Medium Access Control. CSCI370 Lecture 5 Michael Hutt New York Institute of Technology

Transcription

1 Medium Access Control CSCI370 Lecture 5 Michael Hutt New York Institute of Technology

2 The Data Link Layer

3 Logical Link Control (LLC) IEEE Standard RFC 1042 Provides three service options Unreliable datagram service Acknowledged datagram service Reliable connection-oriented service

4 Medium Access Control (MAC) Layer 2 addressing (MAC address or physical address) Channel access control mechanisms for multiaccess channels Determines who goes next on a multiaccess channel

5 Multiple Access Protocols Pure Aloha Transmit whenever data is available to be sent Does not listen before transmitting Listen to find out if frame was destroyed (collision) Slotted Aloha Forces all workstations to agree on time slots Can t start sending in the middle of a time slot Improves throughput to 2x of pure Aloha

6 Multiple Access Protocols Persistent Carrier Sense Multiple Access First listen to see if anyone else is transmitting If collision detected wait random amount of time and start over Propagation delay can have on performance Nonpersistent CSMA Listen first, if no one is sending, begin sending If channel is busy, wait a random period before listening again Better channel utilization, but longer delay

7 Multiple Access Protocols CSMA with Collision Detect CSMA/CD Terminate transmission immediately upon detecting a collision Basis for Ethernet Need to transmit for 2T, where T is the propagation delay between the 2 farthest workstations Transmitting stations must constantly monitor the channel (receive) for collisions - inherently half-duplex

8 Wireless LAN Protocols Can t use CSMA since we need to detect interference at the receiver Can t reliably detect all transmissions due to range of radios Multiple Access with Collision Avoidance (MACA) Force a short transmission from the receiver so stations around the receiver avoid transmitting A sends RTS frame to B. B replies with CTS Stations close to A hear the RTS Stations close to B hear the CTS MACW (MACA for Wireless) Added CSMA for transmitters, and ACKs

9 Ethernet Ethernet Cabling Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm Ethernet Performance Switched Ethernet Data Link Layer Switching Virtual LANs

10 Ethernet Cabling

11 Manchester Encoding

12 Ethernet MAC Sublayer Protocol

13 Ethernet MAC Sublayer Protocol Ethernet II (DIX) Used for IP (Raw) - no type field with with SNAP Used for IP, Appletalk, CDP and other proprietary protocols

14 Ethernet MAC Sublayer Protocol

15 Binary Exponential Backoff Algorithm After n collisions wait somewhere between 0 and 2^n-1 time slots Time slot = 51.2µsec (time it takes to transmit 512 bits) After 16 collisions failure is reported to higher layers

16 Ethernet Performance

17 Switched Ethernet Multiport bridge Each port is in its own collision domain Ports can now operate full duplex Switch disables collision detection in full duplex mode

18 Bus Topology

19 Bridging

20 Spanning-Tree Protocol

21 Bridges Store and forward Buffers the frame and checks the CRC, then forwards to destination port Cut Through DA MAC address appears first. Forward frame immediately to destination port. No CRC check.

22 Bridges

23 Layer 2 Switch

24 Network Devices Repeaters - L1 Hubs - L1 Bridges - L2 Switches - L2 Routers - L3/L4 Gateways - L4/L7

25 Network Devices

26 Virtual LANs

27 802.1Q

28 802.1Q

29 Wireless LANs IEEE Standard a - 5 GHz, 54 Mbps (Net Bit Rate) b GHz, 11 Mbps g GHz, 54 Mbps n - 5 GHz and/or 2.4 GHz, 600 Mbps

30 The Protocol Stack

31 The MAC Sublayer Protocol a) the hidden station problem b) the exposed station problem

32 The MAC Sublayer Protocol

33 The Frame Structure

34 Wi-Fi Security WEP - no longer secure WPA (TKIP) 8/27/09 Computer scientists in Japan say they've developed a way to break the WPA encryption system used in wireless routers in about one minute. WPA2 (AES)