WAN Technologies
| Many of today's network environments are not restricted to a single location or LAN. Instead, many of these networks span great distances, becoming wide area networks (WANs). When they do, hardware and software are needed to connect these networks. This section reviews the characteristics of various WAN technologies. Before we go on to discuss the specific WAN technologies, we must first look at an important element of the WAN technologiesswitching methods. Switching MethodsIn order for systems to communicate on a network, there has to be a communication path or multiple paths between which the data can travel. To communicate with another entity, these paths move the information from one location to another and back. This is the function of switching. Switching provides communication pathways between two endpoints and manages how data is to flow between these endpoints. Two of the more common switching methods used today include:
Packet SwitchingIn packet switching, messages are broken down into smaller pieces called packets. Each packet is assigned source, destination, and intermediate node addresses. Packets are required to have this information because they do not always use the same path or route to get to their intended destination. Referred to as independent routing, this is one of the advantages of packet switching. Independent routing allows for a better use of available bandwidth by letting packets travel different routes to avoid high-traffic areas. Independent routing also allows packets to take an alternate route if a particular route is unavailable for some reason.
In a packet-switching system, when packets are sent onto the network, the sending device is responsible for choosing the best path for the packet. This path might change in transit, and it is possible for the receiving device to receive the packets in a random or nonsequential order. When this happens, the receiving device waits until all the data packets are received, and then it reconstructs them according to their built-in sequence numbers. Two types of packet-switching methods are used on networks: virtual-circuit packet switching and datagram packet switching.
Circuit SwitchingIn contrast to the packet-switching method, circuit switching requires a dedicated physical connection between the sending and receiving devices. The most commonly used analogy to represent circuit switching is a telephone conversation in which the parties involved have a dedicated link between them for the duration of the conversation. When either party disconnects, the circuit is broken and the data path is lost. This is an accurate representation of how circuit switching works with network and data transmissions. The sending system establishes a physical connection, the data is transmitted between the two, and when the transmission is complete, the channel is closed. Some clear advantages to the circuit-switching technology make it well suited for certain applications. The primary advantage is that after a connection is established, there is a consistent and reliable connection between the sending and receiving device. This allows for transmissions at a guaranteed rate of transfer. Like all technologies, circuit switching has downsides. As you might imagine, a dedicated communication line can be very inefficient. After the physical connection is established, it is unavailable to any other sessions until the transmission is complete. Again, using the phone call analogy, this would be like a caller trying to reach another caller and getting a busy signal. Circuit switching can therefore be fraught with long connection delays. Integrated Services Digital Network (ISDN)ISDN has long been an alternative to the slower modem WAN connections but at a higher cost. ISDN allows the transmission of voice and data over the same physical connection. ISDN connections are considerably faster than regular modem connections. To access ISDN, a special phone line is required, and this line is usually paid for through a monthly subscription. You can expect these monthly costs to be significantly higher than those for traditional dial-up modem connections. To establish an ISDN connection, you dial the number associated with the receiving computer, much as you do with a conventional phone call or modem dial-up connection. A conversation between the sending and receiving devices is then established. The connection is dropped when one end disconnects or hangs up. The line pickup of ISDN is very fast, allowing a connection to be established, or brought up, much more quickly than a conventional phone line. ISDN has two defined interface standardsBasic Rate Interface (BRI) and Primary Rate Interface (PRI). BRIBRI ISDN uses three separate channelstwo bearer (B) channels of 64Kbps each and a delta (D) channel of 16Kbps. B channels can be divided into 4 D channels, which allows businesses to have 8 simultaneous Internet connections. The B channels carry the voice or data, and the D channels are used for signaling. The two B channels can be used independently as 64Kbps carriers, or they can be combined to provide 128Kbps transfer speeds.
PRIPRI is a form of ISDN that is generally carried over a T1 line and can provide transmission rates of up to 1.544Mbps. PRI is composed of 23 B channels, each providing 64Kbps for data/voice capacity, and one 64Kbps D channel, which is used for signaling. Table 6.1 compares BRI and PRI ISDN.
Fiber Distributed Data Interface (FDDI)FDDI is an American National Standards Institute (ANSI) topology standard that uses fiber-optic cable and token-passing media access. FDDI is implemented using both multimode and single-mode fiber cable and can reach transmissions speeds of up to 100Mbps at distances of more than 2 kilometers. FDDI combines the strengths of Token Ring, the speed of Fast Ethernet, and the security of fiber-optic cable. Such advantages make FDDI a strong candidate for creating network backbones and connecting private LANs to create MANs and WANs.
Unlike the regular 802.5 network standard, FDDI uses a dual-ring configuration. The first, or primary, ring is used to transfer the data around the network, and the secondary ring is used for redundancy and fault tolerance; the secondary ring waits to take over if the primary ring fails. If the primary ring fails, the secondary ring kicks in automatically, with no disruption to network users.
FDDI has a few significant advantagessome of which stem directly from the fact that it uses fiber-optic cable as its transmission media. These include a resistance to EMI, the security offered by fiber, and the longer distances available with fiber cable. In addition to the advantages provided by the fiber-optic cable, FDDI itself has a few strong points, including
Like every technology, there are always a few caveats:
T-carrier LinesT-carrier lines are high-speed dedicated digital lines that can be leased from telephone companies. This creates an always open, always available line between you and whomever you choose to connect to when you establish the service. T-carrier lines can support both voice and data transmissions and are often used to create point-to-point private networks. Because they are a dedicated link, they can be a costly WAN option. Four types of T-carrier lines are available:
Of these T-carrier lines, the ones commonly associated with networks and the ones most likely to appear on the Network+ exam are the T1 and T3 lines.
It is important to point out that T-carrier is the designation to the technology used in the United States and Canada. In Europe, they are referred to as E-carriers and in Japan, J-carriers. Table 6.2 shows the T/E/J carriers.
SONET/OC-x LevelsBell Communications Research developed SONET, a fiber-optic WAN technology that delivers voice, data, and video at speeds in multiples of 51.84Mbps. Bell's main goals in creating SONET were to create a standardized access method for all carriers and to unify different standards around the world. SONET is capable of transmission speeds between 51.84Mbps and 2.488Gbps. One of Bell's biggest accomplishments with SONET was to create a new system that defined data rates in terms of Optical Carrier (OC) levels, as shown in Table 6.3.
X.25One of the older WAN technologies is X.25, which is a packet-switching technology. Today, X.25 is not as widely implemented as it once was. X.25's veteran status is both its greatest advantage and its greatest disadvantage. On the upside, X.25 is a global standard that can be found in many places. X.25 had an original maximum transfer speed of 56Kbps, which, when compared to other technologies in the mid-1970s, was fast but almost unusable for most applications on today's networks. In the 1980s a digital version of X.25 was released increasing throughput to a maximum 64kbps. This too is slow by today's standards. Because X.25 is a packet-switching technology, it uses different routes to get the best possible connection between the sending and receiving device at a given time. As conditions on the network change, such as increased network traffic, so do the routes that the packets take. Consequently, each packet is likely to take a different route to reach its destination during a single communication session. The devices that make it possible to use X.25 service are called packet assemblers/disassemblers (PADs). A PAD is required at each end of the X.25 connection. Table 6.4 compares the various WAN technologies reviewed in this Chapter.
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