Which of the following is a digital telephone/telecommunication network that carries voice, data,

and video over an existing telephone network infrastructure?

A.
PPP

B.
Frame relay

C.
ISDN

D.
X.25

Explanation:

Integrated Services Digital Network (ISDN) is a digital telephone/telecommunication network that
carries voice, data, and video over an existing telephone network infrastructure. It requires an
ISDN modem at both the ends of a transmission. ISDN is designed to provide a single interface for
hooking up a telephone, fax machine, computer, etc.
ISDN has two levels of service, i.e., Basic Rate Interface (BRI) and Primary Rate Interface (PRI).
Answer option A is incorrect. The Point-to-Point Protocol, or PPP, is a data link protocol commonly
used to establish a direct connection between two networking nodes. It can provide connection
authentication, transmission encryption privacy, and compression. PPP is commonly used as a
data link layer protocol for connection over synchronous and asynchronous circuits, where it has
largely superseded the older, non-standard Serial Line Internet Protocol (SLIP) and telephone
company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25
protocol suite). PPP was designed to work with numerous network layer protocols, including
Internet Protocol (IP), Novell’s Internetwork Packet Exchange (IPX), NBF, and AppleTalk.
Answer option D is incorrect. The X.25 protocol, adopted as a standard by the Consultative
Committee for International Telegraph and Telephone (CCITT), is a commonly-used network
protocol. The X.25 protocol allows computers on different public networks (such as CompuServe,
Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network
layer level. X.25’s protocols correspond closely to the data-link and physical-layer protocols
defined in the Open Systems Interconnection (OSI) communication model.
Answer option B is incorrect. Frame relay is a telecommunication service designed for costefficient data transmission for intermittent traffic between local area networks (LANs) and between
end-points in a wide area network (WAN). Frame relay puts data in a variable-size unit called a
frame. It checks for lesser errors as compared to other traditional forms of packet switching and
hence speeds up data transmission.
When an error is detected in a frame, it is simply dropped. The end points are responsible for
detecting and retransmitting dropped frames.