David Evans explains the challenges and choices faced by those embarking down the M2M path
The ability to send data over a mobile phone network has been available for over 20 years but only in the past few years have we witnessed the exponential growth in M2M (machine-to machine) applications. Increased network coverage, higher data throughput and the continual search for innovation has led to mobile connectivity becoming one of the most substantial growth areas in industry today.
Robin Duke-Woolley, CEO at Beecham Research, made the following observations about future market developments: "M2M has now come centre stage for mobile operators as the next big area for connections growth. Beecham Research sees overall cellular network connections growing rapidly over the next few years to over two billion worldwide by 2020. However, this is a relatively conservative forecast. If we all get used to using five to ten mobile connected devices each by this time then projections of 12 to 15 billion connected mobile devices worldwide by 2020 are achievable. We need to think about the implications of that scenario."
There are only a finite number of ways to make internet connections cost effectively. Most home routers use the twisted pair copper from BT as a carrier for ADSL services or the fibre-optic network delivered by specialists such as Virgin Media (formerly NTL).
The newest internet delivery mechanism, mobile broadband, uses the rapidly evolving and expanding mobile phone networks to provide an internet connection anywhere a mobile phone signal can be received.
In 2001, GPRS was just starting to emerge as an IP based data delivery mechanism providing typical real-world data rates of just 50kbit/s download and 20kbit/s upload. Ten years on, megabit download and upload is a reality thanks to a well established 3G network in the UK.
The rapid increase in over-the-air bandwidth is set to continue with the advent of 4G networks based on LTE (long term evolution) technology. This latest generation of cellular internet connectivity is designed to deliver an all IP based architecture and most importantly, incredible data-rates of tens of megabits per second over the air.
So what fundamental challenges can be addressed with the application of cellular connections in industrial M2M applications?
Instant internet connectivity
Consider an installer of mobile ATM machines. They are dependent on internet connectivity for machines to validate transactions. Conventional dial-up modems require a land-line which could take two months to install and may add a large cost to run a cable to the desired location in a retail outlet.
In this instance a wireless internet connection through a device such as a GPRS modem or GPRS router could reduce installation cabling costs to zero and provide an internet connection in minutes rather than months. In addition, a well-calculated GPRS tariff can provide a lower monthly recurring cost than that of a land-line.
Moving vehicles and machines
The cell-handover capability inherent in mobile networks allows M2M devices to stay connected to the internet on the move. The geographic reach of modern networks provides extensive wireless internet availability. For example, the 2G networks in the UK provide coverage to more than 99% of the population and the figures are similar for most developed countries. From a device connectivity perspective, this service can be thought of as providing a serial (RS232 or RS485) or Ethernet cable the length and breadth of the country.
This is a powerful enabling technology that facilitates applications that were simply impossible 20 years ago.
One of the biggest uses of mobile internet connectivity is in the vehicle tracking sector. GPS and GPRS combine to provide near real-time status of a vehicle's location to a remote observer.
Tracking technology can lead to increased productivity, reduced operational costs, improved customer service and enhanced security for both driver and vehicle.
Billing of cellular internet connections is achieved by measuring the amount of data sent and received across the network. There is a great deal of flexibility in the available tariffs including very low data usage plans from as little as £1 per month. Well designed applications can be developed to reduce data transfer over the air to make mass deployments an affordable reality.
Some systems are able to use SMS messaging for data transfer to a central server. While this method of communicating does not offer the versatility of a TCP/IP based link, it can be very low cost – ideal for applications such as tank level measurement where information is only needed from time to time.
Put simply, what other internet service provider can offer connections for as little as £1 per month?
So how do you internet enable a device or system using the cellular networks? There are three core product types that manufacturers and system integrators can use to internet enable their systems – modules, modems and routers.
At the heart of the M2M industry are the GPRS and 3G modules, see Fig. 2. These surface mount or connectorised components are integrated into a PCB design to provide GSM, SMS and TCP/IP connectivity functions. Modules are configured and driven by AT commands borrowed from the world of the Hayes dial-up modem. Some standardisation of the AT command sets for GSM devices is laid out in the GSM07.05 and GSM07.07 standards.
Most modules require an external micro or Pic to manage the internet connection by sending AT commands at appropriate intervals over an RS232 serial link or a USB connection.
The requirement to have additional processing can be removed by choosing a programmable module. Such a device provides a full Java virtual machine onboard that can support a bespoke user application. Access to all of the hardware on the module is made available to the programmer through APIs. Functions include digital IO, analogue IO, dual RS232 ports, USB port and a quad-band GPRS interface. Other modules on the market employ alternative programming languages such as Python or C/C++.
Modules have a low unit cost with GPRS variants now selling for as little as £15 in volume. However, there is a relatively high integration cost associated with incorporating cellular devices into products at this level. The carrier PCB must be designed, manufactured, tested and the end product may be liable for CE re-testing in adherence to the R+TTE directive.
Using a pre-approved module significantly reduces the approval work required. In Europe, the R&TTE Directive (1999/5/EC) applies to any product incorporating an RF device. If the module has been approved to an appropriate standard – typically EN 301 511 for a GSM, and an antenna listed with that approval is used – then the radio emissions of the device are covered. The module should also have been tested to EN 301 489-7 for a GSM device, this is the EMC part of the radio testing. So providing the testing configuration is representative of the final installation, it is not necessary to repeat the detailed GSM EMC performance tests, only those tests outlined below.
It is also necessary to meet the safety requirements of the Low Voltage Directive (LVD). There is often confusion here, as under the LVD equipment operating under 50V DC does not have to comply. However, under the R&TTE Directive, the requirements of the LVD apply, regardless of the supply voltage. Typically the safety standard EN 60950-1 (safety of IT equipment) is applied.
The EMC standard EN 301 489-7, refers to EN 301 489-1. The requirements for a battery powered product, are just electrostatic discharge (ESD) and RF immunity from 80 to 1000MHz and 1400 to 2700MHz. There is always a requirement for the product not to transmit unintentionally. Also, for equipment that does not provide a continuous link, the manufacturer would supply the specification for acceptable performance, which the product must meet during testing. For mains powered products, conducted emissions, conducted immunity, voltage dips and interruptions, transients, and surge testing is required. Also mains harmonics and possibly flicker testing is required. In an existing product, where a GPRS module has been added, much of this mains port testing may have been done under the EMC Directive. However it is necessary as a minimum to re-test conducted and radiated immunity.
Modules are the logical option for any products that need cellular connectivity and are produced in high volumes such as coffee machines, vehicle trackers and smart meters. For lower volume products, manufacturers may choose to incorporate a finished modem to remove design-in costs.
In essence, a cellular modem is a module with an enclosure, PSU, data and antenna connectors, see Fig. 3. Connection to the modem is typically made with an RS232 or USB cable.
The operation of the modem is dictated by the command set of the module and requires external AT commands to be issued to generate the desired response. For example, the command ATD *99***1# is used to initiate a GPRS connection or AT+CMGS to send a text message.
A new generation of intelligent RS232 modems can be pre-configured to perform a variety of advanced functions including auto-connection to the network and auto-reboot if the signal is lost. This makes connections to remote serial devices very easy – the RS232 port is identified by an IP address and TCP port number which allows the use of virtual com port software on a PC to integrate seamlessly remote serial devices into existing application software.
Typical pricing of such modems is around £100 providing a good halfway house between high integration complexity modules and the more expensive and elaborate routers.
Cellular routers feature a module and an embedded PC to provide high-speed routing of IP datagrams from the lan side (Ethernet) to the wan side (mobile network). Whenever a remote connection is required to an Ethernet device, a router is usually required. Most devices also include an RS232 or RS485 port.
Operation of the router is identical in principle to that of a home broadband router. The major difference stems from the type of carrier on the wan interface. Most home routers use the twisted pair copper from BT as a carrier for xDSL services or a fibre-optic network. The wan interface of a cellular router employs a radio link to the nearest cell tower and on into the service provider's core network where IP traffic is routed to the internet or directly to a private network.
Routers are typically available with one or four Ethernet ports (implemented with a built-in switch) and supplied as GPRS only or 3G and GPRS enabled. Advanced routers have many networking features, including:
• VPN server and client capability using IPSec, L2TP, PPTP and Open VPN;
• VRRP virtual router redundancy protocol for automatic router fail-over;
• Automatic reboot – reboots the modem inside the router if the internet connection is lost; and
• Remote control – raise and lower the 3G/GPRS interface with an SMS message.
Routers can be deployed to provide instant internet connectivity to Ethernet and serial devices. They provide the quickest and simplest method of making an M2M connection but also have the highest cost. Industrial quality GPRS routers sell for around £250 and their 3G counterparts cost in the region of £500.
The choice of sim card (see Fig. 5) is critical to making a successful connection and must be considered at the early stages of system design. Standard sims only provide a dynamic, private IP address to which there is no logical route across the internet. This is a well known issue and often referred to as the “one way problem”. To comprehend fully the issue requires a detailed understanding of the network provider's infrastructure and is beyond the scope of this short article.
To solve the one way problem, it is possible to buy sim cards with a public IP address or with an associated VPN connection that allows direct connection from a PC to a remote device.
While the public IP address sim is very easy to implement, it is not without some drawbacks. The major issue relates to unsolicited data being sent to the device (by virtue of it being visible to anyone on the internet) and large data usage bills being incurred. There are ways to limit this problem such as firewalling all ports except the ones required but there remains a risk. SSL based VPNs offer an alternative to the one way problem but require software to be installed on the local PC to provide a VPN interface.
There is no doubt that M2M is a growth market that will affect us all over the coming years. It brings with it the ability to reduce costs, innovate and, most importantly, will provide a fresh revenue stream for many high technology companies in the near future.
David Evans is technical sales manager for Adaptive Modules Ltd