length
PHY
MAC
LLC
Network Network
PHY
MAC
LLC
802.3 802.3 802.5 802.5
802.3
802.3
802.3 802.5
802.5
802.5
CSMA/CD Token Ring
General Bridge Issues
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Bridge
Network
Physical
Network
LLC
PhysicalPhysicalPhysical
LLC
MAC MACMAC MAC
Bridges of Same Type
Common case involves LANs of same typeBridging is done at MAC level
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Interconnection of IEEE LANs withcomplete transparencyUse table lookup, and
discard frame, if source & destinationin same LANforward frame, if source & destinationin different LANuse flooding, if destination unknown
Use backward learning to build tableobserve source address of arrivingLANs
handle topology changes by removingold entries
Transparent Bridges
Bridge
S1 S2
S4
S3
S5 S6
LAN1
LAN2
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B1
S1 S2
B2
S3 S4 S5
Port 1 Port 2 Port 1 Port 2
LAN1 LAN2 LAN3
Address Port Address Port
S
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B1
S1 S2
B2
S3 S4 S5
Port 1 Port 2 Port 1 Port 2
LAN1 LAN2 LAN3
Address Port
S1 1
Address Port
S1 1
S1 S5
S1 to S5 S1 to S5 S1 to S5 S1 to S5
S
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B1
S1 S2
B2
S3 S4 S5
Port 1 Port 2 Port 1 Port 2
LAN1 LAN2 LAN3
Address Port
S1 1S3 1
Address Port
S1 1S3 1
S3 S2
S3 S2S3 S2 S3 S2
S3 S2 S3 S2
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B1
S1 S2
B2
S3 S4 S5
Port 1 Port 2 Port 1 Port 2
LAN1 LAN2 LAN3
S4 S3
Address Port
S1 1S3 2S4 2
Address Port
S1 1S3 1S4 2
S4 S3
S4 S3
S4 S3S4 S3
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B1
S1 S2
B2
S3 S4 S5
Port 1 Port 2 Port 1 Port 2
LAN1 LAN2 LAN3
Address Port
S1 1S3 2S4 2S2 1
Address Port
S1 1S3 1S4 2
S2 S1
S2 S1
S2 S1
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Adaptive Learning
In a static network, tables eventually store alladdresses & learning stopsIn practice, stations are added & moved all thetime
Introduce timer (minutes) to age each entry & force itto be relearned periodicallyIf frame arrives on port that differs from frame address& port in table, update immediately
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Avoiding Loops
LAN1
LAN2
LAN3
B1 B2
B3
B4
B5
LAN4
(1)
(2)
(1)
l h
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Spanning Tree Algorithm
1. Select a root bridge among all the bridges. root bridge = the lowest bridge ID.
2. Determine the root port for each bridge except the rootbridge
root port = port with the least-cost path to the root bridge3. Select a designated bridge for each LAN
designated bridge = bridge has least-cost path from the LANto the root bridge.
designated port connects the LAN and the designated bridge4. All root ports and all designated ports are placed into a
forwarding state. These are the only ports that areallowed to forward frames. The other ports are placedinto a blocking state.
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LAN1
LAN2
LAN3
B1 B2
B3
B4
B5
LAN4
(1)
(2)
(1)
(1)
(1)
(1)
(2)
(2)
(2)
(2)
(3)
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LAN1
LAN2
LAN3
B1 B2
B3
B4
B5
LAN4
(1)
(2)
(1)
(1)
(1)
(1)
(2)
(2)
(2)
(2)
(3)
Bridge 1 selected as root bridge
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LAN1
LAN2
LAN3
B1 B2
B3
B4
B5
LAN4
(1)
(2)
(1)
(1)
(1)
(1)
(2)
(2)
(2)
(2)
(3)
Root port selected for everybridge except root port
R
R
R
R
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LAN1
LAN2
LAN3
B1 B2
B3
B4
B5
LAN4
(1)
(2)
(1)
(1)
(1)
(1)
(2)
(2)
(2)
(2)
(3)
Select designated bridgefor each LAN
R
R
R
R
D
D
D D
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LAN1
LAN2
LAN3
B1 B2
B3
B4
B5
LAN4
(1)
(2)
(1)
(1)
(1)
(1)
(2)
(2)
(2)
(2)
(3)
All root ports & designatedports put in forwarding state
R
R
R
R
D
D
D D
S R ti B id
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Source Routing Bridges
To interconnect IEEE 802.5 token ringsEach source station determines route todestinationRouting information inserted in frame
Routingcontrol
Route 1designator
Route 2designator
Route mdesignator
Destinationaddress
Sourceaddress
Routinginformation
Data FCS
2 bytes 2 bytes 2 bytes 2 bytes
R t Di
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Route Discovery
To discover route to a destination each stationbroadcasts a single-route broadcast frame Frame visits every LAN once & eventuallyreaches destination
Destination sends all-routes broadcast framewhich generates all routes back to sourceSource collects routes & picks best
D t il d R t Di
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Detailed Route Discovery
Bridges must be configured to
form a spanning treeSource sends single-route frame without route designator fieldBridges in first LAN add incomingLAN #, its bridge #, outgoing LAN# into frame & forwards frameEach subsequent bridge attachesits bridge # and outgoing LAN #Eventually, one single-route framearrives at destination
When destination receives single-
route broadcast frame it respondswith all-routes broadcast framewith no route designator fieldBridge at first hop inserts incomingLAN #, its bridge #, and outgoingLAN # and forwards to outgoingLAN
Subsequent bridges insert theirbridge # and outgoing LAN # andforwardBefore forwarding bridge checksto see if outgoing LAN already indesignator fieldSource eventually receives all
routes to destination station
Fi d t f S1 t S3
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B4
B6
B3 B7LAN 1
B1
B2
S1S2
S3
B5
LAN 2 LAN 4
LAN 3 LAN 5
Find routes from S1 to S3
LAN1 B1
B3
B4
LAN3 B6 LAN5
LAN4
LAN2
B4LAN 2 LAN 4
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B4
B6
B3 B7LAN 1
B1
B2
S1S2
S3
B5
LAN 2 LAN 4
LAN 3 LAN 5
LAN5
B6
B7
LAN3
LAN4
B2
B3
B5
LAN1 B1 LAN2B3
B4 LAN4 B5B7
LAN2 B1B4
LAN1 B2LAN4 B5
B7
LAN4 B4
B7
LAN2 B1B3
LAN1 B2
B4
B5
LAN2 B1
B3 LAN3B2B5B6
LAN1 B1LAN1 B2 LAN3
B3
B5B6
LAN3 B3B2
B6
LAN1
LAN2
B1 LAN2B3B4
B1B4
LAN1 B2
Virtual LAN
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Physicalpartition
Logical partition
Bridge
or
switch
VLAN 1 VLAN 2 VLAN 3
S17
2 3 4 5 61
8
9 Floor n 1
Floor n
Floor n + 1
S2
S3
S4
S5
S6
S7
S8
S9
Virtual LAN
Per Port VLANs
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Logical partition
Bridge
or
switch
VLAN 1 VLAN 2 VLAN 3
S172 3 4 5 61
8
9 Floor n 1
Floor n
Floor n + 1
S2
S3
S4
S5
S6
S7
S8
S9
Per-Port VLANs
Bridge only forwards frames to outgoing ports associated with same VLAN
Tagged VLANs
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Tagged VLANs
More flexible than Port-based VLANsInsert VLAN tag after source MAC address ineach frame
VLAN protocol ID + tag
VLAN-aware bridge forwards frames to outgoingports according to VLAN IDVLAN ID can be associated with a port staticallythrough configuration or dynamically through
bridge learningIEEE 802.1q