qr code c# sample 2: OSI Reference Model in Objective-C

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2: OSI Reference Model
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NT1s or CSU/DSUs built into their WAN interfaces Normally, the terms DTE and DCE are used to describe WAN components, but they are sometimes used to describe LAN connections For instance, in a LAN connection, a PC, file server, or router is sometimes referred to as a DTE, and a switch, bridge, or hub is called a DCE Examples of physical layer standards include the following cable types (as well as many, many others): Category 3, 5 and 5E; EIA/TIA 232, 449, and 530; multimode and single-mode fiber (MMF and SMF); and Type 1 Interface connectors include the following: attachment unit interface (AUI), bayonet nut coupling (BNC), DB-9, DB-25, DB-60, RJ-11, RJ-45, and others An Ethernet hub and a repeater are examples of devices that function at the physical layer
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The physical layer de nes physical properties for connections and communication, including wires (UTP and ber) and connectors (RJ-45 and DB-9) A hub and a repeater are examples of
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devices that function at the physical layer A repeater is used to physically extend a single segment, while a hub known as a multiport repeater connects many segments together
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Remember the devices listed in Table 2-2 and the layers at which they function Table 2-2 provides a reminder of the devices that function at various OSI Reference Model layers
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TABLE 2-2
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Name of Layer
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Network Data link Physical
Device
Routers Switches, bridges, NICs Hubs and repeaters
Devices and the Layers at Which They Function
Encapsulation and De-encapsulation
CERTIFICATION OBJECTIVE 203
Encapsulation and De-encapsulation
Before delving into the mechanics of how information is transferred between computers, you must become familiar with the terminology used to describe the transmitted data Many of the layers of the OSI Reference Model use their own specific terms to describe data transferred back and forth As this information is passed from higher to lower layers, each layer adds information to the original data typically a header and possibly a trailer This process is called encapsulation The data link layer adds both, where the term encapsulation is the most appropriate; however, upper layer protocols add a header, and a few protocols add a trailer, depending on the protocol Both processes are referred to as encapsulating upper layer information and data Generically speaking, the term protocol data unit (PDU) is used to describe data and its overhead Table 2-3 describes the terms used at the various layers of the OSI Reference Model For instance, as data is passed from the session layer to the transport layer, the transport layer encapsulates the data PDU in a transport layer segment For TCP and UDP in the TCP/IP protocol stack, the transport layer adds a header, but no trailer As the PDU information is passed down, each layer adds its own header and, possibly, trailer to the upper layer PDU Once the physical layer is reached, the bits of the data link layer frame are converted into a physical layer signal a voltage, light source, radio wave, or other source according to the type of physical medium that is employed When the destination receives the information, it goes through a reverse process of deencapsulating information basically stripping off the headers and trailers of the PDU information at each layer as the information is passed up from layer to layer of the OSI Reference Model
TABLE 2-3
PDU Term
Data Segment Packet Frame Bits
OSI Reference Model Layer
Application, presentation, and session layers Transport layer Network layer (TCP/IP calls this a datagram) Data link layer Physical layer
PDU Terms
2: OSI Reference Model
Remember the terms in Table 2-3 and that computers deal with data in binary (bits) A quick anagram you
can use to remember these terms is Do Sergeants Pay For Beer (data, segment, packet, frame, bits)
Figure 2-5 shows an example of the process used for encapsulating and deencapsulating PDUs as data is passed down and back up the OSI Reference Model In this example, you can see how the application, presentation, and session layers create the data PDU As this information is passed down from layer to layer, each layer adds its own header (and possibly trailer, as is the case with most layer 2 protocols) The next few sections will help you better understand the process that devices go through as information is transmitted between computers The next section covers the details as to how information is encapsulated and sent down the protocol stack and then placed on the wire to the destination The section following that covers the reverse process: how the information is de-encapsulated at the destination and delivered to the application at the application layer The last part of the chapter looks at a more complex environment, where bridges, routers, and hubs are involved in the communication process to get information from the source to the destination
FIGURE 2-5
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