Chapter 4 Flash Cards - WANs

Question 1

Explain the Bit Rate for a DS0

Answer 1

64 kbps

Question 2

Explain the Bit Rate for a DS1 (T1)

Answer 2

1.544 Mbps (24 DS0s, plus 8 kbps overhead)

Question 3

Explain the Bit Rate for a DS3 (T3)

Answer 3

44.736 Mbps (28 DS1s, plus management overhead)

Question 4

Explain the Bit Rate for a E1

Answer 4

2.048 Mbps (32 DS0s)

Question 5

Explain the Bit Rate for a E3

Answer 5

34.368 Mbps (16 E1s, plus management overhead)

Question 6

Explain the Bit Rate for a J1 (Y1)

Answer 6

2.048 Mbps (32 DS0s; Japanese Standard)

Question 7

Describe an HDLC frame.

Answer 7

1 byte - Flag
1 byte - Address
1 byte - Control
Data
2 Byte - FCS

Question 8

Describe the Proprietary Cisco HDLC Frame.

Answer 8

1 byte - Flag
1 byte - Address
1 byte - Control
2 byte - Type
Data
2 Byte - FCS

Question 9

Define Synchronous

Answer 9

The imposition of time ordering on a bit stream. Practically, a device tries to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice slight variations in the speed on each end and can adjust its speed accordingly.

Question 10

Define Clock Source

Answer 10

The device to which the other devices on the link adjust their speed when using synchronous links.

Question 11

Define CSU/DSU

Answer 11

Channel Service Unit/Data Service Unit. Used on digital links as an interface to the telephone company in the United States. Routers typically use a short cable from a serial interface to a CSU/DSU, which is attached to the line from the telco with a similar configuration at the other router on the other end of the link.

Question 12

Define Telco

Answer 12

Telephone Company

Question 13

Define Four-wire circuit

Answer 13

A line from the telco with four wires, composed of two twisted-pair wires. Each pair is used to send in one direction, so a four-wire circuit allows full-duplex communication.

Question 14

Define T1

Answer 14

A line from the telco that allows transmission of data a 1.544 Mbps

Question 15

Define E1

Answer 15

Similar to a T1, but used in Europe. It uses a rate of 2.048 Mbps and 32 64-kbps channels.

Question 16

List some synonyms for a point to point leased line.

Answer 16

leased line, leased circuit, link, serial link, serial line, point-to-point link, circuit

Question 17

Define LAPF

Answer 17

Link Access Procedure - Frame. Defines the Frame Relay header and trailer.

Question 18

Define Access link

Answer 18

In Frame Relay, the physical serial link that connects a Frame Relay DTE device, usually a router, to a Frame Relay switch. The access link uses the same physical layer standards as do point-to-point leased lines.

Question 19

Define back-to-back link

Answer 19

A serial link between two routers, created without CSU/DSUs, by connecting a DTE cable to one router and a DCE cable to the other. Typically used in labs to build serial links without the expense of an actual leased line from the telco.

Question 20

Define clocking

Answer 20

The process of supplying a signal over a cable, either on a separate pin on a serial cable or as part of the signal transitions in the transmitted signal, so that the receiving device can keep synchronization with the sending device.

Question 21

Define DTE (Layer 1)

Answer 21

Data Terminal Equipment. From a Layer 1 perspective, the DTE synchronizes its clock based on the clock sent by the DCE. From a packet-switching perspective, the DTE is the device outside the service provider’s network, typically a router.

Question 22

Define CSU/DSU

Answer 22

Channel service unit/data service unit. A device that understands the Layer 1 details of serial links installed by a telco and how to use a serial cable to communicate with networking equipment such as routers.

Question 23

Define DCE (Layer 1)

Answer 23

Data Communications Equipment. From a physical layer perspective, the device providing the clocking on a WAN link, typically a CSU/DSU, is the DCE. From a packet-switching perspective, the service provider’s switch, to which a router might connect, is considered the DCE.

Question 24

Define DS0

Answer 24

Digital Signal Level 0. A 64-kbps line or channel of a faster line inside a telco whose origins are to support a single voice call using the original voice (PCM) codecs.

Question 25

Define DS1

Answer 25

Digital Signal Level 1. A 1.544 Mbps line from the telco, with 24 DS0 channels of 64 kbps each, plus an 8 kbps management and framing channel. Also called a T1

Question 26

Define Frame Relay

Answer 26

An international standard data-link protocol that defines the capabilities to create a frame-switched (packet-switched) service, allowing DTE devices (typically routers) to send data to many other devices using a single physical connection to the Frame Relay service.

Question 27

Define HDLC

Answer 27

High-Level Data Link Control. A bit-oriented synchonous data link layer protocol developed by the International Organization for Standardization (ISO).

Question 28

Define Leased Line

Answer 28

A serial communications circuit between two points, provided by some service provider, typically a telephone company (telco). Because the telco does not sell a physical cable between the two endpoints, instead charging a monthly fee for the ability to send bits between the two sites, the service is considered to be a leased service.

Question 29

Define Packet Switching

Answer 29

A generic reference to network services, typically WAN services, in which the service examines the contents of the transmitted data to make some type of forwarding decision. This term is mainly used to contrast with the WAN term circuit switching, in which the provider sets up a (Layer 1) circuit between two devices, and the provider makes no attempt to interpret the meaning of the bits.

Question 30

Define PPP

Answer 30

Point-to-Point Protocol. A protocol that provides router-to-router and host-to-network connections over synchronous point-to-point and asynchronous point-to-point circuits.

Question 31

Define Serial Cable

Answer 31

A type of cable with many different styles of connectors used to connect a router to an external CSU/DSU on a leased-line installation.

Question 32

Define Virtual Circuit

Answer 32

In packet-switched services like Frame Relay, VC refers to the ability of two DTE devices (typically routers) to send and receive data directly to each other, which supplies the same function as a physical leased line (leased circuit), but doing so without a physical circuit. This term is meant as a contrast with a leased line or leased circuit.