Lecture #17 - Personal Area Networks: Part 1
In this lecture we start to investigate technologies used to create
Personal Area Networks (PANs). A PAN is used to interconnect
devices within the range of an individual. Examples of this include connecting
a computer to a printer, connecting a mouse or keyboard to a computer and
connecting a headset to a mobile phone. A number of wired and wireless
technologies can be used for PANs including USB, Bluetooth, IrDA and IEEE1394
(aka Firewire).
USB - The Universal Serial Bus
The Universal Serial Bus (USB) was introduced in 1996 and provides a serial
interface that is extremely easy to use. USB has since become a defacto
standard and can be found in most peripherals and many consumer devices.
USB has been highly successful, with more than 2 billion devices having USB
interfaces in 2008. Some of the reasons for its success include:
- It is relatively simple to use.
- Provides power as well as communications.
- Hot pluggable (no need to power everything off before
connecting/disconnecting).
- Reasonably high data rates.
USB Network Structure
A USB network forms a tree structure of up to seven levels. At the root of the
tree is a host controller and root hub. The host controller is responsible for
controlling all communication between all attached USB devices. The root hub
provides interfaces to which additional USB devices can be attached.
Tiers 2 to 6 of the tree may contain other hubs or "functions" (eg. a mouse,
printer or hard drive). Tier 7 can contain only functions.
USB Physical Layer
All USB data is carried by 4 wire cable. The cables are fitted with a
USB Series A plug on the upstream end (facing the host). The downstream end
may be permanently wired into a device or may be fitted with a USB Series B
plug.
 |
|
 |
| USB Series A Plug |
|
USB Series B Plug |
This forces users to wire their networks correctly, they just can't do it any
other way. In addition to the standard USB-A and USB-B interfaces, the smaller
Mini-A, Micro-A and Micro-B interfaces were introduced, allowing for USB to
be used in consumer devices such as cameras and portable music players. All USB
cables should have an "A" interface on one end and a "B" interface on the other.
Two wires are used to carry 5 volt power to low consumption devices such as
mice (< 500 mA). Higher power consuming devices may have their
own separate power supply.
The other two wires are used to carry half duplex balanced differential data.
Depending on the bit rate, the data circuit may be unterminated or may be
terminated at both ends.
| USB 1.0 Low Speed |
1.5 Mbps |
Unterminated |
| USB 1.0 Full Speed |
12 Mbps |
Terminated |
| USB 2.0 High Speed |
480 Mbps |
Terminated |
USB Bit Level Protocol
USB data signals are "differential" ie. two wires are required to convey one
bit. Usually these wires carry opposite signals. The USB specification defines
two normal data signal states.
| "J" |
D+ line = 400mV, |
D- line = 0mV |
| "K" |
D+ line = 0mV, |
D- line = 400mV |
-
All packets begin with a prominent preamble and synchronising flag
(like Ethernet). This is a bit sequence KJKJ.....KJKJKK.
-
Following the preamble, packets have a header which depends on the type of
the packet. There are 16 different types. See later in this lecture.
-
Packets are NRZI encoded. As detailed previously, NRZI maintains a constant
voltage for the duration of a data bit. Data ONES are encoded as a change
in level at the beginning of a bit. ZEROES are encoded as "no change" at
the beginning of a bit.
-
Packets are "bit stuffed" to guarantee that the preamble and headers are
unmistakable and that there are regular changes in the signal (to aid clock
recovery). Bit stuffing consists of inserting a '0' after any six
consecutive '1' bits.
-
Normally, the two data lines carry a "J" or a "K". Both data lines may be
dropped to zero volts for special signalling purposes.
How USB Works
All communications are initiated by the host controller and must pass through
the host controller.
To discover the existence of new functions, the host regularly polls all the
hubs. Each hub records the connection of a new function. Upon discovering the
new function, the host installs any new drivers needed, has a chat to the
function to assign it an address and carries out any initialisation required
by the function.
Since the host controls all communications, the USB protocol uses a 3 or 4 way
handshake to invite a function to pass data back to the host:
A similar sequence is used to send data to a function:
USB Packet Types
USB defines 16 different major packet types (and many other minor types).
The main ones are:
| OUT |
Address + endpoint number for a
host to endpoint transaction |
| IN |
Address + endpoint number for a
endpoint to host transaction |
| ACK |
Receiver accepts error free data.
There are three other similar handshake packets. |
| DATA |
Four types of data packet. |
| SPLIT |
Provides additional transaction types. |
| SOF |
Distributes timing information. |
IN, OUT and SETUP packets
| PID |
Identifies packet type. This is followed by the complement of the
type (for error checking). If not correct, the packet is rejected.
|
| ADDR |
Address of an endpoint (hub? or function). 127 can be addressed. |
| ENDP |
Endpoint number. Each function can handle 16 different communication
channels. Many functions implement separate control and data channels. |
| CRC |
Five bit CRC covering Address and Endpoint. |
Data Packets
ACK, NAK, STALL, NYET and ERR packets
Start-of-Frame Packets
SOF packets are issued by the host controller at regular intervals (every
1.000ms +/- 0.0005ms for full speed connections or every 125us +/-0.0625us
for high speed connections). Frames are numbered for the use of functions that
need to "know the time".
SOF frames don't cause any receiving function to generate a return packet.
The standard doesn't make obvious what a "transmitting" function does in
response to a SOF. It may be that the function uses the timing information for
its own purposes but doesn't communicate any resulting data until next polled
by the host controller.
Bluetooth
Bluetooth is a wireless networking protocol intended to replace all of the
wire presently used to connect things together. It allows devices to form
small networks (piconets) consisting of a master station and one or more slave
stations. Slave stations may be members of one or more piconets, as may
masters (forming irregular scatternets).
Bluetooth Protocols
Bluetooth is implemented in a layered manner, similar to Ethernet/TCP/IP.
The lowest levels are Bluetooth specific. Higher layers use established
protocols, enabling existing data services to be easily carried:
Bluetooth Radio
Technical stuff:
-
Short range, typically <30m. Transmitter power typically 1mW, but up to
100mW may be used. Higher power devices must implement "power control" ie.
be able to reduce their power automatically if conditions permit.
-
Operating frequency = 2400MHz to 2483.5MHz Industrial Scientific and Medical
(ISM) radio band.
-
Bluetooth uses frequency hopping spread spectrum to allow a number of networks
to be co-located without interference. The hopping sequence is determined by
the Master station. The Master and the Active Slave take turns at sending
packets.
-
Channel spacing = 1MHz.
-
Channels used 23 or 79 depending on location.
-
Modulation Gaussian Frequency Shift Keying (A form of Digital Frequency
Modulation).
- Data rate 1Mbps
-
Hopping rate = 1 every 625us. Normally, each hop conveys one data packet.
Packets can be extended to take five hop periods (staying on the same
frequency).
-
Channel data rates can be up to 723/57.6Kbps assymetric or 440/440Kbps
symmetrical.
Baseband
The Baseband protocol actually controls some of the aspects mentioned already:
- Frequency hopping pattern.
- Formation of piconets.
- Packaging of bits.
In addition the Baseband protocol can reserve time slots to form two different
types of data link:
- Synchronous Connection-Oriented (SCO)
- Asynchronous Connectionless (ACL)
ACL packets are used for data only, while the SCO packet can contain audio
only or a combination of audio and data. All audio and data packets can be
provided with different levels of FEC or CRC error correction and can be
encrypted.
Furthermore, the different data types, including link management and control
messages, are each allocated a special channel.
Logical Link Control and Adaptation Protocol
L2CAP provides connection-oriented and connectionless data services to the
upper layer protocols with protocol multiplexing capability, segmentation and
reassembly operation, and group abstractions.
L2CAP permits higher level protocols and applications to transmit and receive
L2CAP data packets up to 64 kilobytes in length.
Although the Baseband protocol provides the SCO and ACL link types, L2CAP is
defined only for ACL links.
Cable Replacement Protocol - RFCOMM
RFCOMM is a serial "cable replacement" protocol emulating RS-232 control and
data signals over Bluetooth baseband. RFCOMM emulates up to 60 circuits
between two devices (including about 7 RS-232 handshaking signals for each
circuit). Data rates may be artificially limited so that Bluetooth doesn't
overload any slow hardware.
Audio
The audio model is relatively simple within Bluetooth. Any two Bluetooth
devices can send and receive audio data between each other just by opening an
audio link. Audio data is carried in SCO packets and is routed directly to and
from Baseband. It does not go through L2CAP.
Link Manager Protocol
The link manager protocol is responsible for link set-up between Bluetooth
devices. This includes security aspects like authentication and encryption by
generating, exchanging and checking of link and encryption keys and the
control and negotiation of baseband packet sizes.
Furthermore it controls the power modes and duty cycles of the Bluetooth
radio device, and the connection states of a Bluetooth unit in a piconet.
Service Discovery Protocol (SDP)
Discovery services are crucial part of the Bluetooth framework. These services
provide the basis for all the usage models. Using SDP, device information,
services and the characteristics of the services can be queried and after that,
a connection between two or more Bluetooth devices can be established.
IrDA: The Infra Red Data Association
This is a standard for short range (about 1 metre) communication between
portable computers, PDAs (and just about anything else so its proponents
claim).
IrDA links are bi-directional half duplex and can have data rates up to
16Mbps. The protocols used form a layered family like the Internet model.
At the bottom of the heap, the Physical layer details
the characteristics of the infra-red emitters and detectors. It also specifies
the type of modulation used. There seem to be a couple of different encoding
schemes, depending on data rate:
- Up to 1.152 Mbps RZI is used - somewhat like manchester encoding (details
unclear)
- 4 Mbps - Each character consists of 4 "chips". Within each chip,
four possible symbols can be sent (a one bit and 3 zeroes). Thus each
chip can convey two binary bits and 4 chips together convey a byte.
Above this layer, the Data Link Layer defines how link
access and management is done.
Finally, the highest layer implements flow control.
Copyright © 2007-2008 Joel Sing
Copyright © 2006 Phil Rice
