Rahman 1. Application

Transcription

1 Data Link layer Overview of IEEE Application Presentation Session Transport LLC: On transmission, assemble data into a frame with address and CRC fields. On reception, disassemble frame, perform address recognition and CRC validation. MAC controls access to the physical channel according to a predetermined set of rules Network Datalink Physical Logical Link Control (LLC) Medium Access Control (MAC) IEEE Different PHY Medium Access Control (MAC) IEEE IEEE IEEE a b g n N 2.4/5 to N 2.4/5 to N 2.4/5 to Rahman 1

2 802.11N 2.4/5 to n 2.4/5 to Medium Access Control Wireless Medium: A Chaotic Environment Medium Access Control (MAC) IEEE IEEE IEEE IEEE a b g n The Design Challenges are... Medium access to shared medium Reservation-based Two approaches Noisy and unreliable wireless channel Contention-based... Collision detection is not possible always... Mobility changes neighbor set Rahman 2

3 Reservation-based Two approaches - TDMA (slotted access to channel) - requires tight coordination - requires time synchronization - grows with n - FDMA (Frequency is divided) - frequency could be wasted - effective bandwidth decreases with n Reservation-based Two approaches - TDMA (slotted access to channel) - FDMA (Frequency is divided) Contention-based - Works nicely for single shared wireless channel. - More suitable for ad hoc wireless network From cellular network to : IEEE Infrastructure based network Contention based MAC: IEEE Infrastructure based Without infrastructure IEEE with Access Point Hybrid Network Rahman 3

4 But we will discuss on ad hoc wireless networks No Access Point Coordination is more difficult IEEE Two modes of operation: DCF (Distributed Coordination Function) Appropriate for ad hoc wireless networks No controller PCF (Point Coordination Function) Appropriate for networks with AP AP controls medium access Inter Frame Spacing (IFS), Slot time, DIFS (from shortest to longest) (Short Inter frame space) Provides highest priority access to the channel Defined for short control messages For example between data and ACK DIFS (Distributed inter-frame space) Used by stations that are operating under the DCF mode. Some math: < Slot time < DIFS DIFS = + 2*Slot time Parameters at a glance (IEEE b Standard) DIFS duration 50 micro seconds Slot time 20 micro seconds duration 10 micro seconds Rules to consider for MAC protocols When should I access the channel What should I do if collision occurs How would I know whether my transmission was successful How can I ensure that I am sharing the channel fairly with my neighbors We will now see how these rules are set for IEEE Rule 1: When to access the channel? ALOHA - Transmit whenever you want (LIFE IS GOOD.. ;-) - Frequent chances of collisions - Low channel utilization (18%) A B C D Rahman 4

5 Rule 1: When to access the channel? A B C D Slotted ALOHA - Transmit only at the beginning of a slot - Chances of collisions halved - Channel utilization doubled (36%) Rule 1: When to access the channel? CSMA (Carrier Sense Multiple Access) - Listen before you talk CS part Carrier Sensing Sense the channel and transmit if channel is free - Physical carrier sensing If received signal strength on antenna is larger than some threshold then medium is busy (RSSI-received signal strength indicator) - Virtual Carrier sensing Discussed later on Physical Carrier Sensing illustrated Physical Carrier Sensing is not good enough to detect busy channel A B C DIFS- Distributed (DCF) Inter-Frame space duration 50 micro seconds The hidden-station problem Collision occurs Virtual Carrier Sensing Solution: Virtual Carrier sensing Rahman 5

6 Virtual Carrier Sensing Formally VCS DATA DIFS RTS Duration field Sender DATA CTS ACK Receiver Blue/red Node NAV (BSY) Virtual CS RTS CTS Yellow Node NAV (BSY) RTS and CTS frame formats When four-way handshake is used? Not for broadcast packets Doesn t make any sense For short unicast packets (not used) RTS-CTS part introduces overhead RTS CTS For long unicast packets Makes sense Long data packets are protected by RTS-CTS dialogue In summary There is a parameter called RTSThreshold_ If packet size is less than RTSTHreshold_ use two-way handshake (DATA_ACK) Otherwise use four way handshake CSMA/CA CA not /CD Collision Detection is not always possible Full duplex radio device (capable of transmitting and receiving at once) is very costly Successful transmission does not mean a successful reception Rule 1: When to access the channel? Perform Phy-CS and Virtual-CS. If channel is free for DIFS duration Transmit What about ELSE part??? I mean if channel is found BUSY?? Rahman 6

7 Binary Exponential Back off (BEB) Rule 2: What if collision occurs (or channel is busy)? Back off to a random point in time and try again When BEB is used When medium is found busy either by Physical or Virtual CS (to avoid collisions) When collision occurs (to resolve contention) After every successful transmission (to deal with fairness) BEB explained Back-off illustrated Back off Try later IEEE uses binary exponential back off (BEB) Think about visually impaired human being Choose a random number and start counting down Choose a random number between [0, CW] CW is called contention window Suppose the random number is n Count n idle slots before transmission Pause if medium becomes busy during count down Back off for 14 slots Slot time 20 micro seconds Back-off illustrated (previous example ) Why Back-off is called exponential After every unsuccessful attempts double CW CW min = 31 CW max = 1023 BEB DIFS- Distributed (DCF) Inter-Frame space duration 50 micro seconds CW Attempts BEB Rahman 7

8 Rule 3: Detecting successful transmission Collision detection is absent Not a CSMA/CD like Ethernet, it is a CSMA/CA For unicast packets Provision for Acknowledgement Absent of ACK means unsuccessful transmission Take few attempts (don t try for ever) Short retry limit (typically 7) used for short packets Long retry limit (typically 5) used for long packets Probability of failure in consecutive attempts is pretty low For broadcast packets No way to know.. Sender Receiver DIFS RTS ACK illustrated CTS DATA ACK Note that DATA is followed by ACK after duration Rule 4: Fairness issue Rule 4: Fairness issue (unhandled fairness issue) What will happen if station 1 has more packet to transmit at this moment Back off after every successful transmission What will happen if this frame experiences collision Back off after every unsuccessful attempt Parameters at a glance Tunable parameters Short retry limit Long retry limit Physical CS threshold RTS threshold Putting All Pieces Together Work out how duration fields are calculated? Rahman 8

9 State diagram Rahman 9