size of both the collision domains and the
broadcast domains in a network. Routers are the most important
traffic regulating devices on large networks. They enable
virtually any type of computer to communicate with any other
computer anywhere in the world. LANs typically employ a
combination of Layer 1, Layer 2, and Layer 3 devices.
Implementation of these devices depends on factors that are
specific to the particular needs of the organization.
Interactive Media Activity Drag and Drop: Devices Function
at Layers After completing this activity, students will be able
to identify the different OSI layers where networking devices
function.
Content 4.1 Introduction to
Ethernet/802.3 LANs 4.1.2 Factors that impact network
performance Today's LANs are becoming increasingly congested
and overburdened. In addition to an ever-growing population of
network users, several other factors have combined to test the
limits of the capabilities of traditional LANs: - The
multitasking environment present in current desktop operating
systems such as Windows, Unix/Linux and Mac allows for
simultaneous network transactions. This increased capability
has lead to an increased demand for network resources.
- The use of network intensive applications such as the World
Wide Web is increasing. Client/server applications allow
administrators to centralize information, thus making it easier
to maintain and protect information.
- Client/server
applications free local workstations from the burden of
maintaining information and the cost of providing enough hard
disk space to store it. Given the cost benefit of client/server
applications, such applications are likely to become even more
widely used in the future.
Content
4.1 Introduction to Ethernet/802.3 LANs
4.1.3 Elements of Ethernet/802.3 networks The most common
LAN architecture is Ethernet. Ethernet is used to transport
data between devices on a network. These devices include
computers, printers, and file servers. All nodes on a shared
Ethernet media transmit and receive data using a data frame
broadcast method. The performance of a shared medium
Ethernet/802.3 LAN can be negatively affected by several
factors: - The data frame delivery of Ethernet/802.3
LANs is of a broadcast nature.
- The carrier sense
multiple access/collision detect (CSMA/CD) method allows only
one station to transmit at a time.
- Multimedia
applications with higher bandwidth demand such as video and the
Internet, coupled with the broadcast nature of Ethernet, can
create network congestion.
- Normal latency occurs as
the frames travel across the Layer 1 medium and through Layer
1, Layer 2, and Layer 3 networking devices.
-
Extending the distances and increasing latency of the
Ethernet/802.3 LANs by using Layer 1 repeaters.
Ethernet using CSMA/CD and a shared medium can support data
transmission rates of up to 100 Mbps. CSMA/CD is an access
method that allows only one station to transmit at a time. The
goal of Ethernet is to provide a best-effort delivery service
and allow all devices on the shared medium to transmit on an
equal basis. A certain number of collisions are expected in the
design of Ethernet and CSMA/CD. Therefore, collisions can
become a major problem in a CSMA/CD network.
Content
4.1 Introduction to Ethernet/802.3 LANs 4.1.4
Half-duplex networks Originally Ethernet was a
half-duplex technology. Using half-duplex, a host could either
transmit or receive at one time, but not both. Each Ethernet
host checks the network to see whether data is being
transmitted before it transmits additional data. If the network
is already in use, the transmission is delayed. Despite
transmission deferral, two or more Ethernet hosts could
transmit at the same time. This results in a collision. When a
collision occurs, the host that first detects the collision
will send out a jam signal to the other hosts. Upon receiving
the jam signal, each host will stop sending data, then wait for
a random period of time before attempting to retransmit. The
back-off algorithm generates this random delay. As more hosts
are added to the network and begin transmitting, collisions are
more likely to occur. Ethernet LANs become saturated because
users run network intensive software, such as client/server
applications, which cause hosts to transmit more often and for
longer periods of time. The network interface card (NIC), used
by LAN devices, provides several circuits so that communication
among devices can occur.
Content 4.1
Introduction to Ethernet/802.3 LANs 4.1.5 Network
congestion Advances in technology are producing faster and more
intelligent desktop computers and workstations. The combination
of more powerful workstations and network intensive
applications has created a need for greater network capacity,
or bandwidth. The requirements have exceeded the 10 Mbps
available on shared Ethernet/802.3 LANs. Today's networks are
experiencing an increase in the transmission of many forms of
media: - Large graphics files
- Images
- Full-motion video
- Multimedia applications
There is also an increase in the number of users on a
network. All these factors place an even greater strain on the
10-Mbps of available bandwidth. As more people utilize a
network to share larger files, access file servers, and connect
to the Internet, network congestion occurs. This can result in
slower response times, longer file transfers, and network users
becoming less productive. To relieve network congestion, more
bandwidth is needed or the available bandwidth must be used
more efficiently. Interactive Media Activity Drag and
Drop: Bandwidth Requirements When the student has completed
this activity, the student will be able to identify the
bandwidth requirements for different multimedia applications on
a network.
Content 4.1 Introduction to
Ethernet/802.3 LANs 4.1.6 Network latency Latency, or
delay, is the time a frame or a packet takes to travel from the
source station to the final destination. It is important to
quantify the total latency of the path between the source and
the destination for LANs and WANs. In the specific case of an
Ethernet LAN, understanding latency and its effect on network
timing is crucial to determining whether CSMA/CD for detecting
collisions and negotiating transmissions will work properly.
Latency has at least three sources: - First, there is
the time it takes the source NIC to place voltage pulses on the
wire and the time it takes the receiving NIC to interpret these
pulses. This is sometimes called NIC delay, typically around 1
microsecond for a 10BASE-T NIC.
- Second, there is the
actual propagation delay as the signal takes time to travel
along the cable. Typically, this is about 0.556 microseconds
per 100 m for Cat 5 UTP. Longer cable and slower nominal
velocity of propagation (NVP) results in more propagation
delay.
- Third, latency is added according to which
networking devices, whether they are Layer 1, Layer 2, or Layer
3, are added to the path between the two communicating
computers.
Latency does not depend solely on
distance and number of devices. For example, if three properly
configured switches separate two workstations, the workstations
may experience less latency than if two properly configured
routers separated them. This is because routers conduct more
complex and time-consuming functions. A router must analyze
Layer 3 data.
Content 4.1 Introduction to
Ethernet/802.3 LANs 4.1.7 Ethernet 10 BASE-T
transmission time All networks have what is called bit time or
slot time. Many LAN technologies, such as Ethernet, define bit
time as the basic unit of time in which ONE bit can be sent. In
order for the electronic or optical devices to recognize a
binary one or zero, there must be some minimum duration during
which the bit is on or off. Transmission time equals the number
of bits being sent times the bit time for a given technology.