Channel Allocation

1. Channel Allocation

2. Channel Allocation Problem Multiple Access Protocols

3. Two types of transmission technology Point-to-Point Link Broadcast Link

4. Channel AllocationProblem Static Method Dynamic Method

5. LANs: channel allocation • Allocation? Who goes next? • Static: assignment for long duration • Dynamic: stations continuously compete

6. LANs: channel allocation • Static channel allocation • Scheme: • After allocation: • Division of channel • Request channel • Data Transfer • Release channel • Channel is private • Can be used for a long time • FDM: Frequency division multiplexing • TDM: time division multiplexing Data link layer -- June 2004

7. 1 T =  . C -  LANs: channel allocation • Static channel allocation • Simple and efficient • Small & fixed number of users • Heavy load • Problems • Loss of bandwidth • If some users are quiescent • If less users than subchannels • Poor performance

8. LANs: MA Protocols Multiple Access protocols • University of Hawaii Aloha system • Central computer system • Terminals spread over 4 islands • Communication: FM radio • No station to station communication • Shared channel for communication from terminals to computer system • Transmission strategy: • Terminal sends data as data comes available • Collision possible • Retransmission if no ack received

9. ALOHA Idea: Users can transmit whenever they have data to send Aloha!! • Two types of ALOHA: • Pure ALOHA • Slotted ALOHA The main difference between them is time and time and time

10. frame Send & then wait for collision collision? wait a random time yes no end Pure ALOHA continuous time no global time synchronization

11. 2 frames sent at same time  collision both frames destroyed LANs: MA Protocols • Pure Aloha: frames transmitted at arbitrary times Contention system

12. LANs: MA Protocols • Pure Aloha: channel efficiency • Vulnerable period = 2 x packet time • Packet time = time required to transmit 1 frame

13. Slotted ALOHA 1. Time is divided to equal intervals (slots) 2. Need time synchronization 3. Frames can only be transmitted at starts of time slots

14. LANs: MA Protocols • Aloha: channel efficiency • Pure: max 18% • Slotted (time divided in slots; start sending at start of slot)

15. CSMA with Collision Detection (CSMA/CD) How? Idea: A station stop transmitting its frame immediately after a collision is detected to save time and bandwidth This is the basis of Ethernet LAN But: half-duplex

16. LANs: CSMA protocols Carrier Sense Multiple Access CSMA • Aloha type system + ability to test for a carrier i.e. a transmission • Persistent <> Nonpersistent • 1-persistent • If channel is idle, a frame is transmitted • If channel is busy, the channel is continuously checked • Nonpersistent • If channel is idle, a frame is transmitted • If channel is busy, a random time is waited before channel is sensed again • p-persistent (slotted channel only) • If channel is idle a frame is transmitted with probability p • If channel is busy, next slot is sensed again

17. LANs: CSMA protocols Carrier Sense Multiple Access CSMA • Performance

18. LANs: CSMA/CD protocols Carrier Sense Multiple Access Collision Detect • Strategy • Try to detect collisions asap • Listen while transmitting • If collision is detected, abort transmission • Channel model

19. LANs: CSMA/CD protocols Carrier Sense Multiple Access Collision Detect • Contention period • Worst case scenario • Detection = analog process

20. LANs: collision free protocols • Problem? • Very long, high bandwidth protocols • 1 km, 10 Mbps = 5 sec or 50 bits • Bit-Map protocol • Each contention period has N slots (N = #stations) • Station k is assigned slot k; is used to indicate if station k has to send data • Data transmission proceed without collisions • 20 km, 100 Mbps = 100 sec or 10.000 bits • 1 km, 10 Gbps = 5 sec or 50.000 bits

21. LANs: WDMA protocols Wavelength division multiple access • Approach: • Divide channel into subchannels(FDM,…) • Allocate them as needed • 2 channels/station • Narrow: used by other stations to signal the station: • Wide: used by station to output data frames Data link layer -- June 2004

22. LANs: wireless protocols • Common configuration for a wireless LAN • Base stations (access points) wired together • Notebooks with radio transmitter/receiver • A receiver within range of 2 active transmitters receives a garbled signal • Not all stations are in range of one another • CSMA does not work: interference at receiver is important not at sender • Example …

23. A sends to B If C senses medium, it will not hear A If C transmits to B, it will garble the signal at B Hidden station problem B sends to A If C senses the medium, it may falsely conclude it cannot send to D Exposed station problem LANs: wireless protocols

24. A wants to send to B A sends RTS (request to send) frame (short frame) with length of data frame B replies with CTS (clear to send) frame, containing same length LANs: wireless protocols Multiple Access with Collision Avoidance MACA Data link layer -- June 2004

25. A wants to send to B A sends RTS (request to send) frame (short frame) with length of data frame Stations hearing RTS should remain silent to not interfere with CTS B replies with CTS (clear to send) frame, containing same length Stations hearing CTS should remain silent to not interfere with data frame LANs: wireless protocols Multiple Access with Collision Avoidance MACA Collisions possible! Wait random time Binary exponential backoff! Data link layer -- June 2004

26. LANs: wireless protocols MACAW = MACA for Wireless • Optimisations for MACA • Introduce ACK from receiver of data frame to sender: detect loss in DL iso network/transport layer • Add CSMA: avoid sending a RTS by a station close to a station sending to the same destination • Run binary exponential backoff for each destination iso each station • Exchange of information between stations about congestion

27. LANs: IEEE 802.3 or Ethernet • Overview • 1-persistent CSMA/CD • When a station wants to transmit, it listens to the cable • If idle, it transmits immediately • If busy, it waits until the cable goes idle • If collision, it waits a random time • History • Real start: Aloha • 3 Mbps experiment at Xerox  ethernet • Agreement between Intel, DEC, Xerox • Base for IEEE 802.3

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

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

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

31. 802.3 MAC Frame 2 or 6 1 2 or 6 2 4 7 Destination Address Source Address Information FCS Pad Preamble Length SD Synch Start frame 64 to 1518 bytes • Destination address is either single address • or group address (broadcast = 111...111) • Addresses are defined on local or universal basis • 246 possible global addresses 0 Single address Group address 1 0 Local address 1 Global address

32. Ethernet Frame 2 or 6 1 2 or 6 2 4 7 Destination Address Source Address Information FCS Pad Preamble Type SD Start frame Synch 64 to 1518 bytes

33. Switched Ethernet A simple example of switched Ethernet.

34. Different between Hub and Switch • In a hub, a frame is passed along or "broadcast" to every one of its ports. It doesn't matter that the frame is only destined for one port. The hub has no way of distinguishing which port a frame should be sent to. Passing it along to every port ensures that it will reach its intended destination. This places a lot of traffic on the network and can lead to poor network response times.

35. Different between Hub and Switch • A switch, however, keeps a record of the MAC addresses of all the devices connected to it. With this information, a switch can identify which system is sitting on which port. So when a frame is received, it knows exactly which port to send it to, without significantly increasing network response times.

36.       Twisted Pair Ethernet hub Single collision domain (a) switch High-Speed Backplane or Interconnection fabric (b)    

37. Switched Ethernet • Basic idea: improve on the Hub concept • The switch learns destination locations by remembering the ports of the associated source address in a table. • The switch may not have to broadcast to all output ports. It may be able to send the frame onlyto the destination port. •  a big performance advantage over a hub, if more than one frame transfer can go through the switch concurrently.

38. Fast Ethernet The original fast Ethernet cabling.

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

40. Gigabit Ethernet • Full-duplex operation mode  switches • Collisions impossible: CSMA/CD not used • Maximum cable length determined by signal strength • Half -duplex operation mode  hubs • Collisions possible: CSMA/CD required • Cable length not to be reduced: 200m! • Solutions: • Carrier extension: hardware padding of frames to minimum of 512 bytes • Frame bursting: transmit concatenated sequence of frames in a single transmission

41. Wireless LANs – 802.11 • 2 modes: • In the presence of a wired base station – access point • In the absence of a base station – ad hoc networking • Make 802.11 compatible with ethernet

42. Quote of the day You can never cross the ocean until you have the courage to lose sight of the shore. –Christopher Columbus