What is Access control method?

A short-range wireless standard for achieving wideband sensor networks

Access Control Method

Simply put, Access control method is a method of restricting users who can Access networks, systems, data, etc. Furthermore, Access control has three main functions: authentication, authorization, and auditing.

certification

This function controls whether users are allowed to log in or not. Each user attempting Access is authenticated using their ID, password, biometric authentication (fingerprint, face, etc.), electronic certificate, etc., and permission or denial is controlled.

authorization

This function limits the scope of control for each user who is permitted Access. Users perform operations within the scope of their control based on a predetermined Access control list. Access is determined through authentication, and the scope of operations is further narrowed after Access.

audit

It records a log of authentication and authorization. By recording Access history, it is possible to verify the legitimacy of Access authentication and authorization, contributing to improvements. This function makes it possible to respond quickly from the log history even if an unauthorized login occurs. Various signal transmissions take place within the network, and conflicts and collisions can occur. Several Access control methods are used to avoid abnormal communications in such events. This time, we will explain a method for controlling signal transmission at the data link layer to avoid signal collisions that occur on the communication medium when using a Zigbee network.

Beacon and non-beacon modes

ZigBee has two communication methods: one is non-beacon mode, in which all nodes communicate directly with each other using CSMA/CA, and the other is beacon mode, in which beacon signals are transmitted periodically to enable intermittent operation and guaranteed bandwidth communication.

CSMA/CA

ZigBee's Access control method uses CSMA ⁄ CA (Carrier Sense Multiple Access with Collision Avoidance), which is also used over Ethernet. It checks the condition of the transmission line to prevent interference and waits for a random time (backoff) if it is free before sending data. In addition, data can be retransmitted in the event of a collision.

① Check whether there are any signals being transmitted on the network. ② Start data transmission after a random waiting time. ③ After receiving the data, ZC sends an ACK to all devices in the network. ④ After confirming the ACK from ZC, start data transmission after a random waiting time.

In practice, as shown in the diagram below, the CCA (Clear Channel Assessment) method is used to check whether the channel is free (idle slot), and if the backoff is 0, data is sent. However, when CCA determines that the channel is busy, there is an upper limit to the backoff counter value, and if the upper limit is exceeded, the data is discarded. However, if an ACK is not received when sending data, the data will be resent.

Here's a more detailed explanation of the above. Before sending data, the device waits for a backoff period and then starts CCA. The CCA period is determined by the CCA detection time and determines the channel usage status. The backoff period is calculated by generating a random number (integer value) within the range of [0, 2 ^BE-1] and multiplying that random number by the BackoffPeriod. Here, BE (Backoff Exponent) is initialized to BEmin, and if CCA determines that the channel is busy, BE is incremented. However, BE must not exceed BEmax. (By default, BEmin is set to 3 and BEmax to 5. Also, 1 BackoffPeriod is theoretically 320us.)

If CCA determines that the channel is idle, it will send data after the aTurnaroundTime (192us) period has elapsed. On the other hand, if it determines that the channel is busy, it will wait again for the backoff period. If data is received, it will send an ACK after the aTurnaroundTime period has elapsed. There is a maximum limit, MaxBackoff, on the number of backoffs when CCA determines that the channel is busy, and if this limit is exceeded, the data will be discarded. After sending data, if an ACK is not received within the ACK waiting period, the data will be retransmitted. There is a maximum data transmission retry value, which is the maximum number of retransmissions, and if this limit is exceeded, the data will be discarded.

CCA data transmission and reception diagram

Hidden terminal problem

In fact, ZigBee communication has a hidden terminal problem: if terminal A and terminal B attempting to transmit data to a ZigBee coordinator (ZC) are outside each other's communication range, terminal A and terminal B cannot sense the carrier (check the radio wave status), and a collision may occur in ZC.
In the diagram below, terminal A can communicate with ZC, but cannot communicate with terminal B because they are too far apart. Similarly, terminal B can communicate with ZC, but cannot communicate with terminal A because they are too far apart.

FRAMES

ZigBee prevents hidden terminal issues with a mechanism called Super FRAMES. Super FRAMES indicates a FRAMES separated by a beacon and a beacon. ZigBee can optionally use Super FRAMES with the ZigBee Coordinator (ZC). Each terminal communicates using CSMA/CA, and there is a CAP (Contention Access Period) that allows all terminals to Access the beacon interval, and a CFP (Contention Free Period) that allows only terminals with permission from ZC to Access. There is an inactive period in which all devices are banned from Access.

ZC allows the setting of guaranteed time slots called GTS (Guaranty Time Slots) for the purpose of low latency. The CFP is divided into up to seven sections by the GTS mechanism, and can be assigned to terminals that wish to communicate with priority. The diagram above shows the allocation when the GTS mechanism divides the seven time segments into three sections each into GTS1 and GTS2. Terminals that are assigned this period can send and receive data with priority.

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