ISO 20205 pdf download – Space data and information transfersystems -Spacecraft Onboard Interface Systems —Low Data-Rate Wireless Communications for Spacecraft Monitoring and Control.
2.4 DIFFERENTIATING CONTENTION-BASED AND SCHEDULED CHANNEL ACCESS
There arc two predominarn types of medium-access schemes currently utilized in wireless sensor networks: random or conie,rion-bascd access and schedukd acccss (see reference [F2]). Contention-based schemes require no centralized control of network access and are thus well suited for ad-hoc network architectures as well as other situations where it is desirable to minimize network administration overhead and operational complexity. Nodes are allowed to attempt channel access at arbitrary times in an ad-hoc fashion as dictated by local data traffic flow and must therefore contend with one another for access in a fairly random manner. The most common contention-based access technique utilized in sensor networks is Carrier-Sense Multiple Access (CSMA) with Collision Avoidance (CA). generally abbreviated as CSMA-CA or simply CSMA. In contrast, scheduled access schemes require some type of (generally centralized) control mechanism for coordinating network access for all nodes in the network in a synchronized fashion. Typically, this will be based on predetermined or anticipated traffic how so that bandwidth is available in a predictable manner that precludes contention among the nodes. This approach increases network administrative overhead and operational complexity but facilitates QoS guarantees and deterministic network behavior. The most common scheduled access technique utilized in sensor networks is Tirne-l)ivision Multiple Access (Tl)MA).
In terms of application support, CSMA is best suited for situations where tight bounds on packet latency and packet jitter are not required but nodes may sometimes require relatiely large amounts of available channel bandwidth for relatively short periods of time in a relatively unpredictable manner. (‘SMA does not readily support detrrmi,,i.rtic network behavior but does readily support hurstv and aperiodk’ traffic flow, In contrast. TDMA is well suited for applications requiring much tighter bounds on packet latency and jitter but for which the traffic flow from the nodes is more uniform and predictable. TDMA readily supports deterministic network behavior but is generally better suited for applications with less bursty and more periodic traffic how. In addition, interference avoidance schemes such as frequency hopping are far more easily implemented in a scheduled TDMA MAC sublayer than in a contention-based CSMA MAC sublayer. The same applies to maintaining connectivity in a mesh network topology that supports multi-hop relay traffic with battery powered nodes on a low duty cycle (long sleep period, short active period), although multi- hop transport is beyond the scope of the current Recommended Practice.
2.5 SECt:RITY PROVISIOM%G
Wirclcss networks suffer the maladies of both active tampering and passive eavesdropping due to the inherent nature of wireless communications where access to the transmission media is not a physical constraint as within wired communications. In addition. wirciess sensors have severely limited computational processing power and may have no available onboard data storage. Because of the computational complexity of cryptographic algorithms. coupled iiih the limited battery-based lifetime of a wireless sensor node, security provisioning in these types of dcv ices is a pragmatic engineering balance.
The cryptogi-aphic mechanism in this standard is based on symmetric-key cryptography and uses keys that are provided by higher.Iayer processes; the mechanism assumes a secure implementation of cryptographic operations and secure and authentic storage of keying material (reference [I]). For the recommended practice contained in this document, the PHYMAC layer provides services that support data confidentiality, data integrity I authenticity). and replay protection.
ISO 20205 pdf download – Space data and information transfersystems -Spacecraft Onboard Interface Systems —Low Data-Rate Wireless Communications for Spacecraft Monitoring and Control
