Wireless Networking Rudra Dutta CSC NCSU CSC 453

Wireless Networking Rudra Dutta, CSC, NCSU CSC 453 -001, Spring 2016 Slides and figures from various open sources

Context • Often first-hop/last-hop for Io. T-type sensor/actuator is wireless • Wireless medium access issues must be understood • Variety in span, scale, etc. • Technology changing rapidly • Specific characteristic – medium is shared, but not completely or predictably – Typically sender is unable to draw strong conclusions about collision or successful transmission

Wireless Problems • Range of station may be much less than area covered by LAN – Complicated by changing radio characteristics 3 Copyright Rudra Dutta, NCSU,

The MACA Solution • Basic idea: ask the receiver to resolve collision • Timed waits based on RTS and CTS – Minimizes collisions – MACAW: refinements such as MAC layer ACK 4 Copyright Rudra Dutta, NCSU,

Translating to Real Protocol • B and C (but not D) can hear A • D can hear B • Timed waits on the part of C and D enable A and B to communicate 5 Copyright Rudra Dutta, NCSU,

Variations • CSMA – Only detect empty medium • CSMA/CD - looked at this before – Detect collisions and stop – Reliably possible only in wired media • CSMA/CA – Wait a little on empty medium • RTS/CTS – Improves on CSMA/CA – Sometimes called CSMA/Collision Prevention - nomenclature not clear • CSMA/Bitwise Arbitration or Collision Resolution – Essentially bitmapped protocol preceded by CSMA 6 Copyright Rudra Dutta, NCSU,

Wi. Fi (IEEE 802. 11) • LAN – Ethernet replacement – Historically, common roots as Ethernet • Requires significant power • Bitrate in the 50 – low 100’s Mbps range • Span with “normal” equipment in 100’s of meters (realistically less)

802. 11 • LLC to hide difference between variants Copyright Rudra Dutta, NCSU, Fall, 2011

802. 11 - Architecture of an infrastructure network • Station (STA) 802. 11 LAN STA 1 BSS 1 802. x LAN • Basic Service Set (BSS) Portal Access Point ESS – group of stations using the same radio frequency • Access Point Distribution System – station integrated into the wireless LAN and the distribution system • Portal – bridge to other (wired) networks BSS 2 STA 2 – terminal with access mechanisms to the wireless medium and radio contact to the access point • Distribution System 802. 11 LAN STA 3 – interconnection network to form one logical network (EES: Extended Service Set) based on several BSS

802. 11 - Architecture of an ad-hoc network • Direct communication within a limited range 802. 11 LAN STA 1 STA 3 BSS 1 STA 2 BSS 2 STA 5 STA 4 802. 11 LAN – Station (STA): terminal with access mechanisms to the wireless medium – Basic Service Set (BSS): group of stations using the same radio frequency

IEEE standard 802. 11 fixed terminal mobile terminal server infrastructure network access point application TCP IP IP LLC LLC 802. 11 MAC 802. 3 MAC 802. 11 PHY 802. 3 PHY

802. 11 - MAC layer II • Priorities – defined through different inter frame spaces – no guaranteed, hard priorities – SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response – PIFS (PCF IFS) • medium priority, for time-bounded service using PCF – DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS medium busy DIFS PIFS SIFS contention next frame t direct access if medium is free DIFS 7. 13. 1

802. 11 - CSMA/CA access method I DIFS medium busy direct access if medium is free DIFS contention window (randomized back-off mechanism) next frame t slot time – station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) – if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) – if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) – if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

Association by Active Scanning Steps to Association: Station sends Probe. Access Point A Access Point C Initial connection to an Access Point

Association by Active Scanning Steps to Association: Station sends Probe. Access Point A Access Point C Initial connection to an Access Point APs send Probe Response.

Association by Active Scanning Steps to Association: Station sends Probe. Access Point A Access Point C APs send Probe Response. Station selects best AP. Initial connection to an Access Point

Association by Active Scanning Steps to Association: Station sends Probe. Access Point A Access Point C APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP. Initial connection to an Access Point

Association by Active Scanning Steps to Association: Station sends Probe. Access Point A Access Point C APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP. AP sends Association Response. Initial connection to an Access Point

Association by Active Scanning Steps to Association: Station sends Probe. Access Point A Access Point C APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP. AP sends Association Response. Initial connection to an Access Point - Re. Association follows a similar process

Bluetooth: Cable Replacement § • • • 1 Mb/s. Range ~10 meters. PANs Single chip radio. Why not use Wireless LANs? - power - cost – Low power & low cost. 20

Goals • Open spec • Low cost – In order to replace cables, should have similar cost – Cell phone cable is ~ $10 • • • Power efficiency Lightweight and small form factor Easy to use Reliable and resilient to failures Recently, IEEE 803. 15. 1 standard for Wireless PANs (WPANs) – Only MAC and PHY 21

Bluetooth Radio Link 1 Mhz. . . 79 12 3 83. 5 Mhz • MA scheme: Frequency hopping spread spectrum. – 2. 402 GHz + k MHz, k=0, …, 78 – 1, 600 hops per second. – 1 Mb/s data rate. 22

Multiple Access • BT targets large number of independent communications active in the same area at the same time. • Single FH channel: 1 Mb/s. • Each 1 Mb/s channel shared by limited number of participants. – In target user scenarios, it’s unlikely that all units in-range will share data among all of them. – 1 MB/s is reasonable. (is it? ) • Theoretically, total bandwidth is 79 Mb/s. – In practice, < 79 Mb/s since codes are non-orthogonal. 23

Master and Slaves • Communicating devices must agree on hopping sequence. • BT devices can operate as masters or slaves. • Master node defines sequence to be used. • Slave units use master id to pick sequence. • Master also controls when devices are allowed to transmit. – Master allocates slots to slaves. – Allocates total available bandwidth among slaves. 24

Piconets • • BT communication takes place over piconets Piconet formation initiated by master All other participants are slaves Number of participants limited to 8 (1 master and 7 slaves) – Channel capacity and addressing overhead – Each slave assigned a locally unique ID • Master/slave roles last for the duration of the piconet • On a piconet, slaves only have direct links to master 25

BT States Standby Inquiry Transmit Park Unconnected Page Connected Hold Connecting Active Sniff Low power 26 . Initially, all nodes in standby. . Node (master) can begin inquiry to find nearby devices. . Piconet is then formed. . Devices join by paging.

Packet Format 72 bits 54 bits Access code No CRC No retries FEC (optional) Header 0 - 2744 bits Payload header Voice Data CRC ARQ FEC (optional) 625 µs master slave 27

BLE / Bluetooth 4 • Lower-power version – Power not part of standardization • Different set of channels over same frequency band • Different methodology to spread spectrum across band – Direct Sequence • Healthcare and similar devices