Types of Network Topologies
The preceding sections discussed the evolution of today's advanced networks and the building blocks that have evolved to make them what they are today—that is, the OSI reference model and the TCP/IP protocol. The sections on the OSI reference model described the essential means of how data is transported between the various layers that are running on all intranet devices. The TCP/IP section reviewed the protocols' characteristics. This section addresses the media that operates in your network. The sections that follow review both LAN and WAN topologies.
Local-Area Networks
LANs connect workstations, servers, legacy systems, and miscellaneous network-accessible equipment, which are, in turn, interconnected to form your network. The most common types of LANs are as follows:
• Ethernet—A communication system that has only one wire with multiple stations attached to the single wire; the system operates at a speed of 10 Mbps. Ethernet is currently traditionally found based on copper wire. You can contrast this with Fast Ethernet and Gigabit Ethernet, which have been developed on both copper wire and fiberoptic cabling.
Types of Network Topologies 17
• Fast Ethernet—An improved version of Ethernet that also operates with a single wire with multiple stations. However, the major improvement is in the area of speed; Fast Ethernet operates at a speed of 100 Mbps.
• Gigabit Ethernet—Yet another version of Ethernet that allows for operational speeds of 1 Gbps. The functional differences between copper- and fiber-based Gigabit Ethernet can affect design and operation.
• Token Ring—One of the oldest "ring" access techniques that was originally proposed in 1969. It has multiple wires that connect stations by forming a ring and operates at speeds of 4 Mbps and 16 Mbps. Token Ring is mentioned here as a courtesy to IBM (its creator); it is rarely used today.
• Fiber distributed data internetworking (FDDI)—A dual fiberoptic ring that provides increased redundancy and reliability. FDDI operates at speeds of 100 Mbps. FDDI is still in use, but Gigabit Ethernet and Synchronous Optical Network (SONET), mentioned in the next section, might make FDDI obsolete.
Figure 1-6 shows a typical Ethernet LAN.
Figure 1-6 Typical Ethernet LAN
Figure 1-6 Typical Ethernet LAN
For further information on this subject, visit the following website: www.ethermanage.com/ethernet/ethernet.html
Wide-Area Networks
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:
• 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.
18 Chapter 1: Networking and Routing Fundamentals
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.
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
• 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
WAN Options
Dedicated
Switched
Cable Modems DSL
Leased Lines: Fractional T1/E1 T1/E1 T3/E3
Circuit Switched
Packet/Cell Switched
Basic Telephone X.25
Service Frame Relay
ISDN (PVCs & SVCs)
Switched 56 ATM
SMDS
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.
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|
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 |
- |
- |
- |
*STS-1 is electrical equivalent of OC-1
STS-18 = OC18 = STM-6 = 18 times base rate (not used)
STS-24 = OC24 = STM-8 = 24 times base rate (not used)
STS-36 = 0C36 = STM-12 = 36 times base rate (not used)
E1 = 32 64-kbps channels = 2.048 Mbps
E3 = 34 Mbps in or around STM = synchronous transport module (ITU-T) STS = synchronous transfer signal (ANSI) OC = optical carrier (ANSI)
Although an SDH STM-1 has the same bit rate as the SONET STS-3, the two signals contain different frame structures.
*STS-1 is electrical equivalent of OC-1
STS-18 = OC18 = STM-6 = 18 times base rate (not used)
STS-24 = OC24 = STM-8 = 24 times base rate (not used)
STS-36 = 0C36 = STM-12 = 36 times base rate (not used)
E1 = 32 64-kbps channels = 2.048 Mbps
E3 = 34 Mbps in or around STM = synchronous transport module (ITU-T) STS = synchronous transfer signal (ANSI) OC = optical carrier (ANSI)
Although an SDH STM-1 has the same bit rate as the SONET STS-3, the two signals contain different frame structures.
Continue reading here: IP Addressing
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