2 Networking Concepts
Before considering how to configure Cisco routers and switches, you must be introduced to basic networking concepts you’ll need to understand in order to grasp the advanced concepts
discussed in later chapters. The OSI Reference Model is the best place to start, since it will help you understand how information is transferred between networking devices. Of the seven layers in the OSI Reference Model, be especially sure to understand how the bottom three layers function, since most networking devices function at these layers. This chapter discusses information flow, as well as Cisco’s three-tiered hierarchical model, which is used to design
scalable, flexible, and easy-to-troubleshoot-and-maintain networks.
OSI Reference Model
The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to describe how information is transferred from one machine to another, from the point when a user enters information using a keyboard and mouse to when that information is converted to electrical or light signals transferred along a piece of wire or radio waves transferred through the air. It is important to understand that the OSI Reference Model describes concepts and terms in a general manner, and that many network protocols, such as IP and IPX, fail to fit nicely into the scheme explained in ISO’s model. Therefore, the OSI Reference Model is most often used as a teaching
and troubleshooting tool. By understanding the basics of the OSI Reference Model, you can apply these to real protocols to gain a better understanding of them as well as to more easily troubleshoot problems. Advantages
ISO developed the seven-layer model to help vendors and network administrators gain a better understanding of how data is handled and transported between networking devices, as well as to provide a guideline for the implementation of new networking standards and technologies. To assist in this process, the OSI Reference Model breaks the network communication process into seven simple steps. It thus
? Defines the process for connecting two layers, promoting interoperability between vendors.
? Separates a complex function into simpler components.
? Allows vendors to compartmentalize their design efforts to fit a modular design, which eases implementations and simplifies troubleshooting.
A PC is a good example of a modular device. For instance, a PC typically contains the following components: case, motherboard with processor, monitor, keyboard, mouse, disk drive, CD-ROM drive, floppy drive, RAM, video card, Ethernet card, etc. If one component breaks, it is very easy to figure out which component failed and replace the single component. This simplifies your troubleshooting process. Likewise, when a new CD-ROM drive becomes available, you don’t have to throw away the current
computer to use the new device— you just need to cable it up and add a software driver to your
operating system to interface with it. The OSI Reference Model builds upon these premises. Layer Definitions
There are seven layers in the OSI Reference Model, shown in Figure 2-1: application, presentation, session, transport, network, data link, and physical. The functions of the application, presentation, and session layers are typically part of the user’s application. The transport, network, data link, and physical
layers are responsible for moving information back and forth between these higher layers. Each layer is responsible for a specific process or role. Remember that the seven layers are there to help you understand the transformation process that data will undergo as it is transported to a remote networking device. Not every networking protocol will fit exactly into this model. For example, TCP/IP has four layers. Some layers are combined into a single layer; for instance, TCP/IP’s application layer
contains the functionality of the OSI Reference Model’s application, presentation, and session layers.
The following sections go into more detail concerning the seven layers of the OSI Reference Model. Application Layer
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.
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.
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.
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, oh well!
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. The transport layer together with its mechanics is discussed in more depth in the section ―Transport Layer‖ later in this chapter.
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. These are discussed in the section ―Data Link Layer‖
later in this chapter.
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. Routing is discussed in more depth
in Chapters 9, 10, and 11.
Examples of network layer protocols include AppleTalk, DECnet, IPX, TCP/IP (or IP, for short), Vines, and XNS. The network layer is discussed in much more depth in the section ―Network Layer‖ later in this chapter.
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, which is discussed in more depth in the section
―Data Link Layer‖ later in this chapter.
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.
Fiber Cabling LANs typically use either copper or fiber-optic cabling. Copper cabling is discussed in more depth in the section ―Ethernet‖ later in this chapter.
Fiber-optic cabling uses light-emitting diodes (LEDs) and lasers to transmit data. With this transmission, light is used to represent binary 1’s and 0’s: if there is light
on the wire, this represents a 1; if there is no light, this represents a 0.
Fiber-optic cabling is typically used to
provide very high speeds and to span connections
across very large distances. For example, speeds
of 100Gbps and distances of over 10 kilometers
are achievable through the use of fiber—copper
cannot come close to these feats. However, fiberoptic
cabling does have its disadvantages: it is
expensive, difficult to troubleshoot, difficult to install, and less reliable than copper. Two types of fiber are used for connections: multimode and single-mode. Multimode fiber has a fiber thickness of either 850 or 1300 nanometers (nm), and the light signal is typically provided by an LED. When transmitting a signal, the light source is bounced off of the inner cladding (shielding) surrounding the fiber. Multimode fiber can achieve speeds in the hundreds of Mbps range, and many signals can be generated per fiber. Single-mode fiber has a fiber thickness of 1300 or 1550 nm and uses a laser as the light source. Because lasers provide a higher output than LEDs, single-mode fiber can span over 10 kilometers and have speeds up to 100Gbps. With single-mode fiber, only one signal is used per fiber.
The last few years have seen many advances in the use and deployment of fiber. One major enhancement is wave division multiplexing (WDM) and dense WDM (DWDM). WDM allows more than two wavelengths (signals) on the same piece of fiber, increasing the number of connections. DWDM allows yet more wavelengths, which are more closely spaced together: more than 200 wavelengths can be multiplexed into a light stream on a single piece of fiber.
Obviously, one of the advantages of DWDM is that it provides flexibility and transparency of the protocols and traffic carried across the fiber. For example, one wavelength can be used for a point-to-point connection, another for an Ethernet connection, another for an IP connection, and yet another for an ATM connection. Use of DWDM provides scalability and allows carriers to provision new connections without having to install new fiber lines, so they can add new connections in a very short per