Protocol Design Challenges in M2M Communication

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Connecting smart IoT appliances requires a robust and lightweight protocol that facilitates an efficient Machine-to-Machine (M2M) communication. In this post, we explore some of the interesting challenges in the design of protocols for M2M communication needs.

Low Bandwidth

IoT devices deployed in the field are often connected using networks that have low-bandwidth or offer inconsistent throughput. Hence, the protocol overheads need to be very low – such as having small protocol headers, using variable length headers etc.

For example: Field devices are often connected using 2G / 3G carrier networks that offer low bandwidth.

Two-way Communication:

IoT devices often require a two-way communication channel, whereby communication can be initiated by either parties.

For example: Say, a device needs to send telemetry data to a Backend Application, or the Application wants to dispatch commands to this device. Unlike a traditional HTTP Request-Response model, the communication could be initiated by either of the parties. Polling is an inefficient way of doing this over HTTP.

Intermittent Connectivity

Wide area wireless networks (carrier networks) often experience flaky connectivity. So the underlying protocol needs resilience against connectivity problems. After an abrupt disconnection, when a device comes back online later, any pending messages for this device should be automatically delivered to it.

For example: IoT devices can fade in-and-out of network connectivity as their geo-location changes, or they switch to a ‘hibernation mode’ to conserve power.

Low Compute Footprint

Embedded devices often have low compute capabilities – CPUs with lower energy consumption, lower clock rates, and low memory availability. So the protocol implementation should require a minimal compute and memory overhead on those devices.

One-to-Many Communication Model

Unlike conventional protocols such as HTTP which only allow a 1-to-1 communication, an IoT protocol should allow a one-to-N communication model.

For example: Messages from a temperature sensor could be routed to a telemetry logging service, a temperature control logic, and a monitoring dashboard – all at the same time!

Asynchronous Model

Any party can dispatch messages at will, sometimes even without knowing if the recipient is online at that point in time. As devices fade in-and-out of network connectivity, or hibernate from time to time, it would be inefficient for the senders to keep polling recipients over a HTTP request-response model. Hence, the underlying messaging protocol is expected to be asynchronous in nature

For example: Most IoT devices sense real world events and trigger messages based on the occurrence of those events. Sensors can dispatch this information without really knowing if the intended recipients are online / offline at that point in time.

For example: Applications need to instruct devices by dispatch commands to them. Applications could dispatch commands without knowing if the recipient device is online / offline at that point.

Decoupling of Participants

The protocol needs an appropriate ‘routing mechanism’ whereby a message can be routed to a set of interested recipients, without the sender necessarily knowing who the exact recipients are. The set of recipients for a certain type of message could change as the ecosystem evolves.

For example: A logger device may be interested in temperature information today. Tomorrow, a new dashboard may also be interested to receive this information. The temperature emitter (sensor) may be oblivious to who the recipients actually are.

Routing Complexity

The routing complexity and responsibility should be encapsulated within the IoT Cloud (and not within the devices themselves). The device should be offered a simpler model to connect, dispatch, and to receive messages asynchronously.

Security

This is a paramount concern in IoT today. The protocol should support a strong security model to protect data-in-motion using a robust PKI infrastructure. It needs to offer capabilities such as: Secure endpoint identity (Client certificates), real-time authorization controls, and real-time policy enforcements.

Adoption of Standards

Most embedded OSes and all cloud OSes today support the TCP/IP stack. So a suitable IoT protocol is expected to run on top of the standard TCP/IP stack to ensure highest compatibility across multiple hardware and OS platforms.

Wrapping Up…

M2M communication brings its unique set of challenges – Intermittent connectivity, asynchronous communication needs, and a need to decouple participants. Having an intermediate messaging queue (broker) is a great way to address many of these requirements. Hence IoT protocols such as MQTT adopt a pub-sub, queue-based architecture and are thus better suited for IoT applications today.

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