The seventh layer, or topmost layer, of the OSI Reference Model is the application layer. It provides the interface that a person uses to interact with the application. This interface can be command-line-based or graphics-based. Cisco IOS routers and switches have a command-line interface (CLI), whereas a web browser uses a graphical interface.
Note that in the OSI Reference Model, the application layer refers to applications that are network-aware. There are thousands of computer applications, but not all of these can transmit information across a network. This situation is changing rapidly, however. Five years ago, there was a distinct line between applications that could and couldn�t perform network functions.
A good example of this was word processing programs, like Microsoft Word�they were built to perform one process: word processing. Today, however, many applications--MicrosoftWord, for instance--have embedded objects that don�t necessarily have to be on the same computer. There are many, many examples of application layer programs. The most common are telnet, FTP, web browsers, and e-mail.
Presentation Layer
The sixth layer of the OSI Reference Model is the presentation layer. The presentation layer is responsible for defining how information is presented to the user in the interface that they are using. This layer defines how various forms of text, graphics, video, and/or audio information are presented to the user. For example, text is represented in two different forms: ASCII and EBCDIC. ASCII (the American Standard Code for Information Interchange, used by most devices today) uses seven bits to represent characters. EBCDIC (Extended Binary-Coded Decimal Interchange Code, developed by IBM) is still used in mainframe environments to represent characters. Text can also be shaped by different elements, such as font, underline, italic, and bold.
There are different standards for representing graphical information�BMP, GIF, JPEG, TIFF, and others. This variety of standards is also true of audio (WAV and MIDI) and video (WMV, AVI, and MPEG). There are literally hundreds of standards for representing information that a user sees in their application. Probably one of the best examples of applications that have a very clear presentation function is a web browser, since it has many special marking codes that define how data should be represented to the user.
The presentation layer can also provide encryption to secure data from the application layer. however, this it not common with today�s methods of security, since this type of encryption is performed in software and requires a lot of CPU cycles to perform.
Session Layer
The fifth layer of the OSI Reference Model is the session layer. The session layer is responsible for initiating the setup and teardown of connections. In order to perform these functions, the session layer must determine whether data stays local to a computer or must be obtained or sent to a remote networking device. In the latter case, the session layer initiates the connection. The session layer is also responsible for differentiating among multiple network connections, ensuring that data is sent across the correct connection as well as taking data from a connection and forwarding it to the correct application.
The actual mechanics of this process, however, are implemented at the transport layer. To set up connections or tear down connections, the session layer communicates with the transport layer. Remote Procedure Call (RPC) is an example of an IP session protocol, the Network File System (NFS), which uses RPC, is an example application at this layer.
Transport Layer
The fourth layer of the OSI Reference Model is the transport layer. The transport layer is responsible for the actual mechanics of a connection, where it can provide both reliable and unreliable delivery of data. For reliable connections, the transport layer is responsible for error detection and correction: when an error is detected, the transport layer will resend the data, thus providing the correction. For unreliable connections, the transport layer provides only error detection�error correction is left up to one of the higher layers (typically the application layer). In this sense, unreliable connections attempt to provide a best-effort delivery�if the data makes it there, that�s great, and if it doesn�t
Examples of a reliable transport protocol are TCP/IP�s Transmission Control Protocol (TCP) and IPX�s SPX (Sequenced Packet Exchange) protocol. TCP/IP�s User Datagram Protocol (UDP) is an example of a protocol that uses unreliable connections. Actually, IPX and IP themselves are examples of protocols that provide unreliable connections, even though they operate at the network, and not transport, layer. In IPX�s case, if a reliable connection is needed, SPX is used. For IP, if a reliable connection is needed, TCP is used at the transport layer.
Network Layer
The third layer of the OSI Reference Model is the network layer. The network layer provides quite a few functions. First, it provides for a logical topology of your network using logical, or layer-3, addresses. These addresses are used to group machines together. As you will see in Chapter 3, these addresses have two components: a network component and a host component. The network component is used to group devices together. Layer-3 addresses allow devices that are on the same or different media types to communicate with each other. Media types define types of connections, such as Ethernet, Token Ring, or serial.
To move information between devices that have different network numbers, a router is used. Routers use information in the logical address to make intelligent decisions about how to reach a destination.
Examples of network layer protocols include AppleTalk, DECnet, IPX, TCP/IP (or IP, for short), Vines, and XNS.
Data Link Layer
The second layer in the OSI Reference Model is the data link layer. Whereas the network layer provides for logical addresses for devices, the data link layer provides for physical, or hardware, addresses. These hardware addresses are commonly called Media Access Control (MAC) addresses. The data link layer also defines how a networking device accesses the media that it is connected as well as defining the media�s frame type. This includes the fields and components of the data link layer, or layer-2, frame. This communication is only for devices on the same data link layer media type (or same piece of wire). To traverse media types, Ethernet to Token Ring, for instance, typically a router is used.
The data link layer is also responsible for taking bits (binary 1�s and 0�s) from the physical layer and reassembling them into the original data link layer frame. The data link layer does error detection and will discard bad frames. It typically does not perform error correction, as TCP/IP�s TCP protocol does; however, some data link layer protocols do support error correction functions.
Examples of data link layer protocols and standards for local area network (LAN) connections include IEEE�s 802.2, 802.3, and 802.5; Ethernet II; and ANSI�s FDDI.
Examples of WAN connections include ATM, Frame Relay, HDLC (High-Level Data Link Control), PPP (Point-to-Point Protocol), SDLC (Synchronous Data Link Control), SLIP (Serial Line Internet Protocol), and X.25. Bridges, switches, and network interface controllers or cards (NICs) are the primary networking devices functioning at the data link layer
Physical Layer
The first, or bottommost, layer of the OSI Reference Model is the physical
layer. The physical layer is responsible for the physical mechanics of
a network connection, which include the following:
- The type of interface used on the networking device
- The type of cable used for connecting devices
- The connectors used on each end of the cable
- The pin-outs used for each of the connections on the cable
The type of interface is commonly called a NIC. A NIC can be a physical
card that you put into a computer, like a 10BaseT Ethernet card, or a
fixed interface on a switch, like a 100BaseTX port on a Cisco Catalyst
1900 series switch.
The physical layer is also responsible for how binary information is converted
to a physical layer signal. For example, if the cable uses copper as a
transport medium, the physical layer defines how binary 1�s and 0�s are
converted into an electrical signal by using different voltage levels.
If the cable uses fiber, the physical layer defines how 1�s and 0�s are
represented using an LED or laser with different light frequencies.
Data communications equipment (DCE) terminates a physical WAN connection
and provides clocking and synchronization of a connection between two
locations and connects to a DTE. The DCE category includes equipment such
as CSU/DSUs, NT1s, and modems. Data terminal equipment (DTE) is an end-user
device, such as a router or a PC, that connects to the WAN via the DCE
device. In some cases, the function of the DCE may be built into the DTE�s
physical interface. For instance, certain Cisco routers can be purchased
with built-in NT1s or CSU/DSUs in theirWAN 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
or bridge as a DCE.
Examples of physical layer standards include the following cable types:
Category-3, -5, and -5E; EIA/TIA-232, -449, and -530; multimode and single-mode
fiber (MMF and SMF); Type-1; and others. Interface connectors include
the following: AUI, BNC, DB-9, DB-25, DB-60, RJ-11, RJ-45, and others.
A hub and a repeater are examples of devices that function at the physical
layer.
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