IEEE 802.11 ?? LAN

1
IEEE 802.11 ?? LAN

• ?? LANs ???? (??????) ????
• IEEE 802.11 ??
• MAC ??
• ?????????(US) 900Mhz, 2.4Ghz

• ????? (BSS) ??
• ????
• ??? (AP),??
• ?BSS?????? (DS)
• ???(portal)?????

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????(Ad Hoc Networks)

• ???? IEEE 802.11 ???????AP???????
• ??
• ????????????? ??
• ??????????????
• ???

3
????????????

• ??? A, C??????????
• ???, ????
• ?B??????
• 802.11MAC????? ?????B??????
• CSMA/CA ??????(Collision Avoidance)???CSMA

4
802.11 MAC? CSMA/CA

• 802.11 CSMA ???
• - ??????????DISF ?.
• ??????? (?????)
• -???????? ?? ???????(binary bakeoff)
• 802.11 CSMA ???
• ???? OK
• ?? SIFS???ACK

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IEEE 802.11 MAC ??

• 802.11 CSMA ?? ???
• NAV ??????(Network Allocation Vector)
• 802.11 ?????????
• ????(??????) ?????? NAV????????????

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???? RTS-CTS ??

• CSMA/CA ?????????
• ??? ????? RTS request to send
• ??? ????? CTS clear to send??
• CTS ?????????, ?????? (??????) ??
• ??????????

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???? RTS-CTS ??

• ??RTS / CTS???
• ?????,????????
• ??????????????
• IEEE 802.11 ??
• CSMA
• CSMA/CA ????
• ?AP????

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??????????(DLC)

• ???,???,?????????????
• ????????
• ????MAC??
• e.g., ????, ISDN ???
• ??????DLC??
• SLIP (Serial Line Internet Protocol)(p89)
• PPP (point-to-point ??)
• SDLC Synchronous Data Link Control
  (SNA????????????)
• HDLC High level data link control (ISO????????)

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PPP ???? RFC 1557

• ???? ?????????????????
• ?????????????????? (???? IP)
• ?????????
• ???? ??????,????????????
• ???? (????)
• ??????? ???????/?????????

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PPP ??????

• ?????/??
• ?????
• ??????
• ???????? (e.g., ??)

???????????????? ???????????!
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PPP ???

• Flag???(??)
• Address ?? (??????)
• Control ?? ?????????????
• Protocol ??????????? (eg, PPP-LCP, IP, IPCP,
  etc)

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PPP ???

• info ????????
• check CRC ??????

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????(Byte Stuffing)

• ???? ?? ????????????flag???????lt01111110gt
• Q ?????? lt01111110gt???????flag?
• ??? ???lt01111110gt????????????lt01111110gt
• ???
• ???????01111110 ? ???????,??????
• ??01111110 flag ??

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????
flag ??????????
flag ??????????????
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PPP ??????

• ??????????, ???????????
• ?? PPP ??(?????, ???)
• ??/???? (p92)
• ?IP?? ?? IP ???? (IPCP) ?? (protocol field
  8021) ??????/???IP ??

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ATM ??

• 1980s/1990s standard?? ???? (155Mb/s to 622 Mb/s
  ??) Broadband Integrated Service Digital Network
  (B-ISDN)????
• ?? ???, ???? voice, video, data??
• ??voice, video??? timing/QoS ?? (?? Internet
  best-effort ??)
• ??? ???? ???????telephone world
• ???? (??????, ?? cells-??) ?????

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ATM ????

• adaptation layer(???) ???ATM??????
• ????/??(data segmentation/reassembly)
• ?????Internet transport layer
• ATM ? ?? ?
• ????,????
• ???

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ATM ??? ? ??? ?

• ??(Vision)
• ?????? ATM??????
• ATM ???????
• ??(Reality) ????IP?????
• IP over ATM
• ATM ??????, ?? IP ???

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ATM ??? (AAL)

• ATM Adaptation Layer (AAL)????,??????(IP or ATM
  ???????)????( ATM ? )?? ??
• AAL ????????, ??????????
• AAL????? (??/????, ??)????????ATM ??
• ?? TCP?????????IP??

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ATM ??? (AAL) ?

• ?????AAL?,???ATM?????
• AAL1 ?? CBR (Constant Bit Rate,??????) ??, e.g.
  ??????(circuit emulation)
• AAL2 ??VBR (Variable Bit Rate,??????) ??, e.g.,
  MPEG video
• AAL5 ??UBR,???? (eg, IP ??)

User data
AAL PDU
ATM cell
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AAL5 - Simple And Efficient AL (SEAL)????????

• AAL5 ???,????? IP ??
• 4 ???CRC
• PAD ????????(payload)? 48??????
• ???AAL5????????? 48-byte ATM cells

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ATM ?

• ?? ?????ATM??????
• ?? IP ???
• ???????? IP???????

Guarantees ?
Network Architecture Internet ATM ATM ATM ATM
Service Model best effort CBR VBR ABR UBR
Congestion feedback no (inferred via
loss) no congestion no congestion yes no
Bandwidth none constant rate guaranteed rate gua
ranteed minimum none
Loss no yes yes no no
Order no yes yes yes yes
Timing no yes yes no no
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ATM? ???

• VC?? ???????? VC???
• ?????????????,????
• ?????? VC??? (??????ID)
• ?????????????????????????
• ??,???? (??,??) ??????VC ????????????
• ??????(PVCs)
• ???????
• ?? ?IP???????????
• ?????? (SVC)
• ?????????????

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ATM ???

• ATM VC ???
• ???QoS ??????????VC? (??, ??, ????-delay jitter)
• ATM VC ???
• ???????????(???)
• ????/?????PVC??N2 ??
• SVC????????(call setup)??,???????????????

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ATM ? ATM cell(??)

• 5??ATM????
• 48???????(payload)
• Why? ???payload -gt ????????,??????????
• ???32? 64?? (??!)

Cell header
Cell format
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ATM ????

• VCI virtual channel ID(?????)
• ??????????????
• PT Payload type (????)(e.g. RM cell vs. data
  cell)
• CLP Cell Loss Priority bit(???????)
• CLP 1 ?????????, ????????
• HEC Header Error Checksum(??????)
• cyclic redundancy check(CRC)

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ATM ???

• ? ? (??) ??????
• Transmission Convergence Sublayer (TCS)-??????
  ?ATM ?? PMD ???????
• Physical Medium Dependent(????????) ??????????
• ??????(TCS)??
• ??????? 8? CRC
• ????
• ?? ????? PMD??, ??????????????? ,??????

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ATM ???

• ?????? (PMD) ??
• SONET/SDH ????? (???????)
• ???
• ???? (TDM)
• ????? OC1 51.84 Mb/s OC3 155.52 Mb/s OC12
  622.08 Mb/s
• T1/T3(????) ????? (????????????) 1.5 Mb/s 45
  Mb/s
• ????(unstructured) ????(?/?)

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IP-Over-ATM
IP over ATM ?ATM?????? ?? (e.g., LAN ??)
ATM??, IP ??

• ??? IP
• 3??? (e.g., LAN ??)
• MAC (802.3) ? IP ??

ATM network
Ethernet LANs
Ethernet LANs
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IP-Over-ATM
?? IP????ATM AAL5 PDU ?IP????? ATM ?? ??IP?????
802.3 MAC ??!
ATM network
Ethernet LANs
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? IP-over-ATM????????

• ?????
• IP? (??ARP) ??IP?????ATM???????
• ?????AAL5
• AAL5 ?????????,????ATM?
• ATM?? ?VC???????
• ?????
• AAL5 ????????????
• ?? CRC OK, ?????? IP?

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ATM???ARP(??????)

• ATM???????ATM??
• ???????????????
• IP/ATM?????????ATM ARP (Address Resolution
  Protocol)??
• ATM????ARP ?????????????ATM ARP?????
• ????????????ATM?????????

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X.25 ????( Frame Relay)

• ??ATM
• ??????
• ?????
• ??? telephony world
• ???? IP??
• ?????,??????IP??????

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X.25

• X.25 ???????????????VC??
• ?????????????(??,???,????)
• ?????????LAP-B???????? (??????)
• ???HDLC?????
• ????????( credits )??????
• ?????????????????????
• ????????( back pressure )????

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IP vs. X.25

• X.25 ?????????????????
• ??????(intelligence in the network)
• IP ????????, ??????????
• ???????(intelligence in the endpoints)
• IP over X.25(p148)
• X.25?????IP??????

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???(Frame Relay)

• ???1980s?, ?????1990s, ???????????????X.25???
• Frame relay ??
• ?????
• ??????????

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??? (?)

• ??????????LANs????
• ?????? VCs ?????????????????? ????
• ??VCs ??????,?? ATM
• ????????Frame Relay???? FR ?? (eg, Sprint,
  ATT,????)

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??? (?)

• Flag ?, 01111110, ???
• address
• 10 ? VC ID ??
• 3 ?????
• FECN forward explicit congestion notification
  (???????????)
• BECN congestion on reverse path
• DE discard eligibility(????)

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??? VC????(p153)

• Committed Information Rate (CIR)-??????
• ??, ??? ?? VC???
• ?VC??????????
• ???? CIR??
• DE? Discard Eligibility bit (????)
• ???FR ??????VC???? ??DE?
• DE 0 ???, ????????????
• DE 1 ???, ????????

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??? -??????(CIR) ???

• ???? R?(???)?????(????)FR???????????? 64Kbps lt
  R lt 1,544Kbps
• ??,??VC (???????????) ???????????? ??VC???? CIR
• FR?????????VC????? ????( marks)
• (ie DE gt 1) ???CIR?? (????????????)

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????

• ???????
• ????, ??
• ?????? ?????????
• ???????????????, ARP
• ????????
• Ethernet
• hubs, bridges, switches
• IEEE 802.11 LANs
• PPP
• ATM
• X.25, Frame Relay,DDN