CES 520 - WEEK 12 November 7, 2006

CES 520 - WEEK 12 November 7, 2006 - Networks

Why use a network?

To allow people to access the system remotely.

Example: Home control/monitoring system similar to the class project.

To allow communication with other systems.

Example: Banking system. ATMs must communicate with a central server.

Distributed processing: "A collection of spatially and functionally-distributed embedded nodes connected by a communication infrastructure."

Example: Network of sensors and actuators in a factory.
It often makes sense for sensors to include a microcontroller to do some processing locally.
The network connects sensors and actuator nodes to controller nodes

Allows easy upgrading of parts of the system
Reduces cabling requirements
Can provide redundancy for fault-tolerance

Network Types

WAN - Wide Area Network

Devices are far apart. Potentially world-side: The Internet is a WAN.
Connects many LANs together.
Typically uses leased lines.

LAN - Local Area Network

Devices are close together, normally in the same building or group of buildings.
Typically faster data rate than a WAN.

PAN - Personal Area Network

Covers a small area close to a person (a few meters).
Used for connecting computer devices (printers, PDAs, telephones, etc.)
Can be a wired connection using computer bus such as USB or Firewire.
Or wireless, using Bluetooth or infrared (IrDA).

WLAN - Wireless Local Area Network

An Access point is the interface between the wired and wireless LAN.
Can also use peer-to-peer mode to connect directly between wireless terminals.
Uses one or more of the "WiFi" (IEEE 802.11) standards

802.11 legacy - 2.4 GHz band - 1-2 Mbit/s
802.11b - 2.4 GHz band - 6.5-11 Mbit/s
802.11g - 2.4 GHz band - 11-108 Mbit/s
802.11a - 5 GHz band - 25-54 Mbit/s
802.11n - 2.4/5 GHz band - 200-540 Mbit/s (draft standard)

Example Networks

Automobiles

Hard real-time, fault-tolerant systems required for systems such as

Engine and transmission control
Suspension, steering and braking
Airbags

Soft real-time systems (telematics)

Control of body devices (wipers, doors, seats, lights, windows, etc.)
Multimedia systems
Human-machine interface (HMI).

ECU = "Electronic Control Unit"
Mechatronic (mechanical/electronic) devices reduce cost, improve reliability and functionality

Building automation

Little need for real-time
Low traffic volume
Fault tolerant
Network management is an issue

Factory sensor networks

Advantages:

Flexible installation and maintenance
Allows mobile operation (e.g. robots)

Requirements:

High reliability
Low and predictable delay
Support for large number of sensors/actuators
Low power consumption
Must withstand high electrical noise environment

Data acquisition: Precision Agriculture and Habitat Monitoring

Sensors deployed on animals and in the farm environment
Uses wireless network
Real-time not important
Must be low power for long battery life
Small size and weight for least impact on animals and environment
Smart sensors allow local processing

Data compression
Complex filtering and triggering of data acquisition
System health and status monitoring
Nodes may be reprogrammed in the field

Example: Seabird nesting environment on a small island off the coast of Maine, streaming on the web

Healthcare monitoring

Uses wireless network
Monitor patient's condition both in hospital and at home
Low-power, small size and weight

Communication techniques

CSMA/CD - Carrier Sense Multiple Access/Collision Detect

Collisions are detected and data re-sent after waiting a random time
Used in Ethernet
Not usable with wireless networks since the transmitter can't transmit and listen at the same time.

CSMA/CA - CSMA/Collision Avoidance

More deterministic than CSMA/CD, so more suitable for hard real-time systems
When the channel becomes empty, the transmitting station waits a random time before transmitting.
Another collision avoidance scheme is based on RTS/CTS (Request to send / clear to send)

Transmitting node gets permission from receiver before transmitting.
Helps avoid the hidden-transmitter problem on wireless networks.

Used in CAN (automotive bus) and WiFi 802.11 (wireless LAN).

TDMA - Time Division Multiple Access

Each node gets predetermined time slots. Time slots are organized into frames.
Very deterministic, perfect for hard teal-time systems
Energy savings for wireless systems: Node turns off radio during time slots it is not using.

Token-based networks

Only the owner of the token is allowed to transmit
Token ring was IBM's alternative to Ethernet. 4 or 16 Mbits/s.
No collisions allowed. Each node can transmit only when it receives the token.
Transmitting node captures the token, re-transmits the token when its frame comes back around the ring.
FDDI (Fiber Distributed Data Interface) is a fiber optics network standard based on token ring.

Master node technique

The master decides which and when messages may be sent

Mini slotting

Each node has different wait time when the network becomes idle

Field Area Networks

Before fieldbusses, a 4-20 mA analog loop was the common standard to transmit analog data.
Compared to LANs (Local Area Networks), "Field Area Networks" typically

Have lower data rates, smaller packet sizes.
May have deterministic time requirements (control applications).
"Fieldbus" is defined as a digital, two-way, multidrop communications network.

A Programmable Logic Controller (PLC) typically controls each fieldbus.

Often a central computer talks to each PLC over an Ethernet LAN.
Each PLC talks to the hardware devices over the fieldbus.

Some fieldbusses in common use:
Automotive fieldbusses

CAN - Controller Area Network

Originally developed for automotive applications
Up to 1 Mbps
Event-triggered broadcast bus
CSMA/CA
Fixed priority arbitration - each node checks for higher-priority node before transmitting

TT-CAN (Time-Triggered CAN)

Session-layer extension to CAN
All nodes follow predefined static schedule using a TDMA scheme
Can also set aside time periods to support original event-triggered CAN traffic

TTP/C - Time Triggered Protocol, class C

Used for safety-critical automotive applications
TDMA, for truly hard real-time communication
Supports fault tolerance, fast error detection, and consistency checks
Up to 25 Mbps

LIN - Local Interconnect Network

Cheaper, slower alternative to CAN
Often several LIN networks interface to a CAN upper-layer network
UART/SCI interface hardware
Up to 20 kbps
Master node controls bus synchronization for up to 16 slaves
Traffic is time-triggered

FlexRay

Proposed in 1999 as next-generation fieldbus to replace CAN
Up to 10 Mbps
Synchronous (time-triggered) communication using TDMA
Asynchronous communication using mini-slotting
Supports redundant transmission channels and fast error detection

Avionics/aerospace fieldbusses

ARINC 629

Multi-transmitter serial network, developed by Boeing
Supports periodic and sporadic communication
CSMA/CA using mini-slotting
Up to 2 Mbps

MIS-STD-1553

Military variation of ARINC 629
a.k.a. STANAG 3838AVS, STANAG 7155, DEF STAN 00-18, SAE AS15531
75-ohm twisted-pair differential signalling
Half-duplex, 1 Mbit/s
Dual-redundant interface

IEEE 1393

Spaceborne Fiber Optic Data Bus (SFODB)
1 Gbit/s

AFDX - Avionics Full Duplex Switched Ethernet
HDSB - High Speed Data Bus

Fiber optic, 80 Mbit/s

ASCB - Avionics Standard Communications bus

IEC 61158, in 2000 combined several existing protocols into one standard:

Foundation Fieldbus

Intended for plant or factory automation
Version H1: IEC 61158, type 1

31.25 kbit/s
Intended to connect to field devices
Uses twisted-pair cabling

Version HSE: IEC 61158, type 5 - "High Speed Ethernet"

100 Mbit/s
Intended for hosts, gateways, I/O subsystems, etc.
Uses standard Ethernet cabling

ControlNet - IEC 61158, type 2

Milti-master system
Up to 6 Mbit/s
TDMA access allows deterministic real-time response for critical data.
Uses RG-6 cable TV coaxial cable.

Profibus - IEC 61158, type 3

More than 14 million nodes worldwide.

Profibus FMS - Client-server model, for automation devices
Profibus DP - for sensors and actuators (e.g. robotics)
Profibus PA - for process automation

Master-slave with tokens
Up to 12 Mbps

P-Net - IEC 61158, type 4

Launched in 1984
Uses RS-485 shielded twisted pair. Up to 125 devices, up to 1200 meters.
76.8 kbit/s
Multi-master. All communications initiated by the master.

Interbus - IEC 61158, type 6

Serial bus, available since 1987, to connect controllers to sensors and actuators

WorldFIP - IEC 61158, type 8

Used in train control systems
Producer-Distributer-Consumers (PDC) communications model
Up to 5 Mbps

HART (Highway Addressable Remote Transducer Protocol)

An early fieldbus that adds digital signalling to a legacy 4-20 mA analog current loop

LonWorks

ANSI/EIA 709.1
Popular standard for building automation, train control, and industrial automation
Transmission medium can be twisted-pair, power line, fiber optics, or RF
Nodes can be implemented with custom ICs called "Neurons"

DeviceNet

Uses CAN for data link layer
Up to 500 kbit/s
Allows power and signal on the same twisted-pair line

Modbus

Open standard, available since 1979. Much free software exists.
Serial and Ethernet versions. Wireless implementations also exist.
Data may be binary (Modbus RTU) or human-readable (Modbus ASCII)
One master polls up to 254 devices.

TCN - Train Communication Network

IEC 61375
WTB (Wire Train Bus) connects all vehicles in the train. 1 Mbps
MVB (Multifunction Vehicle Bus) within one vehicle. 1.5 Mbps
Supports periodic and sporadic communication

Industrial Ethernet

Uses readily-available, low-cost, standard Ethernet hardware.

Use another standard's upper layers:
Modbus, Profibus, DeviceNet, Foundation Fieldbus,
Wireless standards

Like Ethernet, WLAN (802.11) and Bluetooth are not real-time.
No commercially-available real-time wireless protocols are currently available.
This is an area of active research.

USB, RS-232, RS-485, etc......
Physical networks

Twisted pair
Fiber optic
Power line
Wireless
Infrared

7-layer OSI model

Layer Mnemonic: "Please Do Not Teach Students Pointless Acronyms"

Physical (electrical and physical specifications, hardware flow control) Examples:

Cables and connector pinouts
Voltage levels, data rates
The "air interface" of a wireless network (modulation type, coding, etc.)
Hubs and repeaters are specified at this level.

Data link (packet-level error control, framing, physical addressing, medium access control (MAC))

Examples: Ethernet, HDLC
Bridges and switches operate at this level.

Network (network routing, logical addressing, traffic management)

Example: IP - Internet Protocol
Routers operate at this level.

Transport (data segmentation and reassembly, flow control, error checking) Examples:

TCP - Transmission Control Protocol
UDP - User Datagram Protocol

Session (setup, coordination, and termination of connections between computers)
Presentation (data transmission, compression, encryption) Examples:

MIME encoding
Data translation for XML

Application (user interface, quality of service, authentication) Examples:

Telnet: Terminal program
FTP - File transfer
HTTP - Internet transport language
SMTP - Email

A protocol stack is a communications protocol that uses two or more layers
Each layer only uses services from the layer below it and only provides services to the layer above.

The interfaces between layers are specified by the OSI standards.
Very useful to partition complicated systems - each layer can be designed and understood separately.

Layers 1 and 2 are typically implemented in hardware, the rest in software
For simple real-time systems, there is little need for routing functionality or end-to-end control.

Typically only use layers 1, 2 and 7 (including user layer).
Layers 3/4 generally go to layers 2 or 7 and 5/6 always go to 7.

Ethernet

IEEE 802.3
Invented at Xerox in 1973, standardized by DEC, Intel and Xerox in 1979.
Specifies the two lower layers of the 7-layer model, Physical and Data link (MAC)
Uses so-called RJ-45 (actually 8P8C, 8-position/8-contact) connector and Cat5 cable.
"10/100-baseT" means it can operate at the older 10 Mbit/s or newer 100 Mbit/s rate.
1000 Mbit/s is possible using Cat5e or Cat6 cable, and 10 Gbit/s or more with fiber optics.
All devices (nodes) on a network communicate over a single cable.

Any node can talk to any other node on the same network.
Adding new devices does not require modifying the old ones.

Ethernet is a LAN. Traditionally all devices must be within a few hundred meters.
The Ethernet medium:

Originally coaxial cable in a linear topology

A failure anywhere in the cable takes out the entire segment

Now usually twisted pair in a star configuration

A Hub at the junction of the star retransmits the signals, electrically isolating each device.
The hub includes the 100-ohm termininations, so only one device is allowed per port.

Can also be fiber optic cable or wireless

Each node has a unique 48-bit MAC address.

2⁴⁸ = 281,475 billion possible addresses
Every device manufactured has its own address
Broadcast frames have all-1 addresses. They are received by all nodes on the local network.
Multicast frames go to a subset of addresses on the network.

Each Ethernet frame includes source and destination address
CSMA/CD limits the maximum practical number of devices on a network without excessive collisions.

Bridges connect network segments, only allowing messages for the other segment to get through.

Reduces collisions between nodes.
Also allows different speed Ethernets on different segments

A Switch is like a bridge, but with a dedicated segment for each node.

Eliminates collisions between nodes.
No collision detection is required.
Allows full-duplex. (The node and switch can transmit at the same time.)

A Router is like a bridge, but connecting entire networks rather than network segments.

Does not forward broadcast messages. (Reduces traffic congestion)
Allows connecting networks of different types (e.g. Ethernet and Token Ring)
Routers are the backbone of the Internet

A Gateway is a router that connects to the Internet.

Often this is your home PC, with other PCs and peripherals on the LAN.

Security issues

Standard security protocols such as Secure Sockets Layer (SSL) and IP Security Protocol (IPSec) may demand excessive resources in an embedded system.
SPINS (Security Protocols for Sensor Networks) is a proposed alternative.

The Internet

The Internet Protocol or TCP/IP

Has become the de-facto standard protocol suite.
The protocol stack uses only 5 layers, with different names than the 7-layer OSI model.
The IP layer translates data between different kinds of networks

Also uses ARP (Address Resolution Protocol) to map IP addresses to MAC addresses.
The IP layer is "unreliable" - data integrity is assured by higher layers.

The Transport layer can use TCP or UDP to provide end-to-end services for applications.

TCP - Transmission Control Protocol

Used for browsing the Internet.
Establishes a virtual circuit byte-pipe connection between applications.
Assures data reliability and arranges packets in the proper order.

UDP - User Datagram Protocol

Unlike TCP, datagrams may get lost or arrive out of order.
Because of the lower processing overhead, it is more efficient and closer to real-time than TCP.
Used for broadcast and multicast messages, DNS, VoIP (Voice over IP), etc.

Socket paradigm

Each connection, or socket, is assigned a 16-bit port number from 1-65535 (0x01-0xFF)
Some ports are "well known", i.e. assigned by IANA (Internet Assigned Number Authority)

80 - HTTP
21 - FTP
25 - SMTP (etc.)

IP addresses are 32 bits, divided into four 8-bit octets

Not to be confused with the 48-bit MAC address of a device on an Ethernet network

DNS = Domain Name System

Translates URLs to IP addresses

Example: www.sonoma.edu ==> 130.157.5.73

The DNS "hosts" file on a local PC is normally used only for local addresses
Hierarchy of domain name servers. The highest level is on the right in the URL.

Example: www.sonoma.edu - "edu" is the highest level, "sonoma" is the next level.

The subnet mask restricts the range of the local subnet

Logical AND of the mask and the IP address = Host ID
The rest of the address is the Subnet ID
Example: A mask of 255.255.255.0 restricts the range of sonoma.edu to 130.157.6.0 to 130.157.6.255
All nodes on a network must be in the same subnet mask!

Example: Your computer must be in the same subnet as your Internet service provider (ISP).

A Broadcast Address has all 1's. An all-zero address also has special meaning.

A mask of 255.255.255.0 only has 256-2 = 254 available addresses.

DHCP - Dynamic Host Configuration Protocol

The local node gets its IP address, subnet mask, and default router from a DHCP server.
For example, your Internet connection uses a DHCP server at your ISP.
Most home routers and firewalls can act as DHCP servers.
DHCP is a broadcast-based protocol: Does not cross router boundaries.

Network programming using Dynamic C

Most TCP/IP functions are in library DCRTCP.LIB
Packets are processed each time tcp_tick() is called. - Should be called in the infinite loop.
There must be a socket for each connection. Sockets may be opened passively or actively.

Passive: tcp_listen() Wait for someone to contact your device.
Active: tco_open() Force a connection with another device.

Some good test programs in Samples\tcpip library: STATE.C, PING.C, NIST_TIME.C, ECHO.C
If the Rabbit is connected directly to the Internet (through the SSU LAN), DHCP should be used.

Macro USE_DHCP should be enabled.

If using a direct Ethernet connection from PC to Rabbit board, DHCP should be turned off on the PC.
To turn off DHCP and manually set the IP address and subnet mask: (Windows XP home edition)

Start > Control Panel > Network Connections - Right-click on "Local Area Connection"
Properties > General - Highlight "Internet Protocol" > Properties
Follow instructions on page 32 of RCM3000 Getting Started manual
Both the PC and Rabbit IP addresses must fit within the subnet mask.

Double-click on "Local Area Connection" in the Network Connections window to enable Ethernet.
To point the browser to the Rabbit: "http://10.10.6.100" (Or whatever IP address the Rabbit is using)
If a direct connection is used, a crossover Ethernet cable is required.

Assignments:

Read Embedded Systems Design, Chapter 9.
Read An Introduction to TCP/IP For Embedded Systems Designers (in file "tcpintro.pdf" in Docs/Manuals/TCPIP/Introduction)
Get familiar with the Dynamic C TCP/IP User's Manual. (in file "tcpipuser.pdf" on the disc in the back of the Embedded Systems Design book)
Lab5 : Write a program that serves a web page to display temperature and date/time. (due in 2 weeks)