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									Future Academy
3rd year
Computer Sciences
Spring Semester 2009
Exercise (13) see printed lectures
Question (1) pp 3-23 to 3-25
Discuss the five concepts of data transmission;
   1. Synchronous mode
       uses no start and stop bits but instead synchronizes transmission
speeds at both the receiving and sending end of the transmission using
clock signals built into each component. A continual stream of data is
then sent between the two nodes. Due to there being no start and stop
bits the data transfer rate is quicker although more errors will occur, as
the clocks will eventually get out of sync, and the receiving device
would have the wrong time that had been agreed in protocol
(computing) for sending/receiving data, so some bytes could become
corrupted (by losing bits). Ways to get around this problem include re-
synchronization of the clocks and use of check digits to ensure the byte
is correctly interpreted and received.
-Time is different
- but rate is the

    2. Asynchronous mode
       uses start and stop bits to signify the beginning bit ASCII
character would actually be transmitted using 10 bits e.g.: A "0100 0001"
would become "1 0100 0001 0". The extra one (or zero depending on
parity bit) at the start and end of the transmission tells the receiver first
that a character is coming and secondly that the character has ended.
This method of transmission is used when data is sent intermittently as
opposed to in a solid stream. In the previous example the start and stop
bits are in bold. The start and stop bits must be of opposite polarity.
This allows the receiver to recognize when the second packet of
information is being sent.
-Agreement in the time and rate

    3. Data flow Methods of circuit operation (Simplex, Half duplex, Full duplex)
Buses and networks are designed to allow communication to occur
between individual devices that are interconnected. The flow of
information, or data, between nodes can take a variety of forms:

With simplex communication, all data flow is unidirectional: from the
designated transmitter to the designated receiver. BogusBus is an
example of simplex communication, where the transmitter sent
information to the remote monitoring location, but no information is ever
sent back to the water tank. If all we want to do is send information one-
way, then simplex is just fine. Most applications, however, demand

Half-duplex communication may be likened to two tin cans on the ends
of a single taut string: Either can may be used to transmit or receive, but
not at the same time. Full-duplex communication is more like a true
telephone, where two people can talk at the same time and hear one
another simultaneously, the mouthpiece of one phone transmitting the
earpiece of the other, and vice versa. Full-duplex is often facilitated
through the use of two separate channels or networks, with an
individual set of wires for each direction of communication. It is
sometimes accomplished by means of multiple-frequency carrier waves,
especially in radio links, where one frequency is reserved for each
direction of communication.
   4. Bandwidth (base band, broadband)
Base band:
-A base band transmission is carried over a single wire using digital
-Base band communications are bidirectional. Signals can be both sent
and received (but not simultaneously.)
- Time Division Multiplexing (TDM) allows multiple signals to be sent via
base band transmissions on a single cable. TDM allocates the multiple
signals across time slots.
-Broadband transmissions are analog transmissions.
-The media used in broadband transmissions is split into two channels
to allow signals to be both sent and received.
-Frequency Division Multiplexing (FDM) is used to create multiple
broadband channels

   5. Topology (physical, logical)

Physical Topologies

Physical topology defines how the systems are physically connected. It represents the
physical layout of the devices on the network. There are five main types of physical
topologies that can be used and each has its own strengths and weaknesses. These five
types include:

      Bus
      Ring
      Star
      Hybrid or tree
      Mesh

The bus topology exists when each of the systems is connected in a line, as seen in
Figure 1. In this topology, all the systems are chained to each other and terminated in
some form on each end. This topology was used in the early days of networking
because it was inexpensive to use and relatively easy to set up.

                               Figure 1--Bus Network

When a packet is sent in a bus topology, there is no intermediary to determine who
the packet should go to. Because of this, every packet that is sent in a bus topology is
received by all systems on the network. Normally, if the packet is not for a particular
system, the computer would simply disregard the packet; however, you can see the
security implications of this type of network. If a malicious user were on this network
and utilized a packet capture program, he could see every conversation that occurred
between machines.

The following table identifies additional advantages and disadvantages of the bus

          Advantages                               Disadvantages

Easy to install                       Out-of-date technology

Costs are usually low                 If cable breaks, whole network is down

Easy to add systems to network Can be difficult to troubleshoot

Great for small networks              Unmanageable in a large network
The ring topology exists when each of the systems is connected to its respective
neighbor forming a ring, as seen in Figure 2. This physical topology has many of the
same strengths and weaknesses of the bus topology. The main difference between the
bus and ring is that the ring topology does not require termination. Because the
systems are connected all together in a loop, there is no beginning and end point as
there is with the bus topology. For additional fault tolerance or performance
enhancements, you can add a second ring. This configuration is seen in Fiber
Distributed Data Interface (FDDI) networks.

                              Figure 2--Ring Network

The following table identifies additional advantages and disadvantages of the bus

          Advantages                              Disadvantages

Easy to install                      Out-of-date technology

Costs are usually low                If cable breaks, whole network is down

Easy to add systems to network Can be difficult to troubleshoot

Great for small networks             Unmanageable in a large network

In the previous two topologies, the systems in the network were connected to each
other. In the star topology, instead of being connected to each other, the systems are
now connected to some central device, as seen in Figure 3. In the star topology, one of
the biggest advantages is that when one system goes down, it does not bring the rest
of the network down with it as it does in the bus or ring topologies. The star topology
is the most prevalent topology in use today. The strengths and weaknesses of the star
topology can be seen in Table 3.
                              Figure 3--Star Network

The following table identifies some additional advantages and disadvantages of the
bus topology:

         Advantages                                 Disadvantages

Easy to install                     Costs are usually higher than with bus or ring

Easy to add devices to              If you have only one central device and it
network                             fails, it brings the network down

One break does not bring
whole network down

Easier to troubleshoot

Widely used

Centralized management

Hybrid or Tree
The hybrid or tree topology is simply a combination of the other topologies. Figure 4
shows an example of a hybrid network. In this layout, we have three star networks
that are connected to each other through a bus topology shown by the red line.

                             Figure 4--Hybrid Network
The mesh topology is the last topology we discuss. In this layout, every system is
connected to every other system. The main advantage of this topology is high
availability. The main disadvantage of this topology is cost, both administrative and
physical. Because each system is connected to each other, the amount of cabling and
maintenance necessary can be prohibitive, especially in larger networks. The formula
for determining the amount of cable needed in a mesh network is:

       (N x (N - 1))/2, where N is the number of systems to be interconnected

In our example in Figure 5, we have six systems that require 15 cables to create a
mesh network. This topology is mainly used in Wide Area Network environments or
in environments where high availability outweighs the costs associated with this
amount of interconnection.

                             Figure 5--Mesh Network

The following table identifies additional advantages and disadvantages of the mesh

       Advantages           Disadvantages

Extremely fault tolerant Expensive

Difficult to implement

Difficult to administer

Difficult to troubleshoot

Logical Topologies

The Logical topology defines how the systems communicate across the physical
topologies. In CISSP terms, you may hear logical topology referred to as the LAN
media access method or network access method. There are two main types of logical

       shared media topology
       token-based topology
Shared Media
In a shared media topology, all the systems have the ability to access the physical
layout whenever they need it. The main advantage in a shared media topology is that
the systems have unrestricted access to the physical media. Of course, the main
disadvantage to this topology is collisions. If two systems send information out on the
wire at the same time, the packets collide and kill both packets. Ethernet is an
example of a shared media topology.

To help avoid the collision problem, Ethernet uses a protocol called Carrier Sense
Multiple Access/Collision Detection (CSMA/CD). In this protocol, each system
monitors the wire, listening for traffic. If traffic is detected, the system waits until it
hears no traffic before it sends packets out. If a situation occurs where two systems
send out packets at the same time and a collision occurs, each system waits for a
period of time before it retries. This time period is different for each system, so that
the collision does not occur again.

For small networks, the shared media topology works fine; however, as you begin to
add more systems to the network, there is a greater opportunity for collisions. To help
reduce the number of collisions, many networks are broken up into several smaller
networks with the use of switches or hubs, and each network is then referred to as its
own collision domain.

Shared media networks are typically deployed in a bus, star, or hybrid physical

Token Based
The token-based topology works by using a token to provide access to the physical
media. In a token-based network, there is a token that travels around the network.
When a system needs to send out packets, it grabs the token off of the wire, attaches it
to the packets that are sent, and sends it back out on the wire. As the token travels
around the network, each system examines the token. When the packets arrive at the
destination systems, those systems copy the information off of the wire and the token
continues its journey until it gets back to the sender. When the sender receives the
token back, it pulls the token off of the wire and sends out a new empty token to be
used by the next machine.

Token-based networks do not have the same collision problems that Ethernet-based
networks do because of the need to have possession of the token to communicate.
However, one problem that does occur with token-based networks is latency. Because
each machine has to wait until it can use the token, there is often a delay in when
communications actually occur.

Token-based network are typically configured in physical ring topology because the
token needs to be delivered back to the originating machine for it to release. The ring
topology best facilitates this requirement.
Question (2) pp 3-25 to 3-36
1. What is meant by access method of transmitted data?
It means sending a stream of bits or bytes from one location to another
using any number of technologies, such as copper wire, optical fiber,
laser, radio, or infra-red light. Practical examples include moving data
from one data storage device to another such as accessing a website,
which involves data transfer from web servers to a user's browser.

2. Show main characteristics of each type of standardized LAN and WAN;
code                    Combined with         X.25
IEEE 802.2              ---                   Fast packet switching
IEEE 802.3              Ethernet              Frame relay
IEEE 802.3u             Fast ethernet         ATM
IEEE 802.3z + 802.3ab gigabit
IEEE 802.5              Token ring
IEEE 802.12             100VG-AnyLAN
Apple LocalTalk         ----
FDDI                    ----
Question (3) pp 3-37 to 3-38
1. Ethernet versus fast Ethernet

2. Typical ring topology versus token ring topology
Typical ring topology:
      An electrical arrangement of nodes on a network in a ring
Token Ring
       A network designed by IBM that uses a ring topology and
circulates a token to manage traffic on the network
3. The T-Carrier system (T1, …, T4) versus the E-Carrier system (E1, …, E5)
 sometimes abbreviated as T-CXR, is the generic designator for any of
several digitally multiplexed telecommunications carrier systems
E - Carrier
The E1 signal format carries data at a rate of 2.048 million bits per
second and can carry 32 channels of 64 Kbps* each. E1 carries at a
somewhat higher data rate than T-1 (which carries 1.544 million bits per
second) because, unlike T-1, it does not do bit-robbing and all eight bits
per channel are used to code the signal. E1 and T-1 can be
interconnected for international use.

Question (4)
State some networking situations where you may come across the following
Abbreviation                        Details                          Situation
TDM             Time division multi-plixing                          pp 3-24
FDM             Frequency division multi-plixing                     pp 3-24
CSMA /CD        Carrier sense multiple access/ collision detection   pp 3-25
IEEE            Institute of electrical and electronic engineers     pp 3-25
FDDI            Fiber distributed data interface                     pp 3-26
LLC             Logical link control                                 pp 3-26
MAC             Media access control                                 pp 3-26
MAU             Multi-station access unit                            pp 3-29
MAN             Municipal area network                               pp 3-32
FDDI            Fiber distributed data interface                     pp 3-32

Abbreviation     Details                            Situation
TDM              Time division multi-plixing        a type of multiplexing that combines
                                                    data streams by assigning each
                                                    stream a different time slot in a set.
                                                    TDM repeatedly transmits a fixed
                                                    sequence of time slots over a single
                                                    transmission channel. Within T-
                                                    Carrier systems, such as T-1 and T-3,
                                                    TDM combines Pulse Code
                                                    Modulated (PCM) streams created for
                                                    each conversation or data stream
FDM              Frequency division multi-          A system that allows the
                 plixing                            transmission of more than one signal
                                                    over a common path, by assigning
                                                    each signal different frequency band.
CSMA/CD   Carrier Sense Multiple        a set of rules determining how
          Access / Collision            network devices respond when two
          Detection                     devices attempt to use a data
                                        channel simultaneously (called a
                                        collision). Standard Ethernet
                                        networks use CSMA/CD to physically
                                        monitor the traffic on the line at
                                        participating stations. If no
                                        transmission is taking place at the
                                        time, the particular station can
                                        transmit. If two stations attempt to
                                        transmit simultaneously, this causes
                                        a collision, which is detected by all
                                        participating stations. After a random
                                        time interval, the stations that
                                        collided attempt to transmit again. If
                                        another collision occurs, the time
                                        intervals from which the random
                                        waiting time is selected are
                                        increased step by step. This is
                                        known as exponential back off

IEEE      Institute of electrical and   International non-profit, professional
          electronic engineers          organization for the advancement of
                                        technology related to electricity. It
                                        has the most members of any
                                        technical professional organization
                                        in the world, with more than 365,000
                                        members in around 150 countries.
FDDI   Fiber distributed data   provides a standard for data
       interface                transmission in a local area network
                                that can extend in range up to 200
                                kilometers (124 miles)
                                A FDDI network contains two token
                                rings, one for possible backup in
                                case the primary ring fails. The
                                primary ring offers up to 100 Mbit/s
                                capacity. When a network has no
                                requirement for the secondary ring to
                                do backup, it can also carry data,
                                extending capacity to 200 Mbit/s. The
                                single ring can extend the maximum
                                distance; a dual ring can extend 100
                                km (62 miles). FDDI has a larger
                                maximum-frame size than standard
                                100 Mbit/s Ethernet, allowing better

LLC    Logical link control     According to the IEEE 802 family of
                                standards, Logical Link Control
                                (LLC) is the upper sublayer of the
                                OSI data link layer. The LLC is the
                                same for the various physical media
                                (such as Ethernet, token ring, and
                                The LLC sub layer is primarily
                                concerned with:
                                Multiplexing protocols transmitted
                                over the MAC layer (when
                                transmitting) and demultiplexing
                                them (when receiving).
                                Providing flow and error control
                                The protocol used for LLC in IEEE
                                802 networks and in some non-IEEE
                                802 networks such as FDDI is
                                specified by the IEEE 802.2 standard
MAC   Media Access Control        It provides addressing and channel
                                  access control mechanisms that
                                  make it possible for several terminals
                                  or network nodes to communicate
                                  within a multipoint network, typically
                                  a local area network (LAN) or
                                  metropolitan area network (MAN).

MAU   Multi station access unit   a MAU is also known as an Ethernet
                                  transceiver or MSAU (MultiStation
                                  Access Unit) is a type of adapter,
                                  connector or stand alone device that
                                  enables a network device to be
                                  connected to a token ring network. A
                                  MAU is one form of fault tolerance
                                  that helps prevent issues if a network
                                  device or computer goes down and
                                  are commonly available as either
                                  active or passive. An active MAU is
                                  not powered and does not in any way
                                  strengthen the signal from a device.
                                  A passive MAU is powered and
                                  repeats and strengthens a signal.
MAN   Municipal Area Network      A MAN is a network that
                                  interconnects users with computer
                                  resources in a geographic area or
                                  region larger than that covered by
                                  even a large local area network but
                                  smaller than the area covered by a
                                  wide area network (WAN). The term
                                  is applied to the interconnection of
                                  networks in a city into a single larger
ATM   Asynchronous transfer       A high-speed, cell-based,
      mode                        connection-oriented, packet
                                  transmission protocol for handling
                                  data with varying burst and bit rates.
                                  It provides Virtual Connection (VC)
                                  switching and multiplexing to enable
                                  the uniform transmission of voice,
                                  data, video and other multimedia

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