COMMUNICATION IN HOUSE

To understand the technologies for communication in house we must folow this steps

• To offer an overall image about the communication between different elements used in house and that assure the comfort and main functionality

To describe the different kind of ways used for communication, such as:

• Communication using the network
• Communication of special nets (two/four wires) nets
• IR communication
• Radio communication
• Optical fibers communication

• To present the problems related to the interoperability of different network existing in house: bridges between different networks and telecommunication networks.

• To describe how the household devices and automation elements can be accessed through Internet.

Communication using the network

1.1. Description of Ethernet

a) Definition of Ethernet

Ethernet is the most widely used local area network (LAN) technology and is used for approximately 85 percent of the world’s LAN-connected PCs and workstations. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox and then developed further by Xerox, DEC, and Intel.

An Ethernet LAN may use coaxial cable, special grades of twisted pair wiring, or fiber optic cable. "Bus" and "Star" wiring configurations are supported. Ethernet devices compete for access to the network using a protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD), operating at Layer 2 (Data Link) of the OSI (Open Systems Interconnect) Model.

b) Ethernet Features

Over the years, Ethernet has steadily evolved to provide additional performance and network intelligence. Today, the technology can provide four data rates:

• 10BASE-T EthernetThe original and most popular version of Ethernet delivers performance of up to 10 Mbps over twisted-pair copper cable.
• Fast Ethernetdelivers a speed increase of ten times the 10BASE-T Ethernet specification (100 Mbps) while retaining many of Ethernet’s technical specifications. These similarities enable organizations to use 10BASE-T applications and network management tools on Fast Ethernet networks.
• Gigabit Ethernetextends the Ethernet protocol even further, increasing speed tenfold over Fast Ethernet to 1000 Mbps, or 1 Gbps. Because it is based upon the current Ethernet standard and compatible with the installed base of Ethernet and Fast Ethernet switches and routers, network managers can support Gigabit Ethernet without needing to retrain or learn a new technology.
• 10 Gigabit Ethernet, ratified as a standard in June 2002, is an even faster version of Ethernet. It uses the IEEE 802.3 Ethernet media access control (MAC) protocol, the IEEE 802.3 Ethernet frame format, and the IEEE 802.3 frame size. Because 10 Gigabit Ethernet is a type of Ethernet, it can support intelligent Ethernet-based network services, interoperate with existing architectures, and minimize users’ learning curves. Its high data rate of 10 Gbps makes it a good solution to deliver high bandwidth in wide-area networks (WANs) and metropolitan-area networks (MANs).

c) Environment

Ethernet is increasingly used in applications such as process control, heavy manufacturing, power plants, mining, military COTS, medical systems, HVAC systems, and roadside traffic control. Is used primarily to connect PLC’s, computers, flat panel displays, and other high level components.

From embedded systems to PLCs and factory management systems, every part of the process control and automation industry is now gradually realizing the benefits of utilizing standard Ethernet and TCP/IP. Ethernet has become the dominant network technology spanning the management systems to controller levels, making sensor-to-boardroom manufacturing integration a reality.

d) Ethernet Network Elements

Ethernet LANs consist of network nodes and interconnecting media. The network nodes fall into two major classes:

• Data terminal equipment (DTE)—Devices that are either the source or the destination of data frames. DTEs are typically devices such as PCs, workstations, file servers, or print servers that, as a group, are all often referred to as end stations.
• Data communication equipment (DCE)—Intermediate network devices that receive and forward frames across the network. DCEs may be either standalone devices such as repeaters, network switches, and routers, or communications interface units such as interface cards and modems.

e) Ethernet Network Topologies and Structures

1. The point-to-point interconnection, shown in Figure 1. Only two network units are involved, and the connection may be DTE-to-DTE, DTE-to-DCE, or DCE-to-DCE. The cable in point-to-point interconnections is known as a network link. The maximum allowable length of the link depends on the type of cable and the transmission method that is used.

Figure 1 Example Point-to-Point Interconnection

2. The original Ethernet networks were implemented with a coaxial bus
structure, as shown in Figure 2. Segment lengths were limited to 500 meters, and up to 100 stations could be connected to a single segment.

Individual segments could be interconnected with repeaters, as long as multiple paths did not exist between any two stations on the network and the number of DTEs did not exceed 1024. The total path distance between the most-distant pair of stations was also not allowed to exceed a maximum prescribed value.

Figure 2 Example Coaxial Bus Topology

3. Star-connected topology, shown in Figure 3. The central network unit is either a multiport repeater (also known as a hub) or a network switch. All connections in a star network are point-to-point links implemented with either twisted-pair or optical fibre cable.

Figure 3 Example Star-Connected Topology

f) The IEEE 802.3 Logical Relationship to the ISO Reference Model

Animation 1 shows the IEEE 802.3 logical layers and their relationship to the OSI reference model. As with all IEEE 802 protocols, the ISO data link layer is divided into two IEEE 802 sub layers, the Media Access Control (MAC) sub layer and the MAC-client sub layer. The IEEE 802.3 physical layer corresponds to the ISO physical layer.

Animation 1 Ethernet's Logical Relationship to the OSI Reference Model

The MAC-client sub layer may be one of the following:

• Logical Link Control (LLC), if the unit is a DTE. This sub layer provides the interface between the Ethernet MAC and the upper layers in the protocol stack of the end station. The LLC sub layer is defined by IEEE 802.2 standards.
• Bridge entity, if the unit is a DCE. Bridge entities provide LAN-to-LAN interfaces between LANs that use the same protocol (for example, Ethernet to Ethernet) and also between different protocols (for example, Ethernet to Token Ring). Bridge entities are defined by IEEE 802.1 standards.

The MAC layer controls the node's access to the network media and is specific to the individual protocol. The only requirement for basic communication (communication that does not require optional protocol extensions) between two network nodes is that both MACs must support the same transmission rate.

The 802.3 physical layer is specific to the transmission data rate, the signal encoding, and the type of media interconnecting the two nodes. Gigabit Ethernet, for example, is defined to operate over either twisted-pair or optical fibre cable, but each specific type of cable or signal-encoding procedure requires a different physical layer implementation. The MAC sub layer has two primary responsibilities:

• Data encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception
• Media access control, including initiation of frame transmission and recovery from transmission failure.

The Basic Ethernet Frame Format

The IEEE 802.3 standard defines a basic data frame format that is required for all MAC implementations, plus several additional optional formats that are used to extend the protocol's basic capability. The basic data frame format contains the seven fields shown in Figure 4.4

• Preamble (PRE)—Consists of 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.
• Start-of-frame delimiter (SOF)—Consists of 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.
• Destination address (DA)—Consists of 6 bytes. The DA field identifies which station(s) should receive the frame. The left-most bit in the DA field indicates whether the address is an individual address (indicated by a 0) or a group address (indicated by a 1). The second bit from the left indicates whether the DA is globally administered (indicated by a 0) or locally administered (indicated by a 1). The remaining 46 bits are a uniquely assigned value that identifies a single station, a defined group of stations, or all stations on the network.
• Source addresses (SA)—Consists of 6 bytes. The SA field identifies the sending station. The SA is always an individual address and the left-most bit in the SA field is always 0.
• Length/Type—Consists of 4 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format. If the Length/Type field value is less than or equal to 1500, the number of LLC bytes in the Data field is equal to the Length/Type field value. If the Length/Type field value is greater than 1536, the frame is an optional type frame, and the Length/Type field value identifies the particular type of frame being sent or received.
• Data—Is a sequence of n bytes of any value, where n is less than or equal to 1500. If the length of the Data field is less than 46, the Data field must be extended by adding a filler (a pad) sufficient to bring the Data field length to 46 bytes.
• Frame check sequence (FCS)—Consists of 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames. The FCS is generated over the DA, SA, Length/Type, and Data fields.

Figure 4 The Basic IEEE 802.3 MAC Data Frame Format

Where:

            PRE = Preamble

            SFD = Start-of-frame delimiter

            DA = Destination Address

SA  = Source Address
FCS = Frame check sequence
The field is lengthen in bytes.

Frame Transmission

Sequence by transferring the LLC information into the MAC frame buffer.

• The preamble and start-of-frame delimiter are inserted in the PRE and SOF fields.
• The destination and source addresses are inserted into the address fields.
• The LLC data bytes are counted, and the number of bytes is inserted into the Length/Type field.
• The LLC data bytes are inserted into the Data field. If the number of LLC data bytes is less than 46, a pad is added to bring the Data field length up to 44.
• An FCS value is generated over the DA, SA, Length/Type, and Data fields and is appended to the end of the Data field.

The IEEE 802.3 standard currently requires that all Ethernet MACs support half-duplex operation, in which the MAC can be either transmitting or receiving a frame, but it cannot be doing both simultaneously. Full-duplex operation is an optional MAC capability that allows the MAC to transmit and receive frames simultaneously.

Half-Duplex Transmission—The CSMA/CD Access Method

The CSMA/CD protocol was originally developed as a means by which two or more stations could share a common media in a switch-less environment when the protocol does not require central arbitration, access tokens, or assigned time slots to indicate when a station will be allowed to transmit. Each Ethernet MAC determines for itself when it will be allowed tosend a frame.

The CSMA/CD access rules are summarized by the protocol's acronym:

• Carrier Sense defines that all stations must listen for no traffic on the network for a predefined period of time (determine when gaps between frame transmissions occur) before transmitting data.
• Multiple Access defines that all stations on the network have equal access to transmit data. Also, any station is allowed to repeat the transmit sequence without waiting for other stations to transmit their data. This is unlike Token-Ring or Token-Bus networks that guarantee every other station an opportunity to transmit before a second packet can be sent.
• Collision Detection defines that a transmitting station must detect a collision of data with any other transmitting stations data. This occurs when 2 stations attempt to simultaneously transmit data.

Figure 5 shows the MAC frame format with the gigabit extension field.

Figure 5 MAC Frame with Gigabit Carrier Extension

The extension is automatically removed during frame reception.

Full-Duplex Transmission/Switching Ethernet

Full-duplex operation is an optional MAC capability that allows simultaneous two-way transmission over point-to-point links. Full duplex transmission is functionally much simpler than half-duplex transmission because it involves no media contention, no collisions, no need to schedule retransmissions, and no need for extension bits on the end of short frames.

The result is not only more time available for transmission, but also an effective doubling of the link bandwidth because each link can now support full-rate, simultaneous, two-way transmission.

Transmission can usually begin as soon as frames are ready to send. The only restriction is that there must be a minimum-length inter-frame gap between successive frames, as shown in Figure 6, and each frame must conform to Ethernet frame format standards.

Figure 6 Full Duplex Operation Allows Simultaneous Two-Way Transmission on the Same Link

Flow Control

Full-duplex operation requires concurrent implementation of the optional flow-control capability that allows a receiving node (such as a network switch port) that is becoming congested to request the sending node (such as a file server) to stop sending frames for a selected short period of time.

Control is MAC-to-MAC through the use of a pause frame that is automatically generated by the receiving MAC. If the congestion is relieved before the requested wait has expired, a second pause frame with a zero time-to-wait value can be sent to request resumption of transmission. An overview of the flow control operation is shown in Figure 7.

Figure 7 An Overview of the IEEE 802.3 Flow Control Sequence