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 Methods

In 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 switching

  • Circuit switching

For the Network+ exam, you will be expected to identify the differences between switching methods.


Packet Switching

In 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.

Packet switching is the most popular switching method for networks and is used on most LANs.


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.

  • Virtual-Circuit Packet Switching When virtual-circuit switching is used, a logical connection is established between the source and the destination device. This logical connection is established when the sending device initiates a conversation with the receiving device. The logical communication path between the two devices can remain active for as long as the two devices are available or can be used to send packets once. After the sending process has completed, the line can be closed.

  • Datagram Packet Switching Unlike virtual-circuit packet switching, datagram packet switching does not establish a logical connection between the sending and transmitting devices. The packets in datagram packet switching are independently sent, meaning that they can take different paths through the network to reach their intended destination. To do this, each packet must be individually addressed to determine where its source and destination are. This method ensures that packets take the easiest possible routes to their destination and avoid high-traffic areas.

Circuit Switching

In 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).

BRI

BRI 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.

BRI ISDN channels can be used separately using 64Kbps transfer or combined to provide 128Kbps transfer rates.


PRI

PRI 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.

Table 6.1. BRI and PRI ISDN Comparison

Characteristic

PRI

BRI

Speed

1.544Mbps

128Kbps

Channels

23B+D

2B+D

Transmission carrier

T1

ISDN


ISDN is considered a leased line because access to ISDN is leased from a service provider.


Be ready to answer questions about the characteristics of both BRI and PRI for the Network+ exam.


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.

The Copper Distributed Data Interface (CDDI) standard defines FDDI over copper cable rather than fiber-optic cable. However, the limitations of copper cablesuch as increased EMI risk and attenuationare in effect.


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.

Even though the second ring sits dormant, you can connect network devices to both rings. Network devices that attach to both rings are referred to as Class A stations, or dual attached stations (DASs). Network devices that connect to a single ring are called Class B stations, or single attached stations (SASs).


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

  • Fault-tolerant design By using a dual-ring configuration, FDDI provides some fault tolerance. If one cable fails, the other can be used to transmit the data throughout the network.

  • Speed because of the use of multiple tokens Unlike the IEEE 802.5 standard, FDDI uses multiple tokens, which increase the overall network speed.

  • Beaconing FDDI uses beaconing as a built-in error-detection method, making finding faults, such as cable breaks, a lot easier.

Like every technology, there are always a few caveats:

  • High cost The costs associated with FDDI and the devices and cable needed to implement an FDDI solution are very costly; too costly for many small organizations.

  • Implementation difficulty FDDI setup and management can be very complex, requiring trained professionals with significant experience to manage and maintain the cable and infrastructure.

T-carrier Lines

T-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:

  • T1 T1 lines offer transmission speeds of 1.544Mbps, and they can create point-to-point dedicated digital communication paths. T1 lines have commonly been used for connecting LANs.

  • T2 T2 leased lines offer transmission speeds of 6.312Mbps. They accomplish this by using 96 64Kbps B channels.

  • T3 T3 lines offer transmission speeds of up to 44.736Mbps, using 672 64Kbps B channels.

  • T4 T4 lines offer impressive transmission speeds of up to 274.176Mbps by using 4,032 64Kbps B channels

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.

Because of the cost of a T-carrier solution, it is possible to lease portions of a T-carrier service. Known as fractional T, you can subscribe and pay for service based on 64Kbps channels.


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.

Table 6.2. Comparing T/E/J Carriers

Name

Transmission Speed

T-1

1.544Mbps

T-1C

3.152Mbps

T-2

6.312Mbps

T-3

44.736Mbps

T-4

274.176Mbps

J-0

64Kbps

J-1

1.544Mbps

J-1C

3.152Mbps

J-2

6.312Mbps

J-3

32.064Mbps

J-3C

97.728Mbps

J-4

397.200Mbps

E-0

64Kbps

E-1

2.048Mbps

E-2

8.448Mbps

E-3

34.368Mbps

E-4

139.264Mbps

E-5

565.148Mbps


Ensure that you review the speeds of the various T-carriers for the Network+ exam.


SONET/OC-x Levels

Bell 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.

Table 6.3. OC Levels and Transmission Rates

OC Level

Transmission Rate

OC-1

51.84Mbps

OC-3

155.52Mbps

OC-12

622.08Mbps

OC-24

1.244Gbps

OC-48

2.488Gbps

OC-192

9.953Gbps


Synchronous Digital Hierarchy (SDH) is the International counterpart to SONET.


X.25

One 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.

Table 6.4. Comparing WAN Technologies

WAN Technology

Speed

Supported Media

Switching Method Used

Key Characteristics

ISDN

BRI: 64Kbps to 128Kbps

PRI: 64Kbps to 1.5Mbps

Copper/fiber-optic

Can be used for circuit-switching or packet-switching connections

ISDN can be used to transmit all types of traffic, including voice, video, and data. BRI uses 2B+D channels, PRI uses 23B+D channels. B channels are 64Kbps. ISDN uses the public network and requires dial-in access.

T-carrier (T1, T3)

T1: 1.544Mbps T3: 44.736Mbps

Copper/fiber-optic

Circuit switching

T-carrier is used to create point-to-point network connections for private networks.

FDDI

100Mbps

Fiber-optic

N/A

Uses a dual-ring configuration for fault tolerance. Uses a token-passing media-access method. Uses beaconing for error detection.

X.25

56Kbps/64Kbps

Copper/fiber-optic

Packet switching

X.25 is limited to 56Kbps. X.25 provides a packet-switching network over standard phone lines.

SONET/Ocx

51.8Mbps

Fiber-optic to 2.4Gbps

N/A

SONET defines synchronous data transfer over optical cable.




    Network+ Exam Cram 2
    Network+ Exam Cram 2
    ISBN: 078974905X
    EAN: N/A
    Year: 2003
    Pages: 194

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