WANs are used to connect physically separated applications, data, and resources, thereby extending the reach of your network to form an intranet. The ideal result is seamless access to remote resources from geographically separated end users. The most common types of WAN connectivity technologies include the following:
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Frame Relay—A good, connection-oriented, frame-switched protocol for connecting sites over a WAN. Frame Relay is a great solution for enterprise networks that require a multipoint WAN media.Backbone cable
Node
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Leased lines—A dedicated connection from two distinct points that commonly uses the point-to-point protocol to provide various standards through encapsulation for IP traffic between serial links.•
Asynchronous transfer mode (ATM)—ATM is an International Telecommunications Union–Telecommunication Standardization Sector (ITU-T) standard for cell relay.Information is conveyed in small, fixed-size cells. ATM is a high-speed, low-delay multiplexing and switching technology that can support any type of user traffic, including voice, data, and video applications that are defined by the American National Standards Institute (ANSI) and International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) standards committees for the transport of a broad range of user information. ATM is ideally suited to applications that cannot tolerate time delay, as well as for transporting IP traffic.
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Integrated Systems Digital Network (ISDN)—Consists of digital telephony and data transport services using digitization over a specialized telephone network. The future of ISDN is in question because of the development of digital subscriber line and cable modem technologies.•
Digital subscriber line (DSL)—An always-on Internet connection that is typically billed monthly, usually for a fixed price and unlimited usage. DSL, when installed as a wall socket, looks much like a phone socket. In the United States, the wall socket is, in fact, a phone socket and, for the popular residential type of DSL (asymmetric digital subscriber line [ADSL]), the phone wiring does indeed carry phone and data signals. The key advantage of DSL over dial-up modems is its speed. DSL is from several to dozens of times faster than a dial-up modem connection. DSL is also a great way to save money compared to pay-per-minute ISDN data lines or expensive T1 lines.•
Cable modem—Refers to a modem that operates over the ordinary cable TV network cables. Because the coaxial cable used by cable TV provides much greater bandwidth than telephone lines, a cable modem can be used to achieve extremely fast access to the World Wide Web. The term “Cable Modem” is a bit misleading, as a Cable Modem works more like a LAN interface than as a modem. Basically, you just connect the Cable Modem to the TV outlet for your cable TV, and the cable TV operator connects a Cable Modem Termination System (CMTS) in his end (the Head-End).•
SONET—An optical fiber-based network created by Bellcore in the mid-1980s. It is now an ANSI standard. The international equivalent of SONET is synchronous digital hierarchy (SDH). SONET defines interface standards at the physical layer of the OSI seven-layer model. The SONET ANSI standard defines a hierarchy of interface rates that allow data streams of different rates to be multiplexed from optical carrier (OC) levels, from 51.8 Mbps (about the same as a T-3 line) to 2.48 Gbps. The international equivalent of SONET, standardized by the ITU, is called SDH. SONET is considered to be the foundation for the physical layer of broadband ISDN (BISDN). Asynchronous transfer mode runs can also run on top of SONET as well as on top of other technologies.Types of Network Topologies 19
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Dense wave division multiplexing (DWDM)—An optical multiplexing technique that is used to increase the carrying capacity of a fiber network beyond what can currently be accomplished by time-division multiplexing (TDM) techniques. DWDM replaces TDM as the most effective optical transmission method. Different wavelengths of light are used to transmit multiple streams of information along a single fiber with minimal interference. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream that is transmitted on a single optical fiber. DWDM is also sometimes called wave division multiplexing (WDM). Because each wavelength or channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. DWDM will allow SONET data and ATM data to be transmitted at the same time within the optical fiber.These WAN technologies are only briefly covered in this book. However, their connectivity and protocol characteristics are compared. Figure 1-7 shows some of the basic differences and choices that are considered when switching is involved.
Figure 1-7 Available WAN Technology Options
Table 1-3 summarizes the various carrier speeds and characteristics. This information is a good reference going forward and as the industry develops higher speeds.
WAN Options
Dedicated Switched
Leased Lines:
Fractional T1/E1 T1/E1 T3/E3
Circuit
Switched Packet/Cell Switched
Basic Telephone Service Switched 56ISDN
Frame RelayX.25 (PVCs & SVCs)
SMDSATM Cable Modems
DSL
*STS-1 is electrical equivalent of OC-1 E0 = 64 kbps STS-1 = OC1 = 51.84 Mbps (base rate) 4 * E1 = E2
STS-3 = OC3 = STM-1 = 155 Mbps 4 * E2 = E3
STS-9 = OC9 = STM-3 = 9 times base rate (not used) E3 = 34 Mbps in or around
STS-12 = OC12 = STM-4 = 622 Mbps STM = synchronous transport module (ITU–T) STS-18 = OC18 = STM-6 = 18 times base rate (not used) STS = synchronous transfer signal (ANSI) STS-24 = OC24 = STM-8 = 24 times base rate (not used) OC = optical carrier (ANSI)
STS-36 = 0C36 = STM-12 = 36 times base rate (not used) Although an SDH STM–1 has the same bit rate as the STS-48 = OC48 = STM-16 = 2.5 Gbps SONET STS–3, the two signals contain different frame E1 = 32 64-kbps channels = 2.048 Mbps structures.
Table 1-3 Carrier Rates and Transmission Characteristics*
Digital Signal (DS) Name
Circuit Bit Rate
Number of DS0s Used
Equivalent T-Carrier Name
Equivalent E-Carrier Name
DS0 64 Kbps 1 -
-DS1 1.544 Mbps 24 T-1
-- 2.048 Mbps 32 - E-1
DS1C 3.152 Mbps 48 -
-DS2 6.312 Mbps 96 T-2
-- 8.448 Mbps 128 - E-2
- 34.368 Mbps 512 - E-3
DS3 44.736 Mbps 672, or 28 DS1s T-3
-- 139.264 Mbps 2048 - E-4
DS4/NA 139.264 Mbps 2176 -
-DS4 274.176 Mbps 4032 -
-- 565.148 Mbps 4 E-4 Channels - E-5
SONET Signal Bit Rate SDH Signal SONET Capacity SDH Capacity
OC–1 (STS-1) 51.84 Mbps STM–0 28 DS–1s or 1 DS–3 21 E1s
OC–3 (STS-3) 155.52 Mbps STM–1 84 DS–1s or 3 DS–3s 63 E1s or 1 E4 OC–12 (STS–12) 622.08 Mbps STM–4 336 DS–1s or 12 DS–3s 252 E1s or 4 E4s OC–48 (STS–48) 2.488 Gbps STM–16 1344 DS–1s or 48 DS–3s 1008 E1s or 16 E4s OC–192 (STS–192) 10 Gbps STM–64 5376 DS–1s or 192 DS–3s 4032 E1s or 64 E4s
OC-256 13.271 Gbps - -
-OC-768 40 Gbps - -
-IP Addressing 21
IP Addressing
This section discusses IP addressing methodology, basic subnetting, variable-length subnet masking (VLSM), and classless interdomain routing (CIDR).
In a properly designed and configured network, communication between hosts and servers is transparent. This is because each device that uses the TCP/IP protocol suite has a unique 32-bit IP address. A device reads the destination IP address in the packet and makes the appropriate routing decision based on this information. In this case, a device might be either the host or server using a default gateway or a router using its routing table to forward the packet to its destination. Regardless of what the device is, the communication is easily accomplished and transparent to the user as a result of proper IP addressing.
IP addresses can be represented as a group of four decimal numbers, each within the range of 0 to 255. Each of these four decimal numbers is separated by a decimal point. The method of displaying these numbers is known as dotted decimal notation. Note that these numbers can also be displayed in both the binary and hexadecimal numbering systems.
Figure 1-8 illustrates the basic format of an IP address as determined by using dotted decimal notation.
Figure 1-8 IP Address Format as Determined by Dotted Decimal Notation
IP addresses have two primary logical components, network and host portions, the difference and use of which is extremely important. A third component, the subnet, is also used. A network address identifies the logical network and must be unique; if the network is to be a part of the Internet, the network must be assigned by American Registry for Internet Numbers (ARIN) in North America, Réseaux IP Européens (RIPE) in Europe, and Asia Pacific Network Information Centre (APNIC) in Asia. A host address, on the other hand, identifies a host (device) on a network and is assigned by a local administrator.
! " #
Consider a network that has been assigned an address of 172.24. An administrator then assigns a host the address of 248.100. The complete address of this host is 172.24.248.100.
This address is unique because only one network and one host can have this address.
NOTE In many cases when dealing with advanced networking topics such as OSPF, the latest trend is to write IP addresses as follows: x.x.x.x/8 or /16 or /24. This has become an accepted method of shorthand for IP addressing. The number to the right of the slash (/) represents the number of bits in the subnet mask.