Transport mechanisms

Connecting devices

• Devices can be connected in two ways
  • Wireless
  • Wired
• There are many types of wireless connection
  • Mobile phone (many subtypes)
  • IEE 802.11 (many subtypes)
  • Bluetooth
  • Infra-red
• There are many types of wired connection
  • one-wire
  • two-wire
  • serial cable
  • parallel cable
  • ethernet cable (many subtypes)
  • firewire
  • power cables
  • phone lines (many subtypes)

Wired characteristics

• Nodes must be connected in some topology (e.g star from central hub, linked by single cable)
• Range on each link
• Cell boundaries
• Cell size

Wireless characteristics

• Range in free space
• Attenuation by materials
• Power consumption of transmitter
• Power consumption of receiver
• Static or moving receivers?
• Cell size
• Cell handover

Ethernet

• The main standard for computer to computer connection
• Common speed is 10Mbits/second, though 100Mbps is also popular and higher speeds are possible
• An ethernet card costs A$30 upwards for 10/100Mbps
• Category 5 cable is A$1.15 per metre from Jaycar
• An 8-port 10 Base-T hub is about A$100
• An ADSL modem/router is about $A150

Serial/parallel cable

• 9 core shielded cable is A$2 per metre
• Used to connect computers to external devices
• There are many kits to interface to other devices
  • 8 relay switches and 0-5V analogue output (Dick Smith, A$50)
  • Stepper motor controller (Jaycar, A$50)
  • I/O controller kit with 8 inputs, 8 relay switches (Jaycar, A$57)
• Typical serial bit rates are 9600 baud upwards
• Theoretical max is 290kBps

USB

• Standard connector for PCs
• Max speed of 12Mbps
• Multiple USB devices can be chained together
• USB cable is A$1.40 per metre (Jaycar)

Firewire

• IEEE 1394 standard arising from Apple Computers
• Data transfer rates to 400Mbps, with 3200Mbps planned
• Connect upto 63 devices without needing hubs
• Evergreen PCI card costs US$99
• Evergreen PCMCIA CardBus costs US$149

1-wire

• A proprietary format from iButton
• Low data rates (info is hard to find)
• Good for uses such as sprinkler control, temperature control where low speed and latency delays upto a second are not issues
• iButtons exist for temperature, switches, etc
• There are C and Java APIs for 1-wire

Power cables - X10

• Power cables already exist within the house, so additional cost is zero
• X10 power cable devices send a 120 kHz burst across the 50/60 Hz power line: the presence of a burst per 50 HZ cycle is a one, absence is a zero
• Data rate is 50 bits per second, and a message requires 48 bits (4 bit house code, 4 bit device address, 40 data bits)
• X10 is proprietary
• Communication protocol is one-way: command only, no feedback
• Dick Smith sell X10 kits for A$150

X10 modules

• Modules can replace non-X10 devices, such as an X10 light switch
• Existing devices can plug in to X10 modules, such as a lamp module
• Modules include
  • on/off/dimmer switch
  • timer
  • telephone responder to control other X10 devices from touch-tone buttons
  • serial port plug to allow computer control of X10 devices

WiFi

• IEE802.11b standard, most common for wireless networks
• Data transfer rate upto 11Mbps
• Uses public 2.4Ghz scientific band
• Range of upto 30 metres, depending on power, aerial, walls, etc
• PCI wireless card costs A$100
• PCMCIA wireless card costs A$90
• Wireless Access Point to connect to wired LAN costs A$300
• Intel Centrino chips already include wireless
• Has two modes:
  • Managed: uses Wireless Access Point
  • Adhoc: for "spontaneous" connection between computers
• 802.11a and 802.11g are two extensions that allow 56Mbps

Bluetooth

• Designed for adhoc networks
• Data transfer rate of 1Mbps
• Uses 2.4Ghz scientific band
• Shorter range than WiFi, about 10 metres, uses less power
• Limited to 8 devices in each "pico network"

Infrared

• Line of sight only
• Cheap and unrestricted use
• Commonly used betwen mobile phones and PCs to download email
• Used to talk to toys such as Lego Mindstorms
• Max data rate is 4 Mbps

Consumer infrared

• Used in infrared remote controllers for TVs, etc
• No standards - there are pieces of h/w to listen to remotes and tell you what they transmit, and there are sites listing the codes
• Uses ~40khz signal
• What do toys use?

iRda

• Standard for higher-speed two-way transmission
• Most laptops have iRda
• iRda USB modules are available for desktops

RFID tags

• Like electronic barcodes
• Each RFID tag holds about 16 bytes of data
• Tags may be active (own power supply) or passive (react in a wireless field)
• Passive tags have a range of about 10cm, active upto several metres
• Passive tags can cost cents, active tags a few dollars

Typical system

• Garden watering on 1-wire
• Internal lighting control on X10 power devices (on/off/dim)
• Intrusion detection using IR and micro-switches.
  Intrusion messages passed by
  • coaxial cable
  • power cable
  • wireless
• Video and audio streaming by
  • coaxial cable
  • twisted pair ethernet
  • wireless

Data rates

Making audio available on the internet

• My audio amplifier takes input from a CD player, a turntable, the TV audio and a tuner
• After pre-amplification it is sent to normal loudspeakers, but also to the soundcard of a PC
• The digital signal processor (soundcard) converts this to a digital signal which I can save to file, use to burn CDs - or send to other network devices

Internet loudspeaker

Speaker server code

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define SIZE 1024
char buf[SIZE];

#define SPEAKER_PORT 2013

void copy_to_speaker(int speaker_in) {
    int pfd[2];
    int pid;
    int nread;

    if (pipe(pfd) == -1) {
	perror("pipe failed");
	exit(1);
    }
    if ((pid = fork()) < 0) {
	perror("fork failed");
	exit(2);
    }
    if (pid == 0) {
	close(pfd[1]);
	dup2(pfd[0], 0);
	close(pfd[0]);
	execlp("sox", "sox",
	       "-t", "au", "-",
	       "-t", "ossdsp",
	       "/dev/dsp",
	       NULL);
	perror("play failed");
	exit(3);
    } else {
	close(pfd[0]);

	while ((nread = read(speaker_in, buf, SIZE)) > 0) {
	    write(pfd[1], buf, nread);
	}
	close(pfd[1]);
    }
}

int main(int argc, 
         char *argv[]) {
    int sockfd, client_sockfd;
    int nread, len;
    struct sockaddr_in serv_addr,
	client_addr;
    
    /* create endpoint */
    if ((sockfd =
	 socket(AF_INET,
		SOCK_STREAM, 0))
	< 0) {
	perror(NULL);
	exit(2);
    }
    
    /* bind address */
    serv_addr.sin_family =
	AF_INET;
    serv_addr.sin_addr.s_addr =
	htonl(INADDR_ANY);
    serv_addr.sin_port =
	htons(SPEAKER_PORT);
    
    if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
	perror(NULL);
	exit(3);
    }
    
    /* specify queue */
    listen(sockfd, 5);
    for (;;) {
	len = sizeof(client_addr);
	client_sockfd =
	    accept(sockfd,
		   (struct sockaddr *) &client_addr,
		   &len);
	if (client_sockfd == -1) {
	    perror(NULL);
	    continue;
	}
	
	/* transfer data */
	copy_to_speaker(client_sockfd);
	close(client_sockfd);
    }
}

Sound source code

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/stat.h>
#include <fcntl.h>


#define SIZE 1024
char buf[SIZE];

#define SPEAKER_PORT 2013

void send_to_speaker(int speaker_port) {
    int pfd[2];
    int pid;
    int nread;

    if (pipe(pfd) == -1) {
	perror("pipe failed");
	exit(1);
    }
    if ((pid = fork()) < 0) {
	perror("fork failed");
	exit(2);
    }
    if (pid == 0) {
	close(pfd[0]);
	dup2(pfd[1], 1);
	close(pfd[1]);
	execlp("rec", "rec",
	       "-s", "w",
	       "-c", "2",
	       "-r", "44100",
	       "-t", "wav", "-",
	       NULL);
	perror("play failed");
	exit(3);
    } else {
	close(pfd[1]);

	while ((nread = read(pfd[0], buf, SIZE)) > 0) {
	    write(speaker_port, buf, nread);
	}
	close(pfd[0]);
    }
}

int main(int argc, 
         char *argv[]) {
    int sockfd;
    int audio_file;
    int nread;
    struct sockaddr_in serv_addr;
    
    if (argc != 2) {
	fprintf(stderr,
		"usage: %s IPaddr\n",
		argv[0]);
	exit(1);
    }
    
    /* create endpoint */
    if ((sockfd =
	 socket(AF_INET,
		SOCK_STREAM, 0))
	< 0) {
	perror(NULL);
	exit(2);
    }
    
    /* connect to server */
    serv_addr.sin_family =
	AF_INET;
    serv_addr.sin_addr.s_addr =
	inet_addr(argv[1]);
    serv_addr.sin_port =
	htons(SPEAKER_PORT);
    if (connect(sockfd,
		(struct sockaddr *) &serv_addr,
		sizeof(serv_addr))
	< 0) {
	perror(NULL);
	exit(3);
    }
    
    /* transfer data */
    send_to_speaker(sockfd);
    /*
    fprintf(stderr, "connected\n");
    while ((nread = read(audio_file, buf, SIZE)) > 0) {
	fprintf(stderr, "Writing %d\n", nread);
	write(sockfd, buf, nread);
    }
    close(sockfd);
    close(audio_file);
    */
    exit(0);
}

Speaker cost

• Assuming a PC already exists with soundcard to capture audio
• Add a WiFi PCI wireless card: A$80 for D-Link card
• PC in speaker: strip down an old PC. A$200? Must be new enough to have PCI bus. A 400M disk is big enough to install a stripped down Linux without effort
• Add a WiFi PCI wireless card to the speaker
• Power amplifiers for car systems start at A$100 - pick up a secondhand one for A$40
• Then you have to find a speaker with enough space for a PC motherboard :-(
• How will WiFi cope in the same box as a power amplifier and an 80 watt bass speaker? Will shielding be adequate?
• Open questions:
  • what is a realistic minimum cost?
  • Can uncompressed data be sent?
• Commercial WiFi audio system available from Yamaha MusicCAST - speakers are A$1500