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The Physical Layer
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Once the CRC is calculated and the frame is fully constructed, the Data Link layer passes the frame down to the Physical layer, the lowest layer in the networking food chain. This is the layer responsible for the physical
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transmission of bits, which it accomplishes in a wide variety of ways. The Physical layer s job is to transmit the bits, which includes the proper representation of zeroes and ones, transmission speeds, and physical connector rules. For example, if the network is electrical, then what is the proper range of transmitted voltages required to identify whether the received entity is a zero or a one Is a one in an optical network represented as the presence of light or the absence of light Is a one represented in a copperbased system as a positive or as a negative voltage, or both Also, where is information transmitted and received For example, if pin two is identified as the transmit lead in a cable, which lead is data received over All of these physical parameters are designed to ensure that the individual bits are able to maintain their integrity and be recognized by the receiving equipment. Many transmission standards are found at the Physical layer, including T1, E1, the Synchronous Optical Network (SONET), the Synchronous Digital Hierarchy (SDH), Dense Wavelength Division Multiplexing (DWDM), and the many flavors of the Digital Subscriber Line (DSL). T1 and E1 are longtime standards that provide 1.544 and 2.048 Mbps of bandwidth respectively. They have been in existence since the early 1960s and occupy a central position in the typical network. SONET and SDH provide standards-based optical transmission at rates above those provided by the traditional carrier hierarchy. DWDM is a frequency division multiplexing technique that enables multiple wavelengths of light to be transmitted across a single fiber, providing massive bandwidth multiplication across the strand. It will be discussed in detail later in the chapter on optical networking. DSL extends the useful life of the standard copper wire pair by expanding the bandwidth it is capable of delivering as well as the distance over which that bandwidth can be delivered.
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We have now discussed the functions carried out at each layer of the OSI Model. Layers six and seven ensure application integrity, layer five ensures security, and layer four guarantees the integrity of the transmitted message. Layer three ensures network integrity; layer two, data integrity; and layer one, the integrity of the bits themselves. Thus, transmission is guaranteed on an end-to-end basis through a series of proto-
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Protocols
cols that are interdependent upon each other and that work closely to ensure integrity at every possible level of the transmission hierarchy. So, let s now go back to our e-mail example and walk through the entire process. The Eudora e-mail application running on the PC creates a message at the behest of the human user14 and passes the message to the Application layer. The Application layer converts the message into a format that can be universally understood as an e-mail message, which in this case is X.400. It then adds a header that identifies the X.400 format of the message. The X.400 message with its new header is then passed down to the Presentation layer, which encodes it as ASCII, encrypts it using Pretty Good Privacy (PGP), and compresses it using a British Telecom LempelZiv compression algorithm. After adding a header that details all this, it passes the message to layer five. The Session layer assigns a logical session number to the message, glues on a packet header identifying the session ID, and passes the steadily growing message down to the Transport layer. Based on network limitations and rule sets, the Transport layer breaks the message into 11 packets and numbers them appropriately. Each packet is given a header with address and QoS information. The packets now enter the chained layers, where they will first encounter the network. The Network layer examines each packet in turn and, based on the nature of the underlying network (connection-oriented connectionless ) and the congestion status, queues the packets for transmission. After creating the header on each packet, they are handed individually down to the Data Link layer. The Data Link layer proceeds to build a frame around each packet. It calculates a CRC, inserts a Data Link layer address, inserts appropriate control information, and finally adds flags on each end of the frame. Note that all other layers add a header only; the Data Link layer is the only layer that also adds a trailer. Once the Data Link frame has been constructed, it is passed down to the Physical layer, which encodes the incoming bitstream according to the transmission requirements of the underlying network. For example, if the data is to be transmitted across a T- or E-Carrier network, the data will be encoded using Alternate Mark Inversion and will be transmitted
14 I specifically note human user here because some protocols do not recognize the existence of the human in the network loop. In IBM SNA environments, for example, users are devices or processes that use network resources. No humans are involved.
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