The Physical Layer

The physical layer of the CAD I neutral file for solids describes how the CAD system data structures are mapped onto a sequential file. There are several levels of regarding the same sequential file  

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At the bottom of the OSI model is the Physical layer. This layer describes how the data gets transmitted over a physical medium. It defines how long each piece of data is and the translation of each into the electrical pulses that are sent over the wires. It decides whether data travels unidirectionally or bidirectionally across the hardware. It also relates electrical, optical, mechanical, and functional interfaces to the cable. 

Figure 8.10 shows the complete OSI model. Note the relation of each layer to one another and the function of each layer. Also note that when data is sent from one computer to another, the transmission starts above the Application layer and passed down to the Physical layer. Each layer, as it receives information from the layer above, adds its own information and passes the amended packet to the next layer down. At the bottom, the Physical layer places the packet on the wire. The receiver does the exact opposite procedure. The Physical layer takes the packet off the wire, removes the Physical layer header, and transfers the information to the layer above. Each layer reads the information given to it by the transmitting counterpart layer, removes its header, and passes the remained up the stack until the data being transmitted is received by the Application layer. 

When the data is passing through the OSI model and reaches the physical layer, it must find its way onto the medium that is used to physically transfer data from... 

The network interface card (NIC) provides the physical interface between computer and cabling. It prepares data, sends data, and controls the flow of data. It can also receive and translate data into bytes for the CPU to understand. It communicates at the Physical layer of the OSI model and comes in many shapes and sizes. 

Repeaters operate at the physical layer of the OSI model. Because of this, repeaters can only be used to regenerate signals between similar network segments. I can, for example, extend an Ethernet lOBasel network to 400 meters with a repeater. But I can t connect an Ethernet and Token Ring network together with one. 

Physical layer The first and lowest of the seven layers in the International Standards Organization s Open Systems Interconnection (ISO/OSI) model for computer-to-computer communications. The Physical layer defines the physical, electrical, mechanical, and functional procedures used to connect the equipment. 

The physical layer includes client devices, server devices, and network elements, including LANs, WANs, computing platforms, and systems and applications software. 

The telephone network, which is designed basically for voice transmission, is an example of a circuit-switched system. Circuit-switched systems exist only at the physical layer and use the channel resource to create a bit pipe, a dedicated resource that requires no control once it is created (some control may be required in setting up or bringing down the pipe). Circuit-switched systems, however, are very inefficient for transmitting burst-data traffic. 

Packet-switched systems are very efficient for transmitting data traffic but require control layers in addition to the physical layer that creates the bit pipe. A media-access control (MAC) layer is required for data users to share the bit pipe. A link layer is also necessary to create a reliable link from the error-prone pipe to the network layers so overflows of packet data can be transmitted. The Internet is a good example of a packet-switched network. 

Because all conventional cellular wireless systems, including 3G systems, were fundamentally designed as circuit-switched, voice-transmission systems, they were designed and optimized primarily at the physical layer. The choice of code-division multiple access (CDMA)i as multiple-access technology at the physical layer was also dictated by voice-transmission requirements. Hash-OFDM,... 

In addition, because the various tones in OFDM are orthogonal, different users in the same cell use different resources (tones) and hence do not interfere with each other. This is similar to TDMA, where different users in a cell transmit at different time slots and do not interfere with one another. In contrast, CDMA users in a cell do interfere with each other, thereby increasing the total interference in the system. At the physical layer, therefore, flash-OFDM has the advantages of both CDMA and TDMA and is at least three times as efficient as CDMA. In other words, at the physical layer, flash-OFDM creates the fattest pipe of all cellular technologies. 

In addition to the huge advantage of flash-OFDM at the physical layer, the most significant advantages of flash-OFDM for data transmission are at the MAC and link layers. Flash-OFDM exploits the granular nature of resources in OFDM... 

The data link layer translates the data transmitted by the components of the physical layer, and it is within the physical layer that the most dramatic technological advances made possible by computer engineering have had the greatest impact. The coaxial cable originally used has been replaced by fiber-optic technology and wireless connections. 

These common terms are the result of the layered architecture of the seven-layer OSI model. The architecture breaks the task of two computers communicating to each other into separate but interrelated tasks, each represented by its own layer. As can be seen in Fig. 19.56, the top layer (layer 7) represents the application program running on each computer and is therefore aptly named the application layer. The bottom layer (layer 1) is concerned with the actual physical connection of the two computers or networks and is therefore named the physical layer. The remaining layers (2-6) may not be as obvious but, nonetheless, represent a sufficiently distinct logical group of functions required to connect two computers, as to j ustify a separate layer. 

Internetworking Technology Repeaters Layer 1—The Physical Layer... 

There are seven layers in the OSI model, starting with the physical layer handling the raw data transmission over a physical medium. The most common transmission media are twisted pair (copper wires), coaxial cable, and fiber optics. The data link layer, usually implemented in the network adaptors, is above the physical layer and is concerned with the organization of data into frames and the reliable transportation of these frames over a direct link. The specific problems of multi-access links such as channel allocation and collision detection are handled by the data link sub-layer called medium access control (MAC). Reliable frame delivery, frame ordering, and frame retransmission are provided in the layer by sliding window protocols. This is a set of protocols for full-duplex data frame transmission, in which the sender and the receiver both keep windows of frame acknowledgements and send frames only if a certain number of already sent frames were acknowledged by the receiver. The data link layer also includes some error detection and correction functions such as parity bit code and cyclic redundancy code (CRC). 

As depicted in Fig. 3, the B-ISDN ATM reference model has different layers and a different structure as compared to the OSI model. While the OSI model is two dimensional, the ATM model is three dimensional. ATM s physical layer corresponds to both the physical and data link layer of the OSI model. The physical layer deals with the physical transmission of the bit stream and therefore depends on the physical medium used. Copper cable or fiber optics can be used for ATM. Above the physical layer is the ATM layer that deals with flow control, virtual circuit management, and cell header generation. The ATM layer is functionally equivalent to the OSI data link and network layers. On top of the ATM layer sits the ATM adaptation layer (AAL) that supports the different ATM services. AAL lays somewhere between the transport and session layers in the OSI model and provides assembly and reassembly of packets that are larger than a cell. Four different services are currently defined for ATM, resulting in four different AAL classes. AALl supports circuit... 

Logical Layer this layer groups logical nodes such as software, partitions, virtual links or network messages that implement the functional nodes. All these nodes use resources from the physical layer. The links between the logical and physical layers connect logical nodes with the physical resources that they use. 

Other types of propagations should be taken into account when dealing with security indirect propagation through shared resources. For instance, when a logical item as a piece of software is attacked, if the computer that hosts the software is not protected this attack also contaminates the computer. And then, when the computer is attacked it is also likely that all other pieces of software hosted by the computer are contaminated. Similarly, communication links are shared by several computers. It is likely that an attack on one computer can contaminate the communication link and the connected computers. We have extended the models in order to describe this type of indirect propagation. We added event T contaminate to nodes in the physical layer such that when one... 

In contrast to the SerDes-based interfaces, network-based camera interfaces were inspired by the ever-present connectivity of networks. Currently, two network-based solutions are in use and/or preparation. There are no obvious similarities in the physical layers of the two, but open specifications and second source for silicon is available. 

A MOST Interface NIC consists of the physical layer and low-level system services implemented in hardware. The NIC is connected to the ECU (EHC in MOST terms). The EHC executes the basic-level system services. With the increasing speed of MOST, the majority of the basic layer-system services were implemented in hardware forming an INIC (Fig. 17). 

The architecture of simulations is very similar to the data architecture described in the previous section. As defined in Sect. 10.3, a simulation module can be seen as a black box with an interface. It is an abstract layer in the architecture. The physical layer contains information about implementations of simulation tools interfaces and various configurations. 

Based on a given transmission rate determined by the physical layer, the VeMAC maximum transmission unit (MTU) is defined as the maximum amount of data (without the physical layer overhead) which can be transmitted in the duration of one time slot. The duration of a time slot, t, is chosen such that the MTU is equal to the size of a periodic safety message broadcast by a vehicle plus the maximum size of control information introduced by the VeMAC protocol. For RSUs, if the size of a periodic safety message plus the VeMAC control information exceeds the MTU, the message is fragmented to be transmitted as multiple VeMAC packets, as indicated in... 

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