Network Technology CSE Network Technology CSE3020 Week 9

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

Network Technology CSE Wireless LANs IEEE HIPERLAN 1 HIPERLAN 2 Bluetooth Range (m) Data rate (Mbps)  focus IEEE (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,…).

Network Technology CSE Wireless LAN: IEEE  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

Network Technology CSE IEEE 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.

Network Technology CSE Wireless LAN: IEEE family IEEE : First standard in the family (1997). It uses DSSS or FHSS at 2.4GHz ISM band offering up to 2Mbps. IEEE a: It is operated at 5GHz ISM band. It uses OFDM modulation (multi-carrier) scheme. It offers up to 54Mbps. IEEE b (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

Network Technology CSE IEEE : 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.

Network Technology CSE IEEE : Topologies Ad hoc networks Infrastructure networks Access Point LAN WAN

Network Technology CSE IEEE : 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

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

Network Technology CSE IEEE : 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

Network Technology CSE IEEE 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 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).

Network Technology CSE 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.

Network Technology CSE 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

Network Technology CSE CSMA/CA Initially, W is set to a small value (W=8 according to the IEEE 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 standard, this is known as the “maximum contention window”).

Network Technology CSE 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

Network Technology CSE 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.

Network Technology CSE 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

Network Technology CSE 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

Network Technology CSE 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

Network Technology CSE 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

Network Technology CSE 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

Network Technology CSE Four-way Handshaking Four-way handshaking access method is an optional operation in IEEE 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.

Network Technology CSE IEEE : 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

Network Technology CSE Required Reading W. Stallings, Data and Computer Communications Prentice-Hall. >> Chapter 13.6 & E Chapter 17 7E