Contikimac

Contikimac is a Radio Duty Cycling (RDC) protocol designed to provide an energy efficient method of sending and receiving packets. It is designed for use with the CC2420 however can be used with other radios.

Contikimac relies on retransmissions for reliable communications and it is extremely unlikely that a packet will be received on its first transmission as it turns on and off the radio at a rate of several Hertz (usually 8Hz as specified in the platform-specific configuration) looking for activity. During each turn-on, Contikimac carries out several channel clear checks (by default, two) to see if the channel is clear. Contikimac will then turn off the radio if no activity is detected or, if activity is detected, it will keep the radio on for a set period to try and receive the packet.

Contikimac (is supposed to) pad packets that are short enough to fit between two subsequent CCAs with 0's to ensure that they can be reliably detected and received.

Contikimac Settings

Contikimac uses several settings to control its operation. These are detailed in the following tables.

General Settings

Definition	Internal Variable	Description	Values	Default
CONTIKIMAC_CONF_WITH_PHASE_OPTIMIZATION	WITH_PHASE_OPTIMIZATION	Controls whether TX/RX cycles are synchronised between neighbours.	1, 0	1
CONTIKIMAC_CONF_WITH_CONTIKIMAC_HEADER	WITH_CONTIKIMAC_HEADER	Controls whether a two-byte Contikimac header is added.	1, 0	1
WITH_FAST_SLEEP		Controls whether a more agressive radio sleeping algorithm is used on busy channel.		1
RDC_CONF_HARDWARE_CSMA		Informs Contikimac whether the radio does CSMA and autobackoff.	1, 0	0
RDC_CONF_HARDWARE_ACK		Informs Contikimac whether the radio blocks until it receives an ACK.	1, 0	0
RDC_CONF_MCU_SLEEP		Informs Contikimac whether the MCU can sleep while the radio is off.	1, 0	0
CONTIKIMAC_CONF_CCA_COUNT_MAX	CCA_COUNT_MAX	Sets how many NETSTACK_RADIO.channel_clear() are done for each channel check.		2
CONTIKIMAC_CONF_CCA_COUNT_MAX_TX	CCA_COUNT_MAX_TX	Sets how many NETSTACK_RADIO.channel_clear() are done before each TX.		6
CONTIKIMAC_CONF_SHORTEST_PACKET_SIZE	SHORTEST_PACKET_SIZE	Smallest packet that Contikimac allows.		43

Timining Settings

Definition	Dependant Variable	Description	Default
NETSTACK_CONF_RDC_CHANNEL_CHECK_RATE	NETSTACK_RDC_CHANNEL_CHECK_RATE	Sets the frequency (in Hz) of channel checking. If this is >= 64, WITH_PHASE_OPTIMIZATION is set to 0. This must be a power-of-2.	8 (Set in Platform configuration)
CONTIKIMAC_CONF_CCA_CHECK_TIME	CCA_CHECK_TIME	tr in the Contikimac paper. The rough time it takes to perform a CCA. May be 0, e.g. if the MCU blocks till CCA is done.	RTIMER_ARCH_SECOND / 8192
CONTIKIMAC_CONF_CYCLE_TIME	CYCLE_TIME	Cycle time for CCA checks, e.g. how long each CCA check cycle can take.	(RTIMER_ARCH_SECOND / NETSTACK_RDC_CHANNEL_CHECK_RATE)
CONTIKIMAC_CONF_INTER_PACKET_INTERVAL	INTER_PACKET_INTERVAL	Interval between two successive packet transmissions.	RTIMER_ARCH_SECOND / 2500
CONTIKIMAC_CONF_AFTER_ACK_DETECTECT_WAIT_TIME	AFTER_ACK_DETECTECT_WAIT_TIME	The time to wait before reading out a detected potential ACK.	RTIMER_ARCH_SECOND / 1500

Internal Settings

Definition	Description	Setting
CCA_SLEEP_TIME	tc in the Contikimac paper. This is the time between successive CCA checks	RTIMER_ARCH_SECOND / 2000 or (RTIMER_ARCH_SECOND / 2000) + 1 if RTIMER_ARCH_SECOND is coarse, e.g. <= 8000
INTER_PACKET_DEADLINE	Maximum time a receiver waits for next packet in a burst when FRAME_PENDING is set.	CLOCK_SECOND / 32
CHECK_TIME	The total time to perform CCA_COUNT_MAX CCAs.	(CCA_COUNT_MAX * (CCA_CHECK_TIME + CCA_SLEEP_TIME))
CHECK_TIME_TX	The total time to perform CCA_COUNT_MAX_TX CCAs.	(CCA_COUNT_MAX_TX * (CCA_CHECK_TIME + CCA_SLEEP_TIME))
LISTEN_TIME_AFTER_PACKET_DETECTED	How long to keep checking for activity after a CCA has returned activity. tl + ti + tl	RTIMER_ARCH_SECOND / 80
MAX_SILENCE_PERIODS	The maximum number of silent periods before the radio is turned off.	5
MAX_NONACTIVITY_PERIODS	The maximum number of periods with no packet being received before the radio is turned off when WITH_FAST_SLEEP is enabled.	10
STROBE_TIME	The maximum amount of time to re-transmit a packet for.	(CYCLE_TIME + 2 * CHECK_TIME)
GUARD_TIME	The time before the expected phase of a neighbour that a transmission should start.	10 * CHECK_TIME + CHECK_TIME_TX
MAX_PHASE_STROBE_TIME	The time to transmit repeated packets to neighbours with a phase lock.	RTIMER_ARCH_SECOND / 60
ACK_LEN	The length of an ACK packet.	3

CC1120 Driver

When used with the CC1120 on the Zolertia Z1 with no modifications to any settings, Contikimac does not give the expected results.

The packet time for a 19-byte packet is roughly 4.9ms and the inter-packet time (ti) is in the region of 8.2mS. This does not correspond to the default set time of 0.4mS

With padding, the same packet (padded to 43 bytes) takes 9.1ms to send.

A full 125-byte packet takes 22ms to transmit with preamble, etc.

A full length packet plus five retransmissions with LBT on the initial transmission and ~3ms between each retransmission takes 157ms.

An ACK will take approx 1.28ms

Modifications

• Made LISTEN_TIME_AFTER_PACKET_DETECTED settable from Contiki config as CONTIKIMAC_LISTEN_TIME_AFTER_PACKET_DETECTED rather than just being internally set.
• Made CCA_SLEEP_TIME settable from Contiki config as CONTIKIMAC_CONF_CCA_SLEEP_TIME rather than just being internally set.
• Line 771 to 786: Hardware ACK detection increases the inter-packet interval to inter-packet interval + ACK detection time. To get around this, non-broadcast packets take into account the wait time.

• The CC1120 RSSI valid time is given on P.37 to p.38 of the CC1120 user guide. The calculation of this time is shown in the following table.

Timing	Description	Formula	Value
CSmax	Maximum CS response time.	≤ T0 + (T1 + T2)*(2RSSI_VALID_CNT + 1)	1.23
T0	Start-up delay before RSSI can begin.	D0 + D1 + D3 + D5	0.91
T1	AGC transient decay wait time.	((16.AGC_SETTLE_WAIT + 48) * BB_CIC_DECFACT * Decimation Factor )/fxosc	0.08
T2	AGC measure time.	≤ ((2AGC_WIN_SIZE + 4 * BB_CIC_DECFACT * Decimation Factor) + 46)/fxosc	0.08
D0		((16 * Decimation Factor) + 74)/fxosc	0.0123125
D1		((1 - DCFILT_FREEZE_COEFF) * (62 + CARRIER_SENSE_GATE * (2(5 + x) - 1)) * Decimation Factor * 2)/fxosc	0.775
D3		((16 * BB_CIC_DECFACT - 2) * Decimation Factor * 2)/fxosc	0.0375
D5		(68 * BB_CIC_DECFACT * Decimation Factor)/fxosc	0.085
fxosc	Example	Example	Example
AGC_SETTLE_WAIT	AGC gain adjustment wait time.	AGC_CFG1.AGC_SETTLE_WAIT	1
AGC_WIN_SIZE	AGC integration window size.	AGC_CFG1.AGC_WIN_SIZE	2
Decimation Factor	Example	CHAN_BW.ADC_CIC_DECFACT	20
BB_CIC_DECFACT	Example	CHAN_BW.BB_CIC_DECFACT	2
DCFILT_FREEZE_COEFF	Example	DCFILT_CFG.DCFILT_FREEZE_COEFF	0
CARRIER_SENSE_GATE	Example	MDMCFG1.CARRIER_SENSE_GATE	0
x	DCFILT_CFG.DCFILT_BW when < 5, else 4	DCFILT_CFG.DCFILT_BW	4

• The actual inter-packet interval for the CC1120 with LBT for each retransmission will be a minimum of 6.23ms (the sum of the RSSI valid time plus the mandatory minimum 5ms listening time as required by ETSI EN 300 220-1 V2.3.1). With a busy channel, the actual inter-packet time could be much greater so a timeout is required. This is set at 12.5ms (in CC1120_LBT_TIMEOUT) from the time that the TX strobe is issued. Given that this is a variable amount of time that is quite significant, LBT is only used for the first packet. As long as the time between the end of each transmission and the next retransmission is less than 5ms, no other stations should try to transmit due to the 5ms minimum listen time. As such, the inter-packet interval is set to 3ms.

CC1120 Timings

Timing Value	Description	Time (ms) CC2420	Time (ms) @ 50kbps/50ksps	Time (ms) @ 100kbps/100ksps	Time (ms) @ 200kbps/100ksps	Source/Calc
ts	The minimum transmission time for valid detection.	0.884	5.96	Example	Example	tα + td < ti < tc + 2tr < ts
tα	Time between sending a packet and receiving an ack. this is 12 symbols in 802.15.4	0.192	0.24	0.12	0.12	802.15.4 spec, 12 * symbol rate
ti	Interval between each packet transmission	0.4	3	Example	Example	Must be less than tc. Must be > 5ms + tr + timeout
tr	Time for a valid RSSI.	0.192	1.23	Example	Example	Example
td	Time for detecting an ACK.	0.16	0.8	Example	Example	Preamble + sync delimeter TX, 5-bytes total.
tc	Time between each CCA.	0.5	3.5	Example	Example	Must be > ti
tl	Transmission time of the longest possible packet.	4.1	22	Example	Example

Achieving a 12-symbol turn around for the ACK is not possible with the CC1120 as it takes 1.6ms to start to TX the ACK from the end of the packet (with a 125-byte received packet) and 2.38ms to actually transmit the ACK. 790μS of the 1.6ms is expended reading a 125-byte packet from the FIFO using burst access.