Chapter 7 Review
1) A
2) A, B
3) B, D
4) B
5) D
6) B, D
7) A
8) C
9) C
10) B
11) D
12) D
13) A, C
14) C
15) C
16) B, D
17) C
18) D
19) C, D
20) A, B Key Terms
Switched circuit-A circuit by the phone company in reaction to signaling (the user tapping telephone keys or a modem sending in the same digits), with the telco tearing down the circuit when the user is finished.
Dedicated circuit- An electric circuit created by a telco on behalf of a customer, with the circuit staying up all the time, dedicated for use by the one customer that ordered the circuit. Also known as a line, leased circuit, and point-to-point line.

Circuit switching-The overall process by which a series of telco devices called circuit switches connect a circuit from one customer to the other, with the device’s logic taking incoming bits on one segment in the link and forwarding those bits out the matching outgoing segment, without storing bits.
Packet switching-Process of forwarding customer data in a WAN by looking at the header of the messages sent into the WAN by the customer and making a per-messages decision as to where to forward each message.
Leased line-A physical link between two locations, provided by a telco, that allows two-way communication between sites.
Time-division multiplexing-A type of logic used by some networking devices, including circuit switches in the telco, in which the switch divides a faster line into time channels.
T-carrier system- The name of the combination of different physical line standards (DS0, DS1, DS3, and others), plus circuit switches that use time-division multiplexing (TDM) features, that together allowed the phone company to create digital circuits from end to end and create leased-line services for customers.
DS0- Digital Signal Level 0. One of the physical line standards in the T-carrier system, as originally created by the companies of the Bell System in the United States. DS0 runs at 64 Kbps
DS1- Digital Signal Level 1. One of the physical line standards in the T-carrier system, as originally created by the companies of the Bell System in the United States. DS1 runs at 1.544 Mbps, with a 193-bit frame, 24 DS0 channels, and an 8-Kbps overhead channel.
Frame relay- A widely popular packet-switching technology and service that emerged in the marketin the 1990s, using permanent virtual circuits (PVC) between pairs of routers that can send frames to each other, and data-link connection identifiers (DLCI) to address and identify each PVC.

Acronyms
ATM-Asynchronous transfer mode. A series of networking standards for both LANs and WANs.
CO- Central Office- The term that refers to a telco office space where the telco keeps its equipment. Leased lines physically connect from a telco customer office building into the CO

DCE- Data communications equipment. This term has two uses in networking: 1) In packet – switched networks, the node (device) that sits in the telco network at the edge of the network, on the other side of the physical access link that connects to the customer device (the DTE). 2) A reference to a device that provides clocking on physical cabling that uses clock pin leads, such as on a leased line, in which the CSU/DSU controls the speed a router sends/receives bits by providing clocking to the router.
Demark- Short for demarcation, a legal and business term used by telcos to specify the physical dividing line in the cabling and equipment for a telco service in a customer building, where the customer is responsible for the cabling and devices to one side of the demark, and the telco is responsible for the cabling and equipment on the other side of the line.
DLCI- Data-link connection identifier – The term defined by Frame Relay, particularly the LAPF protocol, for the LAPF header field used as an address field to identify a Frame Relay virtual circuit.
DSO- Digital Signal Level 0. One of the physical line standards in the T – carrier system, as originally created by the companies of the Bell System in the United States. DS0 runs at 64 Kbps.
DSI- Digital Signal Level 1.one of the physical line standards in the T-carrier system, as originally created by the companies of the Bell system in the United States. DS1 runs at 1.544 Mbps, with a 193-bit frame, 24 DS0 channels, and an 8-Kbps overhead channel.
DS3- Digital Signal Level 3. One of the physical line standards in the T-carrier system, as originally created by the companies of the Bell System in the United States. DS3 runs at 44.736 Mbps, with 28 DS1 channels and additional overhead.
DTE- Data terminal equipment, or data circuit- terminating equipment. This term has two uses in networking: 1) In packet-switched networks, the node (device) that sits in the customer site, connected by a physical access link to a node in the telco (the DCE). 2) A reference to a device that slaves its rate of sending and receiving bits to a device on the other end of the cable by watching for changes in the state of the clocking pin leads on a serial cable. This occurs on a leased line, in which a router connected to an external CSU/DSU receives clocking from the CSU/DSU (the DCE), with the router acting as the DTE.
ESS- Electronic Switching System
HDLC- High-Level Data Link Control. An early data-link protocol used on many early variations of WAN services, mainly used today on leased lines.

MetroE- A type of multiaccess WAN service that uses Ethernet as the physicalaccess link and usually uses an Ethernet switch as the customer site device, with the customer sending Ethernet frames from one customer site to the other.
MPLS-
POP- An application protocol commonly used on email client software to retrieve incoming email.
PPP- Point-to-Point Protocol. A data-link protocol used on physical and logical point-to-point communications links.

Other terms
Bell Operating Company- A term that refers to one of the many companies in North America that grew from, and were affiliated with, the companies that Alexander Graham Bell and his partners started after Bell invented the telephone.
Channel- A subset of the capacity of a physical link in the T-carrier system, created by using time-division multiplexing (TDM) technology to separate the links bits into different time slots.
Circuit switch- A networking device used as a node in a networking topology, most often used to connect nearby user devices that need to use a cable to connect to the network.
Data-link connection identifier- The term defined by Frame Relay, particularly the LAPF protocol, for the LAPF header field used as an address field to identify a Frame Relay virtual circuit.
Demultiplex- The opposite of multiplex. For a multiplexer, the process of taking an incoming digital serial bit stream on one link; finding the logical frames and channels inside that bit stream based on line standards like DS0, DS1, and DS3; and splitting out (demultiplexing) the channels in the one bit stream into multiple streams that the multiplexer sends out different ports.
Fractional T1- A leased line that supports less than T1 speed between the two customer routers, using a fraction of the capacity of the T1.

Full mesh- In Frame Relay and other multiaccess networks that use a virtual circuit (VC) concept, a topology in which all devices connect to all others with a VC.
Hub-and-spoke- A term used for WAN topologies in which one central site (the hub) connects to many remote sites (spokes), but the remote sites do not connect directly to each other.
Local loop- The cabling that runs from a telco central office (CO) to a customer home or business to support analog phone calls.
Modem- A device at the customer end of a DSL local loop that encodes and decodes the data on the local loop cable in a DSL connection.
Multi-access- A network service that allows more than two devices to connect to the service.
Multiplex- The opposite of demultiplex. For a multiplexer, the process of taking multiple incoming digital serial bit streams and putting all of them into different channels on a single line that has time channels, like DS1 and DS3 lines.
Multipoint- A WAN topology in which more than two devices can communicate directly over the WAN.
Multiprotocol label switching- A type of WAN service, and the related protocol standards, that acts as a packet-switching service that operates in part based on the IP protocol.
Packet switching- The process of forwarding customer data in a WAN by looking at the header of the messages sent into the WAN by the customer and making a per-message (per-packet) decision as to where to forward each message.
Partial mesh- In packet-switched networks that use a virtual circuit (VC) concept, a design in which a VC exists between a subset of the router pairs, but not all pairs.
Permanent virtual circuit- In packet-switched networks, a logical connection between two customer devices, typically routers, that allows the routers to send messages directly to each other, and specifically the case in which the connection is predefined and never goes away.
RJ-48- The name of the connector used on many leased-line connections for the cabling inside the customer site.