Tap Latency Explained

This page explains why the validator cannot wait for the card issuer authorization before granting access to the transit service despite the fact that this would eliminate some risks. Waiting for the issuer response would impact the transit systems’ throughput and patron experience.

For the beginning, we will determine all components of tap latency, i. e. the period of time between card tapping and green or red light flashing on the validator.

NFC Channel Latency (300 – 640 msec)

This is the time the NFC card and card reader need to communicate with each other. It takes usually up to 100-250 msec, depending on the card and card reader quality.

Even if the validator has a fast card reader, some old contactless cards can be slow. New cards are usually faster, and this latency can be 350 – 500 msec.

Google Pay and Apple Pay wallets can reduce it to 250 – 350 msec.

See more details here.

Wireless Latency ( 70 msec to eternity)

This latency is important for validators installed on vehicles.

If the validator needs to connect to the transit agency host to send a message and receive a response via a wireless network, there will be time elapsed between sending a message to the host and receiving a response.

We assume here that the period of time the transit host needs to process the request is excluded from the Wireless Latency. We also consider that the authorization request and response messages are short.

The current state of the wireless data communication technology demonstrates that the wireless latency can be as low as 70 msec. This latency is demonstrated if a wireless network has nothing else to do at that moment. We will treat this value as a low limit.

Unfortunately, the upper limit can not be determined. It could be up to several seconds or even minutes. Nobody knows how the wireless system will behave if there are several thousand bus validators hammering the network with two requests per second at the rush time. Nobody knows how  the cellular system will behave if there are many phone calls, SMS, or emails at the same time in a specific region. Current open loop pilot projects run relatively small number of open loop transactions via wireless channels and there is no reliable empiric data.  The situation is even worse: there cannot be reliable statistical data or estimates of that kind obtained because of many unpredictable influential factors impacting the wireless data system throughput.

The wireless latency in satellite networks is more predictable but unacceptably high. It is equal to 480 msec. The speed of light is 300,000 km/sec and it takes minimal 120 msec to the signal to reach a satellite on the geostationary orbit which is 36,000 km high. This latency comprises four 120 msec periods because the signal must first reach the satellite, then return back to the Earth to reach the host, and then make the same way back.

Transit Host Latency (70 msec)

The transit agency host will require some time to process the transaction. We should reasonably allow at least 70 msec to process a validator request and prepare the response. Note, that this would be a pretty expensive and very fast host. We do not count here the time the host needs to “talk” to some other system such as Visa or MasterCard.

Authorization Latency (150 msec – 5 sec)

This is the time the transit host needs to obtain the card issuer authorization. Visa and MasterCard have recently made a significant progress in decreasing their network latency but they cannot beat the problem caused by the limited speed of signal propagation. If a cardholder from New Zealand tries his or her card at a point of sale in England, it takes 140 msec only for the signal to travel to the card issuer and back. Another problem is the card issuer host latency. There are fast issuers that have hosts which can process the transaction authorization within 70 msec, and there are slow issuers that require up to 1 second or even more to authorize transactions originated by their cards. The common payment network limit for the authorization latency is 5 sec.

Summarizing this, we can say that the authorization latency range is between 150 msec and 5,000 msec. For overseas issuers, the minimal latency is 290 msec.

Cumulative Tap Latency

The following tables represent the estimates for the time required if the card issuer authorization approval message needs to be obtained

Stationary Validator – Full Authorization

Latency (msec) NFC Channel Transit Host Issuer Approval Total
min 300 70 150 520
max 540 1000 5000 6540

Additional Latency for Wireless Validator

Wireless Latency (msec)
min 70
max none can be guaranteed

Maximal Allowed Transit Tap Latency

The reasonable tap latency in transit environment must not exceed  in average 350-500 msec at subway turnstile and 500 – 1000 msec at bus validator.