• Front-end processor => A small computer dedicated to
communication management. It is attached to the main
or host computer in a network. It performs such special
communications processing as error control, formatting,
editing, controlling and routing, and speed and signal
• Concentrator => A programmable telecommunication
computer that collects and temporarily stores messages
from terminals for batch transmission to the host
• Controller => A specialized computer that supervises
communications traffic between the CPU and the
peripheral devices such as terminals and printers. It
routes output from the CPU to the appropriate
• Multiplexer => A device that enables a single
communications channel to carry data transmissions
from multiple sources simultaneously. The multiplexer
divides the communications channel so that it can be
shared by multiple transmission devices. The
multiplexer can divide a high-speed channel into
multiple channels of slower speed .
Local Area Network
Local Area Network
• LAN is used to connect PCs or resources (e.g. printers) in one
building or several buildings in close proximity.
• LAN has higher transmission capacity than PBX (Private Branch
• LAN can transmit video and graphics.
• LAN is more expensive to install than PBX and less flexible. It
requires new wiring each time a LAN is moved.
• The server acts as a librarian. It stores programs and data files for
network users. The server determines who will get access to what
and in what sequence.
• Sever can be powerful PCs with large hard-disk capacity,
workstations, minicomputers, or mainframes.
• The network gateway connects the LAN to public networks, such
as the telephone network, or to other corporate networks.
Local Area Network
• A gateway is a communications processor that can
connect dissimilar networks by translating from
one set of protocols to another.
• LAN can use twisted wire, coaxial , or fiber-optic
• LAN also can use wireless technology.
• The network operating system (NOS) can reside
on every computer in the network, or it can reside
on a single server.
• The NOS routes and manages communications on
the network and sharing of network resources.
Wide Area Network
• It spans a broad geographical distance, ranging from
several miles to the span of entire continent.
• WAN may consist of a combination of switched and
dedicated lines, microwave, and satellite
• Switched lines are telephone lines that a person can
access from his or her terminal to transmit data to the
• Dedicated lines are continuously available for
transmission. This lines can be leased or purchased
from a common carriers or private communications
• Most WANS are switched.
Value-Added Networks (VANS)
• Value-added networks are private, multipath, data-only, thirdparty-managed network that is used by multiple organizations on
a subscription basis.
• The VAN is set up by a firm that is in charge of managing the
• The subscribers pay only for the amount of data they transmit
plus a subscription fee.
• Customers do not have to invest in network equipment and
software or perform their own error checking, editing, routing,
and protocol conversion.
• The network can use twisted-pair lines, satellite links, and other
communication channels leased by the value-added carrier.
• The most popular LANs are Ethernet LANS, Token
Ring LANS, wireless LANs, and ATM LANs.
• Ethernet is the most widely used local area network
• The protocol was designed in 1973 by Xerox with a
data rate of 10 Mbps (Traditional Ethernet) and a bus
• Today it has a data rate of 100 Mbps (Fast Ethernet)
and 1000 Mbps (Gigabit Ethernet).
• Ethernet is formally defined by IEEE 802.3 standard.
• Access Method: CSMA/CD for traditional Ethernet.
Factors related to CSMA/CD: minimum frame
length, data transmission rate, collision domain.
• Topology: Stations on a traditional Ethernet can be
connected together using physical bus or star
topology, but the logical topology is always a bus =>
only one station at a time can use the medium.
• Layers: The data link layer has two sublayers: the
logical link control (LLC) sublayer and the media
access control (MAC) sublayer. LLC=>flow and error
• Frame: IEEE 802.3 specifies one frame type
containing seven fields; preamble, SFD, DA, SA,
length/type of PDU, Data, and the CRC.
• Addressing: Each station such as a PC, workstation, or printer on
an Ethernet network has its own network interface card (NIC).
NIC provides the station with a 6-byte physical address. Three
types of addresses in Ethernet: unicast, multicast, and broadcast.
• Implementations: The IEEE standard defines several
implementations for traditional Internet.
10BASE5 (thick Ethernet, uses bus topology, uses thick coaxial
cable as transmission medium)
10BASE2 (Thin Ethernet, uses bus topology, uses thin coaxial
cable as transmission medium)
10BASE-T (twisted-pair Ethernet, uses a physical star topology,
uses twisted-pair cable as transmission medium)
10BASE-FL (fiber link Ethernet, uses a star topology, uses fiberoptic cable as transmission medium).
• Fast Ethernet: Fast Ethernet uses the same principle as
traditional Ethernet (CSMA/CD). The transmission rate is 100
Mbps. For CSMA/CD to work, we have to increase the
minimum frame length or decrease the collision domain. In
Fast Ethernet, the collision domain has to be decreased to 250
• Gigabit Ethernet: The transmission rate is 1000 Mbps. To
achieve this rate, the MAC layer has two options: keeping
CSMA/CD or dropping it. To keep CSMA/CD, we need to
decrease collision domain or increase the minimum frame
length. Since a collision domain of 25 meters is unacceptable,
the minimum length of the frame is increased in a very elegant
way. In second option, dropping CSMA/CD, every station is
connected by two separate paths to the central hub. This is
called full-duplex Ethernet with no collision and no
Token Ring LANs
Token ring is a protocol defined in IEEE project 802.5.
Access method: It uses a token (3 bytes) passing access method.
Addressing: Like Ethernet, most Token Ring implementations
use a 6-byte address. Only difference is Token Ring sends the
most significant bit of each byte first.
• Implementations: The ring in a Token Ring consists of a series
of shielded twisted pair sections linking each station to its
immediate neighbors. Configuring the network as a ring
introduces a potential problem. One disabled node could stop
the flow of traffic around the entire network. To solve this
problem, each station is connected to an automatic switch. The
switch can bypass an inactive station
• Wireless communication is one if the fastest growing
technologies. The demand for mobile devices has led to a
need for wireless wide and local area networks.
• Spread Spectrum: Frequency Hopping and Direct Sequence.
• ISM Frequency Band: In 1985, the Federal Communications
Commission (FCC) modified the radio spectrum regulations
for unlicensed devices. It authorizes wireless LANs to operate
in Industrial, Scientific, and Medical (ISM) bands. The use of
these band does not require license from FCC if the
equipment operates under 1 W power.
902 – 928 MHz => I – band
2.4 – 2.48 GHZ => S-band
5.725 – 5.85 GHz => M-band
• Access Method: CSMA/CA
• Architecture: Define architecture for only IEEE 802.11.
• The second type of network that we encounter in the
Internet is the point-to-point wide area network.
• A point-to-point WAN connects two remote devices
using a line available from a public network such as a
• These public companies normally provide the service
at the physical layer, the user is responsible for the
protocol at the data link layer.
• At the physical layer, the point-to-point connection
between two devices can be accomplished using one of
the services available today such as traditional modem
technology with regular telephone line, a DSL line,
cable modem, a T-line, or SONET.
V.90 (56K) modem
• Many end users of the Internet are connected from
home or small businesses to an ISP through a
• One current development is the V.90 (56K) modem
which uses the existing telephone line.
• The subscriber is connected to the switching station of
the telephone company and uses the connection from
the switching station to the ISP computer (server).
• The connection is asymmetric; the user can download
data at 56 kbps but upload at only 33.6 kbps.
Digital Subscriber Line (DSL)
• The digital subscriber line (DSL) is a newer technology
that uses the existing telecommunication networks such as
the local loop telephone line (a connection between
subscriber resident and the telephone company) to
accomplish high-speed delivery of data, voice, and
• DSL is a family of technologies. Five of them are ADSL,
RADSL, HDSL, VDSL, and SDSL.
• Another technology used for remote connection is the
• This technology uses the cable TV services available in
• The technology uses a 500 MHz coaxial cable to deliver
TV channels to residential areas.
• Because each TV channel needs only 6 MHz, more than 75
channels can be simultaneously broadcast through the
cable. Some of these channels can be used to transmit data
through the cable TV provider to the Ethernet.
• T lines are standard digital telephone carriers designed
originally to multiplex voice channels.
• Today, however, T lines can be used to carry data from a
residence or an organization to the Internet.
• They can also be used to provide a physical link between
nodes in a switched wide area network.
• T lines are commercially available in two data rates: T-1
(1.544 Mbps) and T-3 (44.736 Mbps).
• A T-3 line is equivalent to 28 T-1 lines.
• The high bandwidths of fiber-optic cable are
suitable for today’s highest data rate technologies
(such ads video conferencing) and for carrying
large numbers of lower-rate technologies at the
• ANSI created a set of standards called Synchronous
Optical Network (SONET) to handle the use of
• The backbone networks in the Internet are usually a switched
• A switched WAN is a wide area network that covers a large
area (a state or a country) and provides access at several
points to the user.
• Inside the network, there is a mesh of point-to-point networks
that connects switches. The switches, multiple port
connectors, allow the connection of several inputs and
• Switched WAN technology differs from LAN technology in
• Instead of a bus or star topology, switches are used to create
• LAN technology is a connectionless technology, Switch WAN
is a connection-oriented technology.
• Three common switched WANs are: X.25 (almost obsolete),
Frame Relay (will be in use for a few more years) ATM
Switched WANs: X.25
• X.25, introduced in the 1970s, was the first
switched WAN to become popular both in Europe
and the United States.
• Although still used in Europe, it is disappearing
from the United States.
• It was mostly used as a public network to connect
individual computers or LANs.
• It provides an end-to-end service.
Switched WANs: X.25
• Although X.25 was used as the WAN to carry IP packets
from one part of the world to another, there was always a
conflict between IP and X.25. IP is network layer protocol.
An IP packet is supposed to be carried by a frame at the
second (data link) layer. X.25, which was designed before
the Internet, is a three-layer protocol; it has its own network
layer. IP packets had to be encapsulated in the X.25
network packet to be carried from one side of the network
to another. This is analogous to a person who has a car but
has to load it in a truck to go from one point to another.
• Another problem with X.25 is that it was designed at a time
when transmission media were not very reliable. For this
reason, X.25 performs flow and error control at both the
data link layer and the network layer. This makes
transmission very slow.
• For all of these reasons, X.25 will most likely soon
disappear from the Internet.
Switched WANs: Frame Relay
• Frame Relay, a switched technology that provides
low-level service, was designed to replace X.25.
Frame Relay has some advantages:
• High data rate: Frame Relay was originally
developed to provide 1.544 Mbps data rate. Now it
can provide 44.736 Mbps.
• Bursty Data: Some services offered by wide area
network providers assume that the user has a
fixed-rate need. For example, T1 line was
designed for constant data rate 1.544 Mbps. This
type of services are not suitable for bursy data
rates. Bursty data rate requires bandwidth on
demand. Frame relay accepts bursty data.
Switched WANs: Frame Relay
• Less overhead due to improved transmission
media: The quality of transmission media has
improved since the last decade. There is no need
to have a WAN that spends time and resources
double-checking errors. Frame Relay does not
provide error checking in the data link layer.
Instead it is done at the network and transport
Asynchronous Transfer Mode (ATM)
• ATM technology parcels information into uniform
cells, each with 53 groups of eight bytes, eliminating
the need for protocol conversion.
• It can seamlessly and dynamically switch voice, data,
images, and video between users.
• It can pass data between computers from different
• It permits data to be transmitted at any speed the
network handles. ATM can transmit up to 2.5 GBPS.
• It can connect LAN and WAN together more easily.
Internetworking, Connectivity & Open System
• Internetworking => The linking of separate networks,
each of which retains its own identity, into an
• Connectivity => The ability of computers and
computer-based devices to communicate with one
another and share information in a meaningful way
without human intervention.
• Open Systems => Open systems promote connectivity
because they can operate on different hardware
platforms. They are built on public non-proprietary
operating systems, user interfaces, application
standards, and networking protocols.