Chapter 7 - Wide-Area Networks

Question 1

Asynchronous Transfer Mode (ATM)

Answer 1

A Layer 2 WAN technology that interconnects sites using virtual circuits. These virtual circuits are identified by a pair of numbers, called the VPI/VCI pair. A virtual path identifier (VPI) identifies a logical path, which can contain multiple virtual circuits. A virtual circuit identifier (VCI) identifies the unique logical circuit within a virtual path.

Question 2

Basic Rate Interface (BRI)

Answer 2

A BRI circuit contains two 64-kbps B channels and one 16-Kbps D channel. Although such a circuit can carry two simultaneous voice conversations, the two B channels can be logically bonded together into a single virtual circuit (by using PPP’s multilink interface feature) to offer a 128-kbps data path.

Question 3

Challenge Handshake Authentication Protocol (CHAP)

Answer 3

Like PAP, CHAP performs one-way authentication. However, authentication is performed through a three-way handshake (challenge, response, and acceptance messages) between a server and a client. The three-way handshake allows a client to be authenticated without sending credential information across a network.

Question 4

Channel service unit/data service unit (CSU/DSU)

Answer 4

Acts as a digital modem that terminates a digital circuit (for example, a T1 or an E1 circuit).

Question 5

Circuit-switched connection

Answer 5

A connection that is brought up on an as-needed basis. A circuit-switched connection is analogous to a phone call, where you pick up a phone, dial a number, and a connection is established based on the number you dial.

Question 6

Code division multiple access (CDMA)

Answer 6

CDMA offers multiple access, where several transmitters can send information simultaneously over a single communication channel, allowing several users to share a band of frequencies.

Question 7

Customer premise equipment (CPE)

Answer 7

This device resides at a customer site. A router, as an example, can be a CPE that connects a customer with an MPLS service provider.

Question 8

Dedicated leased line

Answer 8

A logical connection interconnecting two sites. This logical connection might physically connect through a service provider’s facility or a telephone company’s central office. The expense of a dedicated leased line is typically higher than other WAN technologies offering similar data rates, because with a dedicated leased line, a customer does not have to share bandwidth with other customers.

Question 9

Demarc

Answer 9

Also known as a demarcation point or a demarc extension, this is the point in a telephone network where the maintenance responsibility passes from a telephone company to a subscriber (unless the subscriber purchased an inside wiring plan). This demarc is typically a box mounted to the outside of a customer’s building (for example, a residence).

Question 10

Digital Subscriber Line (DSL)

Answer 10

A group of technologies that provide high-speed data transmission over existing telephone wiring. DSL has several variants, which vary in data rates and distance limitations. Three of the more popular DSL variants include asymmetric DSL (ADSL), symmetric DSL (DSL), and very high bit-rate DSL (VDSL).

Question 11

E1

Answer 11

A digital transmission format devised by ITU. An E1 circuit contains 32 channels, in contrast to the 24 channels on a T1 circuit. Only 30 of those 32 channels, however, can transmit data (or voice or video). Specifically, the first of those 32 channels is reserved for framing and synchronization, and the 17th channel is reserved for signaling (that is, to set up, maintain, and tear down a session). It transmits data at a rate of 2.048 Mbps.

Question 12

E3

Answer 12

A digital circuit in the same E-carrier family of standards as an E1. An E3 circuit’s available bandwidth is 34.4 Mbps.

Question 13

Edge label switch router (ELSR)

Answer 13

Resides at the edge of an MPLS service provider’s cloud and interconnects a service provider to one or more customers.

Question 14

Frame Relay

Answer 14

A Layer 2 WAN technology that interconnects sites using virtual circuits. These virtual circuits are identified by locally significant data-link connection identifiers (DLCI).

Question 15

Global System for Mobile Communications (GSM)

Answer 15

A standard created to replace first generation (1G) analog cellular networks and was first used with second generation (2G) networks, allowing digital cellular data communication for mobile devices.

Question 16

Integrated Services Digital Network (ISDN)

Answer 16

A digital telephony technology that supports multiple 64-kbps channels (known as bearer channels or B channels) on a single connection. ISDN was popular back in the 1980s for connecting PBXs, which are telephone switches owned and operated by a company, to a telephone company’s central office. ISDN has the ability to carry voice, video, or data over its B channels. ISDN also offers a robust set of signaling protocols: Q.921 for Layer 2 signaling and Q.931 for Layer 3 signaling. These signaling protocols run on a separate channel in an ISDN circuit (known as the delta channel, data channel, or D channel).

Question 17

Label switch router (LSR)

Answer 17

Resides inside a service provider’s MPLS cloud and makes frame forwarding decisions based on labels applied to frames.

Question 18

Local loop

Answer 18

A connection between a customer premise and a local telephone company’s central office.

Question 19

Microsoft Routing and Remote Access Server (RRAS)

Answer 19

A Microsoft Windows server feature that allows Microsoft Windows clients to remotely access a Microsoft Windows network.

Question 20

Multiprotocol Label Switching (MPLS)

Answer 20

A WAN technology popular among service providers. MPLS performs labels switching to forward traffic within an MPLS cloud by inserting a 32-bit header (which contains a 20-bit label) between a frame’s Layer 2 and Layer 3 headers and making forwarding decisions based on the label within an MPLS header.

Question 21

Optical carrier (OC)

Answer 21

Optical networks often use OC levels to indicate bandwidth. As a base reference point, the speed of an OC-1 link is 51.84 Mbps. Other OC levels are multiples of an OC-1. For example, an OC-3 link has three times the bandwidth of an OC-1 link (that is, 3 * 51.84 Mbps = 155.52 Mbps).

Question 22

Packet-switched connection

Answer 22

Similar to a dedicated leased line, because most packet-switched networks are always on. However, unlike a dedicated leased line, packet-switched connections allow multiple customers to share a service provider’s bandwidth.

Question 23

Password Authentication Protocol (PAP)

Answer 23

Performs one-way authentication (that is, a client authenticates with a server). However, a significant drawback to PAP, other than its unidirectional authentication, is its clear-text transmission of credentials, which could permit an eavesdropper to learn authentication credentials.

Question 24

Plain old telephone service (POTS)

Answer 24

A POTS connection connects a customer device (such as a telephone) to the public switched telephone network (PSTN).

Question 25

Point-to-Point Protocol (PPP)

Answer 25

A common Layer 2 protocol offering features such as multilink interface, looped link detection, error detection, and authentication.

Question 26

Point-to-Point Protocol over Ethernet (PPPoE)

Answer 26

Commonly used between a DSL modem in a home (or business) and a service provider. Specifically, PPPoE encapsulates PPP frames within Ethernet frames. PPP is used to leverage its features, such as authentication.

Question 27

Primary rate interface (PRI)

Answer 27

A PRI circuit is an ISDN circuit built on a T1 or E1 circuit. Recall that a T1 circuit has 24 channels. Therefore, if a PRI circuit is built on a T1 circuit, the ISDN circuit has 23 B channels and 1 64-Kbps D channel. The 24th channel in the T1 circuit serves as the ISDN D channel (that is, the channel used to carry the Q.921 and Q.931 signaling protocols, which set up, maintain, and tear down connections).

Question 28

Public switched telephone network (PSTN)

Answer 28

The worldwide telephony network consisting of multiple telephone carriers.

Question 29

Satellite (WAN technology)

Answer 29

Provides WAN access to sites where terrestrial WAN solutions are unavailable. Satellite WAN connections can suffer from long round-trip delay (which can be unacceptable for latency-sensitive applications) and are susceptible to poor weather conditions.

Question 30

Synchronous Optical Network (SONET)

Answer 30

A Layer 1 technology that uses fiber-optic cabling as its media. Because SONET is a Layer 1 technology, it can be used to transport various Layer 2 encapsulation types, such as ATM. Also, because SONET uses fiber-optic cabling, it offers high data rates, typically in the 155-Mbps to 10-Gbps range, and long-distance limitations, typically in the 20-km to 250-km range.

Question 31

T1

Answer 31

T1 circuits were originally used in telephony networks, with the intent of one voice conversation being carried in a single channel (that is, a single DS0). A T1 circuit consists of 24 DS0s, and the bandwidth of a T1 circuit is 1.544 Mbps.

Question 32

T3

Answer 32

In the same T-carrier family of standards as a T1, a T3 circuit offers an increased bandwidth capacity. Although a T1 circuit combines 24 DS0s into a single physical connection to offer 1.544 Mbps of bandwidth, a T3 circuit combines 672 DS0s into a single physical connection, with a resulting bandwidth capacity of 44.7 Mbps.

Question 33

Tip and ring

Answer 33

The red and green wires found in an RJ-11 wall jacks, which carry voice, ringing voltage, and signaling information between an analog device (for example, a phone or a modem) and an RJ-11 wall jack.