Overview + RFID and Motivation + Aloha and Collision Resolution RFID Multiple Access Methods + Commercial Offerings + Comparative Analysis + Conclusion Luc André Burdet Seminar „Smart Environments“ SS04, ETH Zürich Mentor: C. Flörkemeier RFID Multiple Access Methods 2 RFID RFID Challenges Goal is to reliably identify multiple objects, in as short a time as possible Data RFID Reader data-carrying Power However, requirements to drive RFID Tags size and cost down limit ressources ( transceiver ) RFID Tag ( transponder ) coupling Clock element Limited memory and computation capabilities + Little calculation possible at Tag Application + Lack of internal power source + state information unreliable + Collisions difficult to detect (varying signal strengths) + 3 main components: Transponders can‘t be assumed to hear one-another + (passive/active) RFID Tag ( trans mitter/res ponder ) + + Special case of multiple channel access communication problem microchip+coiled antenna; stores data ; power from interrogation signal; Strong FCC regulations on Readers „maximum in Band allowed field strength“ + RFID Reader ( trans mitter/re ceiver ) + ex ISO18000-3: 6,6kbps Tag Reader >> 1,6kbps Reader Tag + RF module, control unit; Reader -> Tag messages must be minimized + Data Processing Subsystem + � Many standard collision-resolution protocols non-applicable or difficultly application, database, ... implemented RFID Multiple Access Methods 3 RFID Multiple Access Methods 4 Overview (Pure) Aloha + RFID and Motivation + Tag transmits upon data ready Tag 1 + Aloha and Collision Resolution Tag 2 + Detect success or collision Tag 3 (Pure) Aloha + + Tag retransmits after random backoff time following collision Slotted Aloha + Shared Medium Framed-slotted Aloha + complete collision partial + Commercial Offerings collision + Comparative Analysis + Conclusion Vulnerable period: Frame0 Collision occurs if t 0 -F ≤ t 1 ≤ t 0 +F Vulnerable period RFID: Tags can‘t detect/sense carrier. Collision is: t 0 -F t 0 t 0 +F t determined by listening for Reader‘s „(N)ACK“ � � ...undetected RFID Multiple Access Methods 5 RFID Multiple Access Methods 6 1
Aloha - 2 Slotted Aloha Tag 1 Switch-off Aloha with an additional constraint: Tag 2 + If Tag response successfully decoded, Tag automatically Tag 1 + Time is divided into discrete time intervals Tag 3 enters Quiet state (slots) Tag 2 + More later under Slotted-Aloha Reader Tag 3 + A Tag can transmit only at Shared Medium the beginning of a slot Slow-down Shared Medium compromise between Aloha and Swith-off Tag 1 + Packets either collide completely or Tag 2 complete collisions (only) + Reader overwhelmed by responses do not collide at all Tag 3 + „Slow-down“ command sent, Reader + Synchronization overhead: Tag adapts its (random) backoff algorithm Shared Reader SOF, EOF � Goal is to diminish Tags‘ reply frequency Medium Frame0 Vulnerable period reduced to size F: „Carrier Sense“ Tag 1 + Vulnerable period + MUTE signal to all Tags when Tag 2 s 0 -1 s 0 s 0 +1 t start of transmission is detected. Tag 3 Reader Frames start at slot limits Shared Medium RFID Multiple Access Methods RFID Multiple Access Methods 7 8 Slotted Aloha - 2 Framed Slotted Aloha „Terminating“ Further discretisation of time: Tag 1 Tag 1 Tag 2 + If Tag response successfully decoded, Tag + Medium access grouped into Frames, automatically enters Quiet state Tag 2 Tag 3 with N slots per frame + Avoids collisions due to Tags replying indefinitely Tag 3 Reader + Tags transmit at most once in a + Tags re-enter Active state upon next “Wake-up” Shared Shared randomly selected slot, within maximum N from Reader Medium Medium + Failure to recognize “Wake-up” a problem: + Little extra synchronization overhead: Tags time-out of sleep mode automatically . Reader SOF, EOF for slots + Also called “Muting” . maximum slot number N set in Tag as default Tag 1 „Early End“ Tag 2 + Slot delimited by Reader SOF, EOF Tag 3 + Reader issues „Next-Slot“ command on no Reader responses received Shared Medium RFID Multiple Access Methods 9 RFID Multiple Access Methods 10 Framed Slotted Aloha - 2 Perspective Adaptive Aloha Tag 1 + easily / quickly adapts to varying number - worst case: never finishes Reader can temporarily expand / contract + of Tags Tag 2 - theoretically proven maximum channel number of slots for upcoming round + simplest Reader design: „listen“ utilisation 18.4% 1 � Number of slots in a round varies with Tag 3 number of Tags in field Shared Slotted-Aloha Medium Previous extensions also applicable: + N = 2 + less of a „free-for-all“ - still only 36.8% medium utilisation 1 N = 4 Terminating / Muting + + doubles the channel utilisation of Aloha - requires synchronisation (overhead) N = 3 (slotted) „Early End“ - Tgas need to count slots + Frame-Slotted Aloha + (automatically) diminishes each Tag‘s - requires synchronization repeat rate to once per frame - „frame size“ needs to be known / transmitted - Tag needs to count frames / slots 1 Theoretical test data, which is based on the assumption of a Poisson arrival RFID Multiple Access Methods 11 RFID Multiple Access Methods 12 2
Overview Philips I*Code „Reader talks first“: first the Reader sends a command + RFID and Motivation + „Timeslot anticollision principle“ – variant of slotted Aloha + + Aloha and Collision Resolution Number of timeslots adjustable by Reader with „Timeslot Index“ command + + Commercial Offerings Timeslot Index 0 1 2 3 4 5 6 7 Philips I*Code + Number of timeslots 1 4 8 16 32 64 128 256 Timeslot Mask (hex) at Tag 00 03 07 0F 1F 3F 7F FF ISO 18000 „MODE 1“ + ISO 18000 „MODE 2“ + Timeslot position := + Comparative Analysis + hashvalue AND TimeslotMask + Conclusion where Tag „Randomness“ is hash value of defined offset (8bit) in serial + number (ID) („I*CODE1“ System Design Guide [SL048611]) RFID Multiple Access Methods RFID Multiple Access Methods 13 14 ISO18000 „MODE 1“ ISO18000-3 „MODE 2“ No default collison management --can be considered as Pure Aloha Combination of Frequency and Time division multiple Access (FTDMA) Tags can select from 8 reply channels (multi-frequency operation) Protocol Extensions (optional) + Extension 1: „non-slotted non-terminating aloha protocol“ [6.1.10.2] (Subcarriers derived by division of powering field‘s frequency) + Tags reply at random with self-determined intervals + Reply as long as in energizing field Freq. Hop Rate: 0 (whole reply transmitted one one frequence) + + Reader doesn‘t influence interrogation process Freq. Hop Sequence: random (in response to valid Reader command) + Extension 2: „slotted terminating adaptive round protocol“ [6.1.10.4] + Shared Continuing dialog between Reader and Tag In all other aspects, Slotted Aloha + + Medium Tags select reply-slot number, from a maximum slot number on each subcarrier + Tag 2 Number of slots in round expands/contracts with number of Tags in field Muting: „mute ratio“ + + Tag 3 (temporarily overridden by Reader) (unmuted, ½, ¾, 31 / 32 , ..., 511 / 512 , fully muted) Tag 1 („RFID for Item Management – Air Interface Part 3“ – ISO 18000-3 MODE 1) („RFID for Item Management – Air Interface Part 3“ – ISO 18000-3 MODE 2) RFID Multiple Access Methods 15 RFID Multiple Access Methods 16 Summary Overview R M R F e + RFID and Motivation u e r T a l a a e d t m r e i p d m e e r l e r i „ / d n C r T S - a e a s a p + Aloha and Collision Resolution l l t i r g o o S n r l y - t t S g i e E T t e t e l o A c a w / r a h d d w S r d l k - i t - M l a A A A c d e y a n - F l l h o u n - p o l o o t E t n i r h h h - o w i n s i e + Commercial Offerings s e n v l t a a a f f n g “ d e s + Comparative Analysis SuperTag TTF + Conclusion Philips I*CODE RTF ext1 RTF ISO18000 MODE1 ext2 ISO18000 MODE2 Auto-ID C1_G2 RTF RFID Multiple Access Methods 17 RFID Multiple Access Methods 18 3
Recommend
More recommend
