1. Network Technology CSE3020 - 2006 1 Network Technology CSE3020 Week 9

2. Network Technology CSE3020 - 2006 2 Wireless LAN  Why Wireless LANs?  Mobility  Flexibility  No cables, save cost  Fast installation Problems:  Limited bandwidth  Noisy channel  Multipath  Security  Power consumption

3. Network Technology CSE3020 - 2006 3 Wireless LANs 100 80 60 40 20 0 110100 IEEE 802.11 HIPERLAN 1 HIPERLAN 2 Bluetooth Range (m) Data rate (Mbps)  focus IEEE 802.11 (USA). HIPERLAN 1 and 2 (Europe). Bluetooth (Industry: Erricson, Nokia, IBM, Intel,…): Provides short distance (around 10m), low data rate (1Mbps) communications between devices (speakers, laptops,…).

4. Network Technology CSE3020 - 2006 4 Wireless LAN: IEEE 802.11  Physical Layer  Topologies  MAC Protocol DCF and PCF Infrastructure and Ad hoc Radio (DSSS,FHSS) and Infrared DSSS: Direct Sequence Spread Spectrum FHSS: Frequency Hopping Spread Spectrum DCF: Distributed Coordination Function PCF: Point Coordination Function

5. Network Technology CSE3020 - 2006 5 IEEE 802.11 Functions Distributed Coordination Function (DCF): Based on CSMA with Collision Avoidance Four-way handshaking access method may be applied (optional). Retransmission is based on Binary Exponential Backoff. Point Coordination Function (PCF): Similar to a polling system.

6. Network Technology CSE3020 - 2006 6 Wireless LAN: IEEE 802.11 family IEEE 802.11: First standard in the family (1997). It uses DSSS or FHSS at 2.4GHz ISM band offering up to 2Mbps. IEEE 802.11a: It is operated at 5GHz ISM band. It uses OFDM modulation (multi-carrier) scheme. It offers up to 54Mbps. IEEE 802.11b (1999): It is operated at 2.4GHz ISM band. It uses CCK & QPSK (with DSSS). It offers 1, 2, 5.5 and 11Mbps. Other standards are still being developed. ISM band: Industrial,Scientific,Medicine band OFDM: Orthogonal Frequency Division Multiplexing

7. Network Technology CSE3020 - 2006 7 IEEE 802.11: Physical Layer Radio Experience gained from building mobile phone and wireless WAN networks can be reused. Large coverage and deep penetration. Very limited license-free frequency bands. Very noisy. Infrared Cheap. No licenses needed. Interference by sunlight, heat sources. Low bandwidth. Signals may be blocked by many surrounding objects.

8. Network Technology CSE3020 - 2006 8 IEEE 802.11: Topologies Ad hoc networks Infrastructure networks Access Point LAN WAN

9. Network Technology CSE3020 - 2006 9 IEEE 802.11: Architecture of an Infrastructure Network ESS BSS STA BSS STA 802.X LANs AP Portal Radio coverage STA = Station AP = Access Point BSS = Basic Service Set ESS = Extended Service Set

10. Network Technology CSE3020 - 2006 10 IEEE 802.11: Infrastructure Design issues: Handover and roaming. Frame Forwarding. Frame conversion from/to existing LANs’ frame format. AP LAN WAN AP

11. Network Technology CSE3020 - 2006 11 IEEE 802.11: Ad hoc Design issue: Multihop relaying (Routing): How can we connect cells if we do not have an access point in each cell? A B C D E F G

12. Network Technology CSE3020 - 2006 12 IEEE 802.11 MAC Protocol A protocol is needed to define rules for all stations to access the common channel without conflict. Many proposals were submitted to IEEE 802.11 working group, CSMA/CA was chosen (1997). CSMA/CA is similar to CSMA/CD (used in Ethernet). A station must make sure the common channel is clear before any transmission attempt. Unlike CSMA/CD (Ethernet), a station cannot detect a collision. The receiver must reply with an acknowledgement immediately after receiving a frame. A station must choose a random future time for all its transmission (new or collided frames).

13. Network Technology CSE3020 - 2006 13 CSMA/CA To transmit a frame, a station first picks a random integer, r (or counter) from range [0,W-1]. The value r is decreased by one when the common channel is detected idle for a short period of time known as slot time (e.g. 50  sec). The station transmits the frame when r reaches zero. If the channel is detected busy before r reaches zero, the station stops to decrease r. The countdown will be reactivated when a long period of idle channel is detected (this period is known as DIFS – Distributed Interframe Space). After the completion of the transmission, the station must wait for a very short period (known as SIFS – Short IFS) for the acknowledgement (ack). The return of the ack confirms the transmission, otherwise, the station must repeat the first step.

14. Network Technology CSE3020 - 2006 14 Ack not received transmission completed Channel turns busy CSMA/CA The station transmits the frame To transmit a frame, a station first picks a random integer, r (or counter) from range [0,W-1] The r value is frozen. The station will continue to monitor the channel. The countdown will be reactivated when a long period of idle channel is detected (known as DIFS) the station must wait for a very short period (known as SIFS) for an acknowledgement (ack). For every slot time (eg. 50  sec) where the common channel is sensed idle, r =r-1 when r =0 channel turns idle for a DIFS START DONE ack received REPEAT START

15. Network Technology CSE3020 - 2006 15 CSMA/CA Initially, W is set to a small value (W=8 according to the IEEE 802.11 standard, this is known as the “minimum contention window”). As a station experiences collision, W is doubled. When W reaches a large value, it stays at that value (W=256 according to the IEEE 802.11 standard, this is known as the “maximum contention window”).

16. Network Technology CSE3020 - 2006 16 CSMA/CA Operation: Example-1 B D C Consider this ad hoc WLAN: Scenario: B is attempting to transmit a frame to D. The transmission is successful. time B was ready, B picked r = 3 r = 0 B transmitted its frame ACK from D Slot time SIFS

17. Network Technology CSE3020 - 2006 17 CSMA/CA Operation: Example-2 B D C Consider this ad hoc WLAN: Scenario: B, C are attempting transmissions to D. Both transmissions are successful. time B transmitted its frame. C detected a busy channel, r c was frozen. ACK from D Slot time B, C are ready, B picked r B = 1 and C picked r c = 3 DIFS r c was reactivated C transmitted its frame here when r c = 0.

18. Network Technology CSE3020 - 2006 18 CSMA/CA Operation: Example-3 Scenario: B, C are attempting transmissions to D. A collision occurs before both transmissions are successful. time Transmissions of B and C collided here. Slot time B, C are ready, B picked r B = 1 and C picked r c = 1 No ACK from D DIFS B, C repeated the operation. B picked r B = 1 and C picked r c = 13 B transmitted its frame here when r B =0. r c was frozen here. ACK from D... B D C

19. Network Technology CSE3020 - 2006 19 Hidden Station Problem When A is transmitting a frame to B, since D is not in A’s coverage, D is a hidden station that D doesn’t sense a busy channel, thus D may start a transmission that collides with A’s transmission Hidden Station Problem in ad hoc Wireless LANs: ABCD A’s coverage B’s coverage

20. Network Technology CSE3020 - 2006 20 Exposed Station problem While A is transmitting a frame to C, B senses a busy channel and concludes that it may not transmit any frame to D which is incorrect Exposed Station Problem in ad hoc Wireless LANs: ABCDA’s coverage B’s coverage

21. Network Technology CSE3020 - 2006 21 Solution to Hidden/Exposed Station Problem Four-way Handshaking time senderreceiver data ACK Basic operation Additional operation RTS CTS RTS: Ready to send CTS: Clear to send

22. Network Technology CSE3020 - 2006 22 RTS/CTS Operation B D C Consider this ad hoc WLAN: Scenario: B is attempting to transmit a frame to D. The transmission is successful. time B was ready, B picked r = 3 r = 0 B transmitted its frame after receiving CTS ACK from D Slot time B transmitted RTS D replied with CTS

23. Network Technology CSE3020 - 2006 23 Four-way Handshaking Four-way handshaking access method is an optional operation in IEEE 802.11. It is also used to improve performance (by reducing the bandwidth wastage due to a collision). Short frames are transmitted using Basic access method, and long frames are recommended to transmit using Four- way handshaking access method.

24. Network Technology CSE3020 - 2006 24 IEEE 802.11: DCF & PCF SIFS PIFS DIFS SIFS = Short Interframe space PIFS = Point IFS DIFS = Distributed IFS PCF (optional) DCF SIFS PDataPC DIFS PCF Operation: PIFS time Backoff DataBusy SIFS PIFS time

25. Network Technology CSE3020 - 2006 25 Required Reading W. Stallings, Data and Computer Communications Prentice-Hall. >> Chapter 13.6 & 14.5 6E Chapter 17 7E