Intro to Computer Systems

Chapter 6b: Peripheral Expansion

Wireless Expansion

Wireless external peripherals are far more convenient to their wired counterparts - they allow for an extra degree of freedom in how they can be organised, and reduce cable clutter on a desk.

Legacy wireless devices were often limited to keyboards and mice, and used a part of the 27 MHz radio spectrum typically used for low end radio-controlled cars, and analog cordless phones. These were very susceptible to both interference, and interception: these simple wireless peripheral protocols were unprotected and the information transmitted across them can be very easily "sniffed".

This YouTube video shows a basic example of 27 MHz keyboard sniffing.

Modern wireless protocols are securely encrypted, and typically make use of wireless communication features like code-hopping and spread spectrum transmission to minimise its sensitivity to interference and its susceptibility to being intercepted.

Bluetooth

Bluetooth is a communications standard for short-distance wireless transmissions typical for peripherals that make up part of a personal area network (a collection of devices on or about the person). It was developed as a general-purpose wireless standard and thus has a number of transmission standards and classes, and utility profiles, to suit a diversity of uses.

The Bluetooth ecosystem as part of a personal area network. Diagram: Bluetooth SIG
The Bluetooth ecosystem as part of a personal area network. Diagram: Bluetooth SIG

The initial Bluetooth standard (1.2) provided a peak bandwidth of 1 Mbit/sec, with Bluetooth 2.0 including an "Enhanced Data Rate" option which increased this to 24 Mbit/sec. Future versions of Bluetooth further extended this to 24 Mbit/sec.

Transmission Standards and Profiles

Bluetooth is meant to be a widely appropriate wireless protocol, and to facilitate this it needs many different forms - what is appropriate for a wireless keyboard and mouse (very low bandwidth, short range) is not useful for tethering a mobile phone to a laptop (high bandwidth, low range) or attaching a phone to a hands-free speaker or a car's entertainment system (low bandwidth, medium range).

Bluetooth Classes

The first segregation for Bluetooth peripherals is the idea of a device class - analogous to the appropriate range of the device. There are three such classes in the standard:

Class
Transmit Power
Approx. Range
1
100 mW
100 metres
2
2.5 mW
10 metres
3
1 mW
1 metre

An appropriate transmission power (and range) is important as portable devices are heavily power-constrained, and the radio transmitter is typically the most power-hungry component of the entire device. For example, a wireless mouse may be a class 2 device; compared to if it was a class 1 device, this yields up to a 40-fold reduction in radio power consumption.

Bluetooth Profiles

The other segregation of Bluetooth peripherals is the idea of a device profile - this is the actual communications protocol(s) that the device supports, each tailored for a specific purpose. There are a large number of device profiles, the most common being:

Bluetooth profile settings in a car's entertainment system.
Bluetooth profile settings in a car's entertainment system.

Bluetooth devices can support many profiles, even simultaneously. For example, a set of Bluetooth headphones with a microphone may support A2DP (to act as high quality headphones) and HSP (to act as a wireless hands-free).

Bluetooth Connections

Bluetooth connections rely on an initial process called pairing to associate one device with another.

The first step in pairing two devices is for one to be made discoverable. What this does is make the device broadcast a number of details, such as its name, device class, and the profiles that it supports. At this connection stage, the pairing device will list the discoverable devices it can detect.

The pairing process itself begins by sharing a link key between the two devices - this is to ensure that there is a secure link between two specific Bluetooth devices that is protected from eavesdropping or accidental communication with another nearby device.

In addition to the various device-related information, each Bluetooth device has a Media Access Control address (MAC) which uniquely identifies it.

As part of this process, the two devices will authenticate by way of a PIN code. (Some devices, such as hands-free sets, may have a hard-coded PIN such as 0000 or 1234.) Once connected, the devices will set themselves up to work on the various profiles that have been enabled by the user controlling them.

Once a device has been paired, they are considered bonded. What this means is that the pairing is remembered - if the devices are in proximity in the future, they will automatically re-connect through the information saved during the initial pairing process.

Personal Area Networks

As miniaturisation of smart devices continues, wearable computing is a concept that is becoming mainstream. Where five years ago a person may have a smartphone on them; they may now also have a network of devices connected to it: for example, a heart-rate monitor when exercising, wireless headphones to listen to music, and a smartwatch to be discreetly notified of events - all in constant communication.

Even Bluetooth's traditional low power classes are too power-hungry for these types of activities, and new standards have been developed to facilitate this type of networking without too many consequences on battery life.

ANT

The first commercially feasible market for this type of personal area network was in fitness equipment, where biometric devices can measure information about the performance and health of an athlete and transmit this back to a central "head unit" for display, logging and diagnosis.

A crank-mounted bicycle power meter that supports ANT+ and Bluetooth Smart.
A crank-mounted bicycle power meter that supports ANT+ and Bluetooth Smart.

The ANT network technology was the first open standard for this type of communication, being used for heart rate monitors, speed and power measuring devices on bicycles, and GPS logging watches for runners. An extension to the standard, ANT+, was further developed to allow interoperability and sharing of information between sensors specific to the health and fitness sector.

Bluetooth Low Energy

The Pebble smartwatch uses Bluetooth Smart to connect to a smartphone.
The Pebble smartwatch uses Bluetooth Smart to connect to a smartphone.

A recent version (4.0) of the Bluetooth connection standard included an optional operating mode known as Bluetooth Low Energy, also known as "Bluetooth Smart". This, like ANT, is a very low-power, battery efficient means of communicating in a personal area network and is tailored towards the smartwatch, healthcare and sports sectors.

Given the ubiquity of Bluetooth controllers on mobile phones and tablets, BT Low Energy is supported on many modern devices, whereas the ANT protocols are typically confined to task-specific devices such as bicycle computers and fitness smartwatches.

Proprietary PAN Protocols

There also exist proprietary PAN protocols that are used by specific manufacturers, typically due to their significance in a certain market sector, or their use of PAN-like features before open standards such as ANT or BTLE were available.

One such example is Polar, a heart-rate monitor company, which developed a protocol called WIND that is very similar but incompatible with ANT. (Their legacy analog protocol, on the 27 MHz spectrum, is a de facto standard for gym equipment.)