CMPE 257 Wireless Networking SET 3 Medium Access Control Protocols Spring 2005 UCSC CMPE257 1 MAC Protocol Topics IEEE 802 11 Fairness Directional antennas Multiple channels Modeling and performance analysis very limited Power efficiency and synchronization issues Scheduled channel access Spring 2005 UCSC CMPE257 2 IEEE 802 11 Standard PHY MAC standard for wireless LANs First standardized in 1997 Meet great success in starting from 1999 Several working groups IEEE 802 11a high speed extension to the 5GHz band 802 11b g high speed extension to the 2 4 GHz band 802 11e Quality of service QoS enhancement still active 802 11i Security enhancement 802 11s Mesh networking support Spring 2005 UCSC CMPE257 3 Requirements Single MAC to support multiple PHYs Handle hidden terminal problem Provisions time bounded service PHYs Direct sequence Frequency hopping Infrared never implemented Spring 2005 UCSC CMPE257 4 Architecture Infrastructure mode Basic Service Set BSS Access Point AP and stations STA take different roles Distribution system DS interconnect multiple BSSs to form a single network not specified in the standard Ad hoc mode Independent Basic Service Set IBSS Single hop the standard makes this assumption either explicitly or implicitly Spring 2005 UCSC CMPE257 5 Access Points Stations select an AP and associate with it Support roaming not part of the standard Provide other functions time synchronization beaconing power management support point coordination function PCF Traffic typically but not always flows through AP direct communication possible in IEEE 802 11e Spring 2005 UCSC CMPE257 6 802 11 Protocol Entities LLC MAC MAC Layer Management MAC Sublayer PLCP Sublayer PHY PHY Layer Management Station Management PMD Sublayer Spring 2005 UCSC CMPE257 7 802 11 Protocol Architecture MAC Entity MAC Layer Management Entity basic access mechanism fragmentation encryption synchronization power management roaming MAC MIB Physical Layer Convergence Protocol PLCP PHY specific supports common PHY SAP provides Clear Channel Assessment CCA signal carrier sense Spring 2005 UCSC CMPE257 8 802 11 Protocol Architecture cont Physical Medium Dependent Sublayer PMD PHY Layer Management modulation and encoding channel tuning PHY MIB Station Management interacts with both MAC Management and PHY Management Spring 2005 UCSC CMPE257 9 MAC in Detail Channel access mechanism Distributed Coordination Function DCF Carrier sense multiple access CSMA with immediate MAC level ACK RTS CTS exchange optional Point Coordination Function PCF Polled access through AP and distributed access Contention free period CFP and contention period CP Seldom implemented in practice Synchronization and power management Spring 2005 UCSC CMPE257 10 CSMA CA Explained Free access when medium is free longer than DIFS DIFS Contention Window PIFS DIFS Busy Medium SIFS Backoff Window Next Frame Slot time Select Slot and Decrement Backoff as long as medium is idle Defer Access Reduce collision probability where mostly needed Efficient Backoff algorithm stable at high loads Stations are waiting for medium to become free Select Random Backoff after a Defer resolving contention to avoid collisions Exponential Backoff window increases for retransmissions Backoff timer elapses only when medium is idle Implement different fixed priority levels Spring 2005 To allow immediate responses and PCF coexistence UCSC CMPE257 11 CSMA CA ACK protocol DIFS Src Data SIFS Dest Ack DIFS Contention Window Next MPDU Other Defer Access Backoff after Defer Defer access based on Carrier Sense CCA from PHY and Virtual Carrier Sense state Direct access when medium is sensed free longer then DIFS otherwise defer and backoff Receiver of directed frames to return an ACK immediately when CRC correct When no ACK received then retransmit frame after a random backoff up to maximum limit Spring 2005 UCSC CMPE257 12 RTS CTS Based Access DIFS Data RTS Src Ack CTS Dest CW Other Next MPDU NAV RTS NAV CTS Defer Access Backoff after Defer Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Vector NAV NAV Defer on either NAV or CCA indicating Medium Busy Busy Use of RTS CTS is optional but must be implemented Use is controlled by a RTS Threshold parameter per station To limit overhead for short frames Spring 2005 UCSC CMPE257 13 Frame Formats Bytes 2 2 Frame Control 6 Duration ID Addr 1 6 Addr 2 6 2 6 Sequence Control Addr 3 Addr 4 0 2312 4 Frame Body CRC 802 11 MAC Header Bits 2 Protocol Version 2 4 Type SubType 1 To DS 1 1 1 1 1 1 1 From DS More Frag Retry Pwr Mgt More Data WEP Rsvd Frame Control Field MAC Header format differs per Type Control Frames several fields are omitted Management Frames Data Frames Includes Sequence Control Field for filtering of duplicate caused by ACK mechanism Spring 2005 UCSC CMPE257 14 Address Field Description To DS 0 0 1 1 Address 1 DA DA BSSID RA Address 2 SA BSSID SA TA Address 3 BSSID SA DA DA Address 4 N A N A N A SA Addr 1 All stations filter on this address Addr 2 Transmitter Address TA From DS 0 1 0 1 Identifies transmitter to address the ACK frame to Addr 3 Dependent on To and From DS bits Addr 4 Only needed to identify the original source of WDS Wireless Distribution System frames Spring 2005 UCSC CMPE257 15 Comments on CSMA CA IEEE 802 11 cannot avoid collisions of data packets see FAMA97 CSMA CA works fine when hidden terminals are just a few For most single hop wireless LANs RTS CTS is not useful turned off by default in practice Spatial reuse is reduced in multi hop networks Spring 2005 UCSC CMPE257 16 Synchronization and Power Management Synchronization finding and staying with a WLAN Synchronization functions TSF Timer Beacon Generation Power Management sleeping without missing any messages Power Management functions Spring 2005 periodic sleep frame buffering Traffic Indication Map UCSC CMPE257 17 Synchronization in 802 11 Timing Synchronization Function TSF Used for Power Management Used for Point Coordination Timing Beacons sent at well known intervals All station timers in BSS are synchronized TSF Timer used to predict start of Contention Free burst Used for Hop Timing for FH PHY TSF Timer used to time Dwell Interval All Stations are synchronized so they hop at same time Spring 2005 UCSC CMPE257 18 Synchronization Approach All stations maintain a local timer Timing Synchronization Function keeps timers from all stations in synch AP controls timing in infrastructure