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26 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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What are common carriers, local exchange carriers, and interexchange

A common carrier is a private company that sells or leases
communication services and facilities to the public. Common carriers are
oriented, and their primary products are services for voice and data
transmissions, both over traditio
nal wi

circuits as well as cellular
services. Common carriers often supply a broad range of computer
services, such as the manufacturing and marketing of specialized
communication hardware and software. A common carrier that provides
local telepho
ne services (e.g., BellSouth) is commonly called a local
exchange carrier (LEC), while one that provides long distance services (e.g.,
MCI) is commonly called an interexchange carrier (IXC). As
the LECs move
into the long distance market and IXCs move into

the local telephone
market, this distinction may disappear.


Explain how cloud architecture works.

Cloud architecture is represented as a cloud with connection
points. Users of circuit switched services lease the connection points
(e.g., telep
hone lines) into the common carrier’s network, which is
called the cloud. This architecture is very flexible and hides its internal
details. Circuits can be established as needed between any computers
attached to the cloud at any point. However, data can o
nly be
transmitted while a circuit is established, and only to the one location it
connects to. These designs are simpler for the organization because they
hide the common carrier’s burner of network design and management
inside the cloud. Network managers

do not need to worry about the
of traffic sent between each computer, they just need to specify
the amount of traffic entering and leaving each

computer and buy the
appropriate size and number of connections into the PSTN.

ed designs can be more expensive because users
must pay for each connection into the network and pay based on the
amount of time each circuit is used. Cloud
based designs are often used
when network managers are uncertain of network demand, particularly

a new or rapidly growing network.

9. Compare and contrast circuit
switched services, dedicated circuit
services, and packet
switched services.

For both circuit switched and dedicated circuit networks, a
circuit is established
between the two communicating computers. This
circuit provides a guaranteed data transmission capability that was
available for use by only those two computers and is assigned solely

that transmission. No other transmission is possible until the circuit

closed. In contrast, packet switched services enable multiple
connections to exist simultaneously between computers over the same
physical circuit or even over different physical circuits.

In the POTS and ISDN circuit switched networks each connection
through the regular telephone network on a different circuit. These
circuits may vary in quality, meaning that while one connection will be
fairly clear, the next call may be noisy. The data transmission rates on
these circuits are usually low. Usuall
y, transmission rates range from
28.8 Kbps to 56 Kbps for dialed POTS circuits to 128 Kbps to 1.5 Mbps
for ISDN circuits. You usually pay per use for circuit switched services.

With a dedicated circuit network, you lease from common
rriers for
their exclusive use twenty
four hours per day, seven days
per week. All connections are point to point, from one building in one
city to another building in the same or a different city. The carrier
installs the circuit connections at the two en
d points to the circuit and
makes the connection between them. The circuit still runs through the
common carrier’s cloud, but the network behaves as if you have your
own physical circuit running from one point to another. Dedicated
circuits are billed at a

flat fee per month and the user has unlimited use
of the circuit. Once you sign a contract, making changes can be
expensive because it means rewiring the building and signing a new
contract with the carrier. Therefore, dedicated circuits require more

in network design than switched circuits both in terms of locations
and the amount of capacity you purchase.

With packet switched services, the user again buys a
connection into the common carrier cloud. The user pays a fixed fee for
e connection into the network depending upon the type and capacity
of the service and is charged for the number of packets transmitted.

13. Distinguish among T1, T2, T3, and T4 circuits

T1 circuit is sometimes called a DS
1 circuit. It provi
des a data
rate of 1.544 Mbps. T1 circuits can be used to transmit data, but often
are used to transmit both data and voice. In this case, a time division
multiplexer (TDM) provides 24 64
Kbps circuits. Digitized voice using
pulse code modulation (PCM) req
uires a 64 Kbps circuit so T1 circuit
enables 24 simultaneous voice channels.

T2 circuit transmits data at a rate of 6.312 Mbps. Basically; it is
a multiplexed bundle of four T1 circuits. T3 circuits allow transmission at
a rate of 44.376 M
bps, although most articles refer to this rate as 45
megabits per second. This is equal to the capacity of 28 T1 circuits. T3
circuits are becoming popular as the transmission medium for corporate
MANs and WANs because of their higher data rates. At low sp
eed, these
T3 circuits can be used as 672 different 64 Kbps channels or voice
channels. A T4 circuit transmits at 274.176 Mbps, which is equal to the
capacity of 178


circuits. Obviously, an organization using either T3 or
T4 circuits must have a tremend
ous need to transmit very large
quantities of data.

15. How do packet
switching services differ from other WAN services?

Packet switched services are quite different from the other
type of network services. For each of these three, dialed
circuit services,
dedicated circuit services, and circuit switched services, a physical circuit
was established between the two communicating computers. This
circuit provides a guaranteed data transmission capability that was
available for use by only thos
e two computers.

In contrast, packet switched services enable multiple
connections to exist simultaneously between computers. With packet
switched services, the user again buys a connection into the common
carrier network. The user pays a fi
xed fee for the connection into the
network and charged for the number of packets transmitted.

22. Compare and contrast X.25
, frame relay, ATM, SMDS, and Ethernet


It is the oldest packet switched service.

A standard developed by ITU

ffers datagram, SVC and PVC services

Uses LAPB and PLP protocols at the data link and network layers,


Requires protocol translation at PADs ( for those users who use
different protocols at their LANs)


A reliable protocol (It performs error
control and retransmits bad


Widely used in Europe


Not in widespread use in North America


Low data rates (64 Kbps)

Frame Relay:

Another standardized technology

Faster than X.25 but slower than ATM

Encapsulates packets


Packets delivered unchanged through the network

Unreliable, like ATM


Up to the end
points to control the errors

NO QoS support (under development)

Common CIR speeds:

56, 128, 256
, 384 Kbps, 1.5, 2, and 45 Mbps


Uses fixed length, 53 byte “cells”


5 bytes of overhead and 48 bytes of user data


More suitable for real time transmissions

Provides extensive QoS information


Enables setting of precise
priorities among different types of
transmissions (i.e. voice, video & e

Data Rates


Same rates as SONET:51.8, 466.5, 622.08 Mbps


New versions: T1 ATM(1.5 Mbps), T3 ATM(45 Mbps


A non
standardized technology


Developed by
Telcordia for local phone companies

Unreliable, like ATM

Encapsulates packets

Originally developed for MANs, but could be used for WANs as well

Transmission speed offered:


56 Kbps to 45 Mbps

Uncertain future


Not standardized; competition
from FR, ATM, and others

Ethernet services:

Most organizations use Ethernet and IP in the LAN and BN

Ethernet services differ from MAN/WAN packet services like X.25, ATM,
Frame Relay or SMDS

Currently offer CIR speed from 1 Mbps to 1 Gbps a
t ¼ the cost of more traditional

No need to translate LAN protocol (Ethernet/IP) to the protocol used in
MAN/WAN services


X.25, ATM, Frame Relay and SMDS use different protocols requiring
translation from/to LAN protocol

Emerging technology; expec
t changes

25. How do VPN services differ from common carrier services?

A new type of VAN, called a virtual private network (VPN) has recently
emerged. VPNs provide circuit that run over the Internet but appear to the user
to be private ne
tworks. Internet access is inexpensive compared to the cost of
leasing dedicated circuits, circuit switched services, or packet switched services
from a common carrier.

Different VPNs provide different services, but most offer packets
ing hardware that will communicate via the Internet, or VPN services which
you lease from the VPN in much the same way as leasing a service from a
common carrier. These VPN hardware (or services) take your data, encrypt it, and
send it through the Internet

through a series of “tunnels”, a virtual circuit through
the Internet which constrains the source and destination to only those within the

29. Describe five important factors in selecting WAN services.

Five important factors in selecting WAN services are vendor capabilities,
network capacity, flexibility, control, and reliability. The best vendors provide high
quality service, quickly respond to network problems, adapt to changing customer
needs, and pro
vide useful network management services along with the data
transmission services.

33. It is said that packet
switched services and dedicated circuit services are
somewhat similar from the perspective of the network designer. Why?

managers build packet
switched networks using PVCs, most
switched networks behave like dedicated circuit networks. At first glance,
the basic architecture looks very similar to the cloud mesh of switched
services, and in fact, they are very
similar because data can move from any
computer attached to the cloud to any on the cloud. However, because virtually
all data
intensive networks use PVCs, this means that the network is actually build
using virtual circuits that are the software equivalen
t of the hardware
dedicated circuits.