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WAN Link Options

A variety of different methods can be used to link networks together. To select an appropriate WAN technology, you need the ability to compare one WAN technology to another.

Home and Small Office

Home and small office networks can connect to other networks and the Internet through a variety of methods.

Dialup

Dialup connectivity is done through the Public Switched Telephone Network (PSTN), which is composed of multiple telephone carriers from around the world. Although the bandwidth available on the PSTN is limited, it is more likely to be available in a given location than other wired WAN solutions. A dialup connection can be used to access the Internet by connecting a computer to a modem, connecting the modem to an analog phone line, and dialing in to a service provider. The service provider can then connect to the Internet, as shown in Figure 29-3. Modems in the United States and Canada are limited to 53.3Kbps download and 48.0Kbps upload.

Figure 29-3

Figure 29-3 PSTN Dialup Topology

Table 29-2 describes some of the common terminology used to describe PSTNs.

Table 29-2 Common PSTN Terms

Term

Definition

Telco

Abbreviation for telephone company.

Local loop

A local loop is a connection between a customer’s premises and his local telephone central office (CO).

Central office (CO)

A building containing a telephone company’s telephone switching equipment is referred to a central office (CO).

Tip and ring

The tip and ring wires are the red and green wires found in an RJ-11 wall jack, which carry voice, ringing voltage, and signaling information between an analog device and a telephone’s wall jack.

Demarc

A demarc, short for demarcation point, is the point in a telephone network where the maintenance responsibility passes from a telephone company to the subscriber.

Smart jack

A smart jack is a type of network interface device that adds features such as converting between framing formats on digital circuits, supporting remote diagnostics, and regenerating a digital signal.

ISDN

ISDN connections are considerably faster than regular modem connections. ISDN is a digital technology that supports multiple 64Kbps channels on a single connection. ISDN circuits are classified as either a basic rate interface (BRI) circuit or a primary rate interface (PRI) circuit:

  • BRI—A BRI circuit consists of two 64Kbps bearer (B) channels and one 16Kbps delta (D) channel. The B channels carry voice, video, and data. They can carry two separate voice conversations or they can be combined using PPP multilink. The D channel carries Layer 2 (Q.921) and Layer 3 (Q.931) signaling.
  • PRI—A PRI circuit is equivalent to a 1.544Mbps T1 circuit. Therefore, it consists of 23 B channels and one 64Kbps D channel.

The components of an ISDN network are shown in Figure 29-4.

Figure 29-4

Figure 29-4 ISDN Topology

Table 29-3 lists a brief description of the ISDN components shown in Figure 29-4.

Table 29-3 ISDN Components

Term

Definition

R reference point

The R reference point resides between a non-ISDN device and a terminal adapter (TA).

S/T reference point

The S/T reference point resides between a network termination 1 (NT1) and a terminal endpoint 1 (TE1).

U reference point

The U reference point resides between a network termination 1 (NT1) and the wall jack connecting back to an ISDN service provider.

Terminal adapter (TA)

A TA performs protocol conversion between a non-ISDN device and a terminal endpoint 1 (TE1) device.

Terminal endpoint 1 (TE1)

A TE1 is a device (such as an ISDN phone) that natively supports ISDN.

Terminal endpoint 2 (TE2)

A TE2 is a device (such as a PC) that does not natively support ISDN.

Network termination 1 (NT1)

An NT1 is a device that interconnects a four-wire ISDN circuit and a two-wire ISDN circuit.

DSL

Digital Subscriber Line (DSL) is a group of technologies that provide high-speed data transmission over existing telephone wiring. There are many different varieties of DSL. Together, all these variations are referred to as xDSL. Popular variants include the following:

  • Asymmetric DSL (ADSL)—ADSL is the most popular variant. It include a channel for analog voice conversations, a channel for uploads, and a channel for downloads. It is called asymmetrical because the download channel is faster than the upload channel.
  • Symmetric DSL (SDSL)—SDSL is more suited to business applications because it offers the same speeds for uploads and downloads.
  • Very High Bit-Rate DSL (VDSL)—VDSL is a variant of ADSL that provides data speeds of up to 13Mbps.

Table 29-4 summarizes the maximum speeds for all the DSL variants.

Table 29-4 DSL Speeds

DSL Variation

Upload Speed

Download Speed

ADSL

1Mbps

3Mbps

ADSL2

1.3Mbps

12Mbps

ADSL2+

1.4Mbps

24Mbps

SDSL

1.5Mbps

1.5Mbps

IDSL (ISDN DSL)

144Kbps

144Kbps

RADSL (Rate-adaptive DSL)

1Mbps

7Mbps

VHDSL

1.6Mbps

13Mbps

HDSL (High bit rate DSL)

768Kbps

768Kbps

PPP

Point-to-Point Protocol (PPP) is a common Layer 2 protocol used on dedicated leased lines. It can carry multiple Layer 3 protocols, such as IPv4 and IPv6. IP uses PPP’s IP control protocol (IPCP). When a link is configured with IP and PPP, IPCP initiates the Link Control Protocol (LCP), which can provide any or all of the following features:

  • Multilink interface—PPP multilink allows multiple interfaces to be combined into one logical interface.
  • Looped link detection—A Layer 2 loop (of PPP links) can be detected and prevented.
  • Error detection—Frames containing errors can be detected and discarded by PPP.
  • Authentication—PPP provides three methods of authentication methods:

    • Password Authentication Protocol (PAP)—PAP performs one-way authentication. Passwords are sent in clear text.
    • Challenge Handshake Authentication Protocol (CHAP)—CHAP performs one-way authentication using a three-way handshake (challenge, response, and acceptance). Passwords are not sent across the link.
    • Microsoft Challenge Handshake Authentication Protocol (MS-CHAP)—MS-CHAP is a Microsoft-enhanced version of CHAP, offering a collection of additional features, including two-way authentication.

Broadband Cable

Broadband cable is an always-on Internet access method available in areas that have digital cable television. Connectivity is achieved by using a device called a cable modem. It has a coaxial connection for connecting to the provider’s outlet and an unshielded twisted-pair (UTP) connection for connecting directly to a system or to a hub, switch, or router.

The provider’s infrastructure probably has a mix of coaxial and fiber cabling called hybrid fiber-coax (HFC). A broadband cable network is shown in Figure 29-5.

Figure 29-5

Figure 29-5 Broadband Cable Topology

Wireless

Wireless WAN connection options include satellite, Worldwide Interoperability for Microwave Access (WiMAX), and several varieties of cellular technologies.

Satellite

Some locations do not have the WAN connectivity options, such as DSL connections or cable modems, commonly available in urban areas. However, these locations might be able to connect to the Internet or to a remote office using satellite communications, where a transmission is bounced off of a satellite, received by a satellite ground station, and then sent to its destination using either another satellite hop or a wired WAN connection.

Two different types of broadband Internet satellite services are deployed:

  • One-way satellite system—A one-way satellite system requires a satellite card and a satellite dish installed at the end user’s site. Outgoing requests are sent using a phone line. Inbound traffic returns on the satellite link.
  • Two-way satellite system—A two-way satellite system, in contrast, provides data paths for both upstream and downstream data. Bidirectional communication occurs directly between the end user’s site and the satellite.

WiMAX

WiMAX (Worldwide Interoperability for Microwave Access) provides wireless Internet broadband access to fixed locations (as an alternative to technologies such as DSL or cable). Depending on the WiMAX service provider, WiMAX coverage areas could encompass entire cities or small countries. Based on the IEEE 802.16 standard, WiMAX can provide data rates up to 1Gbps. Although WiMAX can send data up to 31 miles (50 km), it is most effective within one mile. WiMAX is a popular choice for connecting cell towers in cellular networks.

Cellular Technologies

Some cellular phone technologies (for example, Long-Term Evolution [LTE], which supports a 100Mbps data rate to mobile devices and a 1Gbps data rate for stationary devices) can be used to connect a mobile device (such as a smartphone) to the Internet. Other technologies for cellular phones include the older 2G edge, which provides slow data rates. 2G edge was improved upon with 3G, in addition to the newer 4G, LTE, and Evolved High-Speed Packet Access (HSPA+). Code division multiple access (CDMA) and Global System for Mobile Communications (GSM) are the two major radio systems used in cell phones.

SONET

Synchronous Optical Network (SONET) is a fiber-optic WAN technology that delivers voice, data, and video at speeds starting at 51.84Mbps. SONET uses dense wavelength-division multiplexing (DWDM), which uses erbium-doped fiber amplifiers (EDFA) to amplify the signal and allow it to travel greater distances. An alternative to DWDM is CWDM (coarse wavelength-division multiplexing), which is commonly used with television cable networks. SONET is classified into various Optical Carrier (OCx) levels, as shown in Table 29-5.

Table 29-5 Optical Carrier Data Rates

OCx Level

Data Rate

OC-1

51.84Mbps

OC-3

155.52Mbps

OC-12

622.08Mbps

OC-24

1.244Gbps

OC-48

2.488Gbps

OC-96

4.976Gbps

OC-192

9.953Gbps

OC-768

39.813Gbps

SONET can connect as many as 16 other devices in a linear fashion (similar to a bus topology) or in a ring topology. A metropolitan area network (MAN), as depicted in Figure 29-6, often uses SONET in a ring topology.

Figure 29-6

Figure 29-6 SONET Ring Topology

Dedicated Leased Lines

A dedicated leased line is typically a point-to-point digital circuit interconnecting two sites. All the bandwidth on that circuit is available to those sites. These circuits can use multiplexing technology to simultaneously carry multiple conversations in different 64Kbps channels. A single 64Kbps channel is called a Digital Signal 0 (DS0). WAN technologies commonly used with dedicated leased lines include the following:

  • T1—T1 circuits were originally used in telephony networks, with the intent of one voice conversation being carried in a single channel. A T1 circuit is composed of 24 DS0s, which is called a Digital Signal 1 (DS1). The bandwidth of a T1 circuit is 1.544Mbps.
  • T3—T3 circuits combine 672 DS0s into a single physical connection, which is called a Digital Signal 3 (DS3). A T3 circuit has a bandwidth capacity of 44.7Mbps.
  • E1—E1 circuits are popular outside of North America and Japan. They contain 32 channels, in contrast to the 24 channels on a T1 circuit, for a bandwidth capacity of 2.048Mbps.
  • E3—E3 circuits have a bandwidth capacity of 34.4Mbps, which is less than a T3 circuit.

Dedicated leased lines are terminated at the customer’s premises with a channel service unit/data service unit (CSU/DSU), as shown in Figure 29-7.

Figure 29-7

Figure 29-7 Terminating a Leased Line

Enterprise WANs

Initially, dedicated leased lines were used by enterprises to connect various sites. However, as network traffic and the number of required WAN connections grew, leased lines became cost prohibitive. Several technologies emerged to answer the need, including Frame Relay, Asynchronous Transfer Mode (ATM), Multiprotocol Label Switching (MPLS), and Metro Ethernet.

Frame Relay

Frame Relay sites are interconnected using virtual circuits (VCs). Frame Relay is a Layer 2 technology that uses locally significant identifiers called data-link connection identifiers (DLCI). DLCIs identify the VC. A single router interface can have multiple VCs, as shown in Figure 29-8.

Figure 29-8

Figure 29-8 Frame Relay Topology

Unlike a dedicated leased line, Frame Relay shares a service provider’s bandwidth with other customers of its service provider. Therefore, subscribers might purchase a service level agreement (SLA) to guarantee a minimum level of service. Part of the Frame Relay SLA would be a minimum bandwidth guarantee called a committed information rate (CIR).

During times of congestion, the service provider manages transmission rates using the backward explicit congestion notification (BECN) and forward explicit congestion notification (FECN) bits in the frame relay header. The BECN and FECN bits inform the customers to slow down the transmission rates.

If the service is not congested, a customer might have an SLA that allows transmission rates higher than the CIR. In such cases, the discard eligibility (DE) bit is set in each frame. If the service becomes congested, these frames can be discarded by the Frame Relay service provider.

ATM

ATM is a Layer 2 technology that uses VCs. However, ATM uses a fixed-length frame, called a cell, which includes 48 bytes of data and a 5-byte header, as shown in Figure 29-9.

Figure 29-9

Figure 29-9 ATM Cell Format

Table 29-6 details the fields of the ATM cell header.

Table 29-6 ATM Header Fields

Field

Description

GFC (4 bits)

The Generic Flow Control (GFC) field is used to indicate congestion.

VCI (16 bits)

The Virtual Circuit Identifier (VCI) field indicates a VC.

VPI (8 bits)

The Virtual Path Identifier (VPI) indicates the virtual path, which could contain multiple VCs.

PTI (3 bits)

The Payload Type Indicator (PTI) indicates the type of payload (for example, user data versus ATM management data).

HEC (8 bits)

The Header Error Control (HEC) field is used to detect and correct errors in the header.

ATM VCs are identified with a VPI/VCI pair, as shown in Figure 29-10.

Figure 29-10

Figure 29-10 Identifying ATM VCs

MPLS

Multiprotocol Label Switching (MPLS) can accommodate both Frame Relay and ATM on the same backbone. It does this by inserting a 32-bit header between the Layer 2 and Layer 3 headers. The 32-bit header contains a 20-bit label. This label is used to make forwarding decisions within an MPLS cloud. Therefore, the process of routing MPLS frames through an MPLS cloud is commonly referred to as label switching. Figure 29-11 shows an MPLS topology.

Figure 29-11

Figure 29-11 MPLS Topology

Table 29-7 lists the definitions for the components of the MPLS topology shown in Figure 29-11.

Table 29-7 MPLS Components

Component

Description

CPE

A customer premises equipment (CPE) device resides at a customer site.

ELSR

An edge label switch router (ELSR) resides at the edge of an MPLS service provider’s cloud and interconnects a service provider to one or more customers.

LSR

A label switch router (LSR) resides as part of a service provider’s MPLS cloud and makes frame-forwarding decisions based on labels applied to frames.

Metro Ethernet

Metro Ethernet is simply Ethernet technology extended into the metropolitan area network (MAN). The customer connects to the service provider through an Ethernet connection (using an RJ-45 connector). This effectively extends the LAN into the MAN. The service provider is responsible for configuring the logical connections between the customer sites. The technologies used with the service provider network are hidden from the customer.

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