Applications like environmental sensing, and health and activity sensing, aresupported by networks of devices (nodes) that send periodic packettransmissions over the wireless channel to a sink node. We look at simpleabstractions that capture the following commonalities of such networks (a) thenodes send periodically sensed information that is temporal and must bedelivered in a timely manner, (b) they share a multiple access channel and (c)channels between the nodes and the sink are unreliable (packets may be receivedin error) and differ in quality. We consider scheduled access and slotted ALOHA-like random access. Underscheduled access, nodes take turns and get feedback on whether a transmittedpacket was received successfully by the sink. During its turn, a node maytransmit more than once to counter channel uncertainty. For slotted ALOHA-likeaccess, each node attempts transmission in every slot with a certainprobability. For these access mechanisms we derive the age of information(AoI), which is a timeliness metric, and arrive at conditions that optimize AoIat the sink. We also analyze the case of symmetric updating, in which updatesfrom different nodes must have the same AoI. We show that ALOHA-like access,while simple, leads to AoI that is worse by a factor of about 2e, in comparisonto scheduled access.
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