DSRC-based fare collection systems inside and outside
Toll roads in Russia are becoming a harsh reality. Someone is pleased with this, given the obvious fact that the toll road operator provides "better" conditions of travel. Someone points to multi-kilometer queues at the points of payment, but someone basically does not like the idea of ​​paying for what was previously offered for free. Regardless of our attitude to the issue of toll roads, they already exist, and they will be created in the future. In this article, I would like to consider in more detail the electronic means of non-stop fare, used on modern Russian toll roads.
As a means of non-stop fare payment in Europe and in Russia, DSRC transponders are used - inexpensive “boxes” attached to the windshield and providing information exchange over the radio channel with antennas at the charging points. Colleagues from the OSSP company, the operator of the M-4 route, provided me with one of their transponders for the experiments, which we will analyze for the purpose of study. But before you start breaking the transponder, you need to pay tribute to the subject of the hub and consider the architecture underlying the system of non-stop payment of travel.
The transponder and antenna implement the stack of protocols from the 1st, 2nd and 7th levels of the OSI model, as shown in the figure.
DSRC stack architecture (ISO 12834-2003)
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The main difference between DSRC and its closest relative Wi-Fi is that the DSRC is “sharpened” for communication with a fast moving object. That is, while the car is passing through the antenna, the transponder should wake up, establish a connection and exchange information. By experience, at a speed of 90 km / h, you can safely transfer 1 MB of information. It turned out to transfer and up to 4 MB, but the risks of disconnection already appeared. The Japanese are actively testing the DSRC of a new standard, when it is possible to reliably exchange 10–20 MB of information per session, but they have not yet come to industrial implementation.
During the passage of the car under the antenna is about the following:
Establishing a DSRC antenna and transponder connection
The following picture shows the encapsulation of data frames of different levels.
DSRC frame format
The preamble is needed to synchronize the transponder and antenna.
Start flag - 0111 1110.
LID is a link identifier for brokast 11111111, for other cases - four octets of numbers randomly selected during connection establishment to identify the exchange channel with a specific transponder.
The MAC control field contains information about the contents of the packet — an uplink or downlink, a command or response to a command, etc.
LLC control contains the type of command or command response, LLC status, respectively, contains the result of the command
LPDU - in fact, information of the application level (we consider it separately). An application unit can be transmitted in parts if it does not fit in one physical packet. In general, for those who know the TCP / IP stack, there is nothing fundamentally new here.
The CRC frame ends with a checksum and stop bits similar to the start flag.
At the application level, information is exchanged within the respective applications. Currently the most common applications are:
At the points of payment, the EFC application is used to organize non-stop travel.
During the interaction of the antenna and the transponder, a journey transaction is formed, which can be supplemented with vehicle measurement data. Depending on the tariffs entered and the charging rules, a car may be classified at a point of payment, photographing, license plate recognition and even weight control. Each transponder, prior to being issued to the client, passes through an initialization stage, during which an EFC information block is recorded in its memory, which the transponder must transmit during the information exchange.
The information block in the “maximum configuration” consists of about 50 standardized attributes that can be divided into the following groups:
In Russia, DSRC is used at a payment point in St. Petersburg (North-Western High-Speed ​​Diameter), in Moscow (M-1 highway bypassing Odintsovo), M-4 (in the Domodedovo area), and also on the same route closer to Voronezh. On the roads being designed and under construction will also be created a strip of electronic fare payment.
The main problem with the use of EFC is the inability to travel with one transponder on all paid routes. If at the technological level this problem is completely solvable, then at the application level a large amount of additional work is needed related to the organization of the exchange of encryption keys and the organization of netting between the companies-operators.
EFC tariff schemes are minimally applied. That is, the transponder itself is not a means of payment and is not attached to the vehicle. In fact, our transponder is just a digital label of a subscriber. At the point of collection, each vehicle is classified and the tariff amount is deducted from the account linked to the transponder, depending on the results of the classification. That is, the transponder can be easily rearranged from the car to the truck and back.
Such a simplified approach has its advantages and disadvantages. The advantage is the ability to initialize the transponders during production. A bar code is applied to each transponder, which is matched with the user's contract during the sale, after which the user can freely use the transponder. If, for example, the tariff were tied to a car, it would be necessary to install special programmers at the points of sale, allowing you to record the necessary attributes in the transponder’s memory, for example, the car number and its class.
The second advantage of the simplified scheme is the simplicity of ensuring compatibility of transponders with different operators. Regardless of the classification system of each operator, the journey transaction is formed without errors. Otherwise, we would have to align the principles of classification of cars for all operators.
And the disadvantages include the impossibility of autonomous control of payment (without contacting the users' registration database).
The circuit works in this way. During the sale, as I have already said, a comparison of the transponder number and the user's contract takes place. The operator’s accounting system maintains a balance in the electronic account of the transponder. Transponders with a balance amount below a certain level fall into the warning zone - the “orange list”. If the amount is not enough to pay for travel, such transponders fall into the "red" or "black" list. The “color” transponder lists are distributed to all toll stations and loaded into the controllers of the lanes (or immediately into the memory of antennas of some manufacturers).
When a vehicle approaches a toll point in a special lane, it falls within the range of the antenna of the lane, which reads the transponder ID. The band controller (or antenna software) performs a check with the "color" list. If the transaction was formed normally and the transponder number is not listed, the strip controller software gives a signal to open the barrier. In this case, the car passes the point of payment at a speed of about 30 km / h without stopping. If the transponder is in the “orange” zone, the driver receives a warning, for example, the special “Low Balance” sign lights up and the barrier opens. If the funds are not enough, then the barrier does not open, and the user has to pay the fare in cash.
DSRC antennas in the area of ​​payment of the charging point M-4
To pay for travel on the M-4, Kapsch production TS3203 transponders are used. Fully compatible Q-Free, Norbit, GEA, etc. transponders can also be used.
The appearance of the transponder TS3203
In agreement with the road operator, the manufacturer will put a bar code and a number on the transponder body. These transponder identifiers are transmitted as a file along with a batch of transponders. Data from the file is loaded into the operator’s settlement system for subsequent comparison with contracts during sales. Transponders keep in memory an encrypted data block of the operator, which it can read with its keys. Keys are uploaded and stored on EFC band antennas. Thus, transponders are completely ready for sale and use.
Transponder Marking
Inside, we see a simple board and a speaker-squeaker, through which the transponder reports a successful transaction or an error. As you can see, the protection against opening is completely absent. There are no fuses or photoresistors.
Transponder board
On the other side of the board is a battery, tightly pressed into the holder. According to the Kapsch specification , the battery should last for 7 years with 2000 transactions per year.
Reverse side of the board
The core of the transponder is a proprietary ASIC chip containing memory, processor and all logic. The Kapsch analog core is traditionally called Ella, the digital core is Alex. In previous versions, these cores were implemented as separate chips ( photo ).
Fee close up
As we have seen, the DSRC transponder is a reliable and cheap device. That is why electronic charging is universally made using the DSRC. DSRC technology has only one drawback - to calculate the tariff, it is necessary to install antennas at all exits from the road (or in the middle of each section, as in Austria). If we want to close the “large-scale network” with a “paid” one, then we need to carefully look towards satellite-based systems, which I already wrote about ( with a continuation ).
As a means of non-stop fare payment in Europe and in Russia, DSRC transponders are used - inexpensive “boxes” attached to the windshield and providing information exchange over the radio channel with antennas at the charging points. Colleagues from the OSSP company, the operator of the M-4 route, provided me with one of their transponders for the experiments, which we will analyze for the purpose of study. But before you start breaking the transponder, you need to pay tribute to the subject of the hub and consider the architecture underlying the system of non-stop payment of travel.
DSRC stack
The transponder and antenna implement the stack of protocols from the 1st, 2nd and 7th levels of the OSI model, as shown in the figure.
DSRC stack architecture (ISO 12834-2003)
')
The main difference between DSRC and its closest relative Wi-Fi is that the DSRC is “sharpened” for communication with a fast moving object. That is, while the car is passing through the antenna, the transponder should wake up, establish a connection and exchange information. By experience, at a speed of 90 km / h, you can safely transfer 1 MB of information. It turned out to transfer and up to 4 MB, but the risks of disconnection already appeared. The Japanese are actively testing the DSRC of a new standard, when it is possible to reliably exchange 10–20 MB of information per session, but they have not yet come to industrial implementation.
During the passage of the car under the antenna is about the following:
- The transponder receives a beacon signal and wakes up. The beacon signal contains a BST data structure with a list of services (applications) provided that are supported at a given point. The time between receiving the first antenna signal (any, not necessarily containing BST) and the transponder being ready for operation is 5 ms.
- The antenna and transponder define the channel through which the exchange will take place. There are many cars on the road, and channel separation is necessary.
- The transponder, using the VST data structure, reports the application (or applications) it needs. For example, EFC is an electronic fare payment.
- The antenna and transponder establish a secure connection and exchange data within the selected application.
Establishing a DSRC antenna and transponder connection
The following picture shows the encapsulation of data frames of different levels.
DSRC frame format
The preamble is needed to synchronize the transponder and antenna.
Start flag - 0111 1110.
LID is a link identifier for brokast 11111111, for other cases - four octets of numbers randomly selected during connection establishment to identify the exchange channel with a specific transponder.
The MAC control field contains information about the contents of the packet — an uplink or downlink, a command or response to a command, etc.
LLC control contains the type of command or command response, LLC status, respectively, contains the result of the command
LPDU - in fact, information of the application level (we consider it separately). An application unit can be transmitted in parts if it does not fit in one physical packet. In general, for those who know the TCP / IP stack, there is nothing fundamentally new here.
The CRC frame ends with a checksum and stop bits similar to the start flag.
DSRC stack application layer
At the application level, information is exchanged within the respective applications. Currently the most common applications are:
- Actually, EFC is electronic charging, AID = 1 (application identifier in the VST table). The application layer is described in detail in ISO 14906-2011
- Localization augmentation communication (LAC) - recording of the antenna location data in the CU, AID = 21, ISO 13141-2010 standard
- Compliance Checking Communication (CCC) - the exchange of control information about the vehicle in order to verify compliance with the rules of charging, ISO 12813-2009
At the points of payment, the EFC application is used to organize non-stop travel.
During the interaction of the antenna and the transponder, a journey transaction is formed, which can be supplemented with vehicle measurement data. Depending on the tariffs entered and the charging rules, a car may be classified at a point of payment, photographing, license plate recognition and even weight control. Each transponder, prior to being issued to the client, passes through an initialization stage, during which an EFC information block is recorded in its memory, which the transponder must transmit during the information exchange.
The information block in the “maximum configuration” consists of about 50 standardized attributes that can be divided into the following groups:
- Information about the user's contract. These attributes are always filled, since without them it is impossible to form a transaction and debit funds from the user’s account.
- Information for the check (financial part, accompanying information on the requirements of local legislation, etc.)
- Information about the car - something similar to the data of our TCP, only in digital form. Only the attributes required for billing and control are filled out.
- Information about the transponder: factory number, smart card number (for the CU into which the smart card is inserted - basically this scheme is used in Japan), the status of the CU.
- Information about the driver and passengers (if the number of passengers is taken into account in the fare)
- Information about means of payment, if the transponder is also a means of payment (as in Japan and in some Asian countries)
- Backup fields for other applications DSRC - LAC, CCC and what else will come up in the future.
How is it all physically implemented
In Russia, DSRC is used at a payment point in St. Petersburg (North-Western High-Speed ​​Diameter), in Moscow (M-1 highway bypassing Odintsovo), M-4 (in the Domodedovo area), and also on the same route closer to Voronezh. On the roads being designed and under construction will also be created a strip of electronic fare payment.
The main problem with the use of EFC is the inability to travel with one transponder on all paid routes. If at the technological level this problem is completely solvable, then at the application level a large amount of additional work is needed related to the organization of the exchange of encryption keys and the organization of netting between the companies-operators.
EFC tariff schemes are minimally applied. That is, the transponder itself is not a means of payment and is not attached to the vehicle. In fact, our transponder is just a digital label of a subscriber. At the point of collection, each vehicle is classified and the tariff amount is deducted from the account linked to the transponder, depending on the results of the classification. That is, the transponder can be easily rearranged from the car to the truck and back.
Such a simplified approach has its advantages and disadvantages. The advantage is the ability to initialize the transponders during production. A bar code is applied to each transponder, which is matched with the user's contract during the sale, after which the user can freely use the transponder. If, for example, the tariff were tied to a car, it would be necessary to install special programmers at the points of sale, allowing you to record the necessary attributes in the transponder’s memory, for example, the car number and its class.
The second advantage of the simplified scheme is the simplicity of ensuring compatibility of transponders with different operators. Regardless of the classification system of each operator, the journey transaction is formed without errors. Otherwise, we would have to align the principles of classification of cars for all operators.
And the disadvantages include the impossibility of autonomous control of payment (without contacting the users' registration database).
The circuit works in this way. During the sale, as I have already said, a comparison of the transponder number and the user's contract takes place. The operator’s accounting system maintains a balance in the electronic account of the transponder. Transponders with a balance amount below a certain level fall into the warning zone - the “orange list”. If the amount is not enough to pay for travel, such transponders fall into the "red" or "black" list. The “color” transponder lists are distributed to all toll stations and loaded into the controllers of the lanes (or immediately into the memory of antennas of some manufacturers).
When a vehicle approaches a toll point in a special lane, it falls within the range of the antenna of the lane, which reads the transponder ID. The band controller (or antenna software) performs a check with the "color" list. If the transaction was formed normally and the transponder number is not listed, the strip controller software gives a signal to open the barrier. In this case, the car passes the point of payment at a speed of about 30 km / h without stopping. If the transponder is in the “orange” zone, the driver receives a warning, for example, the special “Low Balance” sign lights up and the barrier opens. If the funds are not enough, then the barrier does not open, and the user has to pay the fare in cash.
DSRC antennas in the area of ​​payment of the charging point M-4
To pay for travel on the M-4, Kapsch production TS3203 transponders are used. Fully compatible Q-Free, Norbit, GEA, etc. transponders can also be used.
The appearance of the transponder TS3203
In agreement with the road operator, the manufacturer will put a bar code and a number on the transponder body. These transponder identifiers are transmitted as a file along with a batch of transponders. Data from the file is loaded into the operator’s settlement system for subsequent comparison with contracts during sales. Transponders keep in memory an encrypted data block of the operator, which it can read with its keys. Keys are uploaded and stored on EFC band antennas. Thus, transponders are completely ready for sale and use.
Transponder Marking
Inside, we see a simple board and a speaker-squeaker, through which the transponder reports a successful transaction or an error. As you can see, the protection against opening is completely absent. There are no fuses or photoresistors.
Transponder board
On the other side of the board is a battery, tightly pressed into the holder. According to the Kapsch specification , the battery should last for 7 years with 2000 transactions per year.
Reverse side of the board
The core of the transponder is a proprietary ASIC chip containing memory, processor and all logic. The Kapsch analog core is traditionally called Ella, the digital core is Alex. In previous versions, these cores were implemented as separate chips ( photo ).
Fee close up
As we have seen, the DSRC transponder is a reliable and cheap device. That is why electronic charging is universally made using the DSRC. DSRC technology has only one drawback - to calculate the tariff, it is necessary to install antennas at all exits from the road (or in the middle of each section, as in Austria). If we want to close the “large-scale network” with a “paid” one, then we need to carefully look towards satellite-based systems, which I already wrote about ( with a continuation ).
Source: https://habr.com/ru/post/240047/
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