Capitolul 4. Subnivelul Controlul Accesului la Mediu (MAC=Medium Access Control)

Transcription

1 Capitolul 4 Subnivelul Controlul Accesului la Mediu (MAC=Medium Access Control) 1

2 The Channel Allocation Problem Static Channel Allocation in LANs and MANs Dynamic Channel Allocation in LANs and MANs Alocare statica: - cu multiplexare cu divizarea frecventei ( N utilizatori, banda de frecventa impartita în N canale) - cu multiplexare cu divizarea timpului (se alocă fiecărui untilizator, pe rând, câte o durată de timp) Alocare dinamică - ALOHA pur transmite fiecare când doreşte, timpul e considerat continuu, cadrele implicate în coliziuni se retransmit eficienţă 0,2, - ALOHA cuantificat timpul e împărţit în intervale discrete de lungimea unui cadru, cadrele au lungime fixă, începutul transmisiei se poate face doar la începutul unui interval de timp, cadrele implicate în coliziuni se retransmit eficienţă 0,37, -CSMA Carrier Sense Multiple Access acces multiplu cu detecţie de purtătoare staţiile ascultă linia pentru a detecta o purtătoare (o transmisie), Dacă alcineva transmite se reia transmiterea cadrului după un timp aleator. Eficienţa creşte pe măsură ce creşte timpul. -CSMA-1 persistent staţia ascuultă linia, dacă e liberă transmite un cadru, dacă în timpul transmisiei cadrului apare o coliziune, cadrul e retransmis după un timp aleator. I se spune 1 persistent deoarece probabilitatea de a transmire un cadru atunci când nu e purtătoare este 1. Eficienţa 0,5 -CSMA-p persistent, p este probabilitatea de a transmite un cadru atunci când nu se detectează purtatoare. Cu timpul eficienţa e mai mare decât la CSMA nepersistent. -CSMA/CD - Carrier Sense Multiple Access with Collision Detection este un CSMA daor că staţiile se opresc imediat ce sesizează o coliziune nu transmit întregul cadru supus coliziunii de vreme ce pot detecta coliziunea încă de la începutul cadrului. 2

3 Dynamic Channel Allocation in LANs and MANs 1. Station Model. 2. Single Channel Assumption. 3. Collision Assumption. 4. (a) Continuous Time. (b) Slotted Time. 5. (a) Carrier Sense. (b) No Carrier Sense. 3

4 Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols (Protocoale fără coliziune) Limited-Contention Protocols (Protocoale cu conflict limitat) Wavelength Division Multiple Access Protocols (Protocoale cu acces multiplu cu divizarea frecvenţei) Wireless LAN Protocols 4

5 Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times. 5

6 Pure ALOHA (2) Vulnerable period for the shaded frame. 6

7 Pure ALOHA (3) Throughput versus offered traffic for ALOHA systems. 7

8 Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols. 8

9 CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle. 9

10 Wireless LAN Protocols A wireless LAN. (a) A transmitting. (b) B transmitting. Raza acoperită depinde de puterea fiecărei staţii şi de poziţia relativă a transmiţătorului 10

11 Wireless LAN Protocols (2) The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A. MACA Multiple Access with colizion Avoidance acces mutiplu cu evitarea coliziunii. A doreşte să transmită un cadru lui B 11

12 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 12

13 Ethernet Cabling The most common kinds of Ethernet cabling. 13

14 Ethernet Cabling (2) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T. 14

15 Ethernet Cabling (3) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. 15

16 Ethernet Cabling (4) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. 16

17 Ethernet MAC Sublayer Protocol Frame formats. (a) DIX Ethernet, (b) IEEE Preambul 7 octeţi de forma Prin codare Manchester aceasta conduce la o undă dreptunghiulară de 10 MHz timp de 5,6 microsecunde. SOF Start of Frame un octet de forma care marchează începutul propriu-zis al cadrului. Lengh câţi octeţi se găsesc în câmpul de date PAD câmp de completare, DATA + PAD minimum 46 octeţi. 17

18 Ethernet MAC Sublayer Protocol (2) Collision detection can take as long as 2 τ. 18

19 Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times. 19

20 Switched Ethernet A simple example of switched Ethernet. 20

21 Fast Ethernet The original fast Ethernet cabling. 21

22 Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet. 22

23 Gigabit Ethernet (2) Gigabit Ethernet cabling. 23

24 IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats. LLC - Logical Link Control realizează şi acceptă un format unic de la şi spre nivelul reţea indiferent de ce tip de protocol 802 este utilizat sub el 24

25 Wireless LANs The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Services 25

26 The Protocol Stack Part of the protocol stack. 26

27 The MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem. 27

28 The MAC Sublayer Protocol (2) The use of virtual channel sensing using CSMA/CA. 28

29 Broadband Wireless Comparison of and The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure 29

30 The Protocol Stack The Protocol Stack. 30

31 The Physical Layer The transmission environment. 31

32 The Physical Layer (2) Frames and time slots for time division duplexing. 32

33 Bluetooth Bluetooth Architecture Bluetooth Applications The Bluetooth Protocol Stack The Bluetooth Radio Layer The Bluetooth Baseband Layer The Bluetooth L2CAP Layer The Bluetooth Frame Structure 33

34 Bluetooth Architecture Two piconets can be connected to form a scatternet. 34

35 Bluetooth Applications The Bluetooth profiles. 35

36 The Bluetooth Protocol Stack The version of the Bluetooth protocol architecture. 36

37 The Bluetooth Frame Structure A typical Bluetooth data frame. 37

38 Data Link Layer Switching Bridges from 802.x to 802.y Local Internetworking Spanning Tree Bridges Remote Bridges Repeaters, Hubs, Bridges, Switches, Routers, Gateways Virtual LANs 38

39 Data Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN. 39

40 Bridges from 802.x to 802.y Operation of a LAN bridge from to

41 Bridges from 802.x to 802.y (2) The IEEE 802 frame formats. The drawing is not to scale. 41

42 Local Internetworking A configuration with four LANs and two bridges. 42

43 Spanning Tree Bridges Two parallel transparent bridges. 43

44 Spanning Tree Bridges (2) (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree. 44

45 Remote Bridges Remote bridges can be used to interconnect distant LANs. 45

46 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers. 46

47 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) (a) A hub. (b) A bridge. (c) a switch. 47

48 Virtual LANs A building with centralized wiring using hubs and a switch. 48

49 Virtual LANs (2) (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches. 49

50 Summary Channel allocation methods and systems for a common channel. 50