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Datacommunications Tutorial
Datacommunications Terms
Asynchronous Transfer Mode (ATM): A very high speed network
utilizing SONET optical transmission methods through the public
telecommunications system.
Baseband Transmission: A transmission method where direct current
signals are placed directly onto the transmission medium (cable).
Ethernet is a baseband network type, hence, the “Base”in 10Base-T, etc.
Baud Rate: A measure of signal changes per second. Commonly used
to rate the speed of a modem.
Bridge: A networking component that links two or more network
segments. Bridges are used to split busy networks into separate, less
congested segments.
Broadband Transmission: A transmission method where multiple
channels are modulated onto separate carrier frequencies. The result is
multiple communications channels that occupy specific frequency ranges.
Bus: Also called a "Daisy Chain". A network topology where each node
is connected to one another in line. A major disadvantage is that when
there is a break in the bus the entire network goes down.
Client Server: A network architecture where multiple user
workstations (Clients) communicate with backend servers through a
network. Clients are fully operating systems that are capable of
processing data.
CODEC: An acronym for COder/DECoder. A device used for
converting analog signals to digital signals. For example, telephone
companies use codecs to convert binary signals transmitted on their
digital networks to analog signals converted on their analog networks.
Converter: A device used to convert from one transmission media to
another (Ex. Fiber/Copper Media Converter). Converters are usually
externally powered as they physically “repeat” or regenerate the signal.
CSMA: (See Basic Ethernet Theory - right).
Ethernet: (See What is Ethernet - right).
Fiber Distributed Data Interface (FDDI): A 100Mbps fiber optic
cabling standard developed by ANSI. FDDI utilizes a dual counter
rotating ring topology for network redundancy.
Hub: A network device that receives a signal from one station and
retransmits to all other connected stations.
Local Area Network (LAN): A network that remains within one facility
(department, office, building, campus).
MODEM: An acronym for MOdulator/DEModulator. Modems are
data communications devices that convert digital signals to analog
signals for transmission over analog public telephone networks.
Network Interface Card (NIC): Network devices that are installed in
computers so that they can be connected to a network. Ethernet NICs
come in different speeds as well as with connections to different media types.
Node: A device or station connected to a network.
Peer to Peer: A network architecture where computers connect directly
with other computers without the need for servers.
Redundancy: Utilizing multiple access methods so that if one goes
down the systems still operate.
Repeater: A network device that regenerates the signal to increase a
cabling run.
Router: A network device that interconnects networks. Routers provide
traffic control and filtering functions, they are commonly used to
connect a LAN to the Internet.
Server: A computing device that provides a service to users on a
network (clients). An example is a file server that stores and maintains
documents for retrieval.
Small Computer System Interface (SCSI): A peripheral interface that
is used to connect devices to a computer.
Star: The most common network topology where each node is
connected to a central point. Advantageous because if one part of the
star is lost the network remains intact.
Switch: A switch is a multiport bridge that segregates different portions
of a network for faster network access (See Basic Ethernet Theory - right).
Synchronous Optical Network (SONET): A Bellcore and ANSI
standard that defines transmission of synchronous and time sensitive
(ex: real time video) information. SONET provides a way for
worldwide carriers to connect equipment.
Token Ring: A networking standard that utilizes a ring topology.
Information is put onto the ring which is then passed (Token Passing)
to the different stations. The amount of time that a station possesses
the token is variable which gives some users priority on the network.
Token Ring was standardized by IEEE under the 802.5 standard.
Transceiver: A device used to change one media type to
another. Transceivers usually get their power from the NIC.
Wide Area Network (WAN): A network that spans a greater distance
and needs the involvement of a public carrier.
1.
Ethernet operational theory is quite easy to understand and a simple analogy is helpful to visualize the basics. Imagine a long hallway
lined with offices.The hallway represents the physical network, the offices represent the attached stations.When an occupant wishes to
speak to another occupant they would lean into the hallway,listen to make sure no one else is engaging in a conversation,then speak out
addressing the desired recipient.All other occupants hear the conversation but ignore it knowing it is not directed to them.
In essence,the above analogy describes the medium access method standardized under IEEE802.3 known as Carrier Sense Multiple Access
with Collision Detection (CSMA/CD). Carrier Sense is analogous to the ability to listen to all conversations (network traffic).Multiple Access
is the ability for multiple users to access the medium while data is transmitted.
2.
Returning to our analogy, what if two or more occupants decide to speak at the same time? Naturally the overlapping voices would
become garbled and indistinguishable.With Ethernet this is known as a collision.In the CSMA/CD method,CD stands for Collision Detection.
If a collision is detected by a transmitting station(s) the rule states:stop transmitting immediately, transmit a jamming signal to inform all
other stations to stop,then wait a random period (binary exponential backoff) and re-transmit.
Unfortunately, as the quantity of stations increases so does the amount of collisions. This causes the average access time to increase
proportionally.This is referred to in the industry as network congestion.
3.
Fortunately,there are several ways to alleviate network congestion.One way is that the entire network can be upgraded to Fast Ethernet
(100Mbps) which represents a 10 fold increase in transmission speed.This,however requires upgrading of all components and can be rather
expensive.Another approach is to add an Ethernet Switch.
In the above analogy, the long hallway represents the network.Adding a two port switch is analogous to dividing the hallway into two
shorter hallways separated by a door. This creates two segments (collision domains) where messages that are directed for occupants
within the same area are not allowed to pass through the door.Only when a message is directed to another segment does the door open
allowing the message to pass.
An Ethernet switch reduces collisions by creating multiple collision domains.This,in most cases,is the most economical approach.A switch
can be added to any network without the need to upgrade existing equipment.
1
2
3
SWITCHED
ETHERNET
NETWORK
(two collision domains):
CONGESTED
ETHERNET
NETWORK
(one collision domain):
SIMPLE ETHERNET
NETWORK
(one collision domain):
What is the difference between standard/fast Ethernet and
switched/shared equipment?
This chart explains the basic
advantages and disadvantages
to using 10 or 100Mbps
equipment in a shared (hubs)
or switched environment.
• Medium speed
• Segment bandwidth
between power users
and low volume users
• Economical
• Lowest speed
• Good for workgroups
using e-mail, small file
transfer and printing
• Most economical
• Highest speed
• Segment high bandwidth
links between power users
• Most expensive
• Medium/high speed
• Good for all around high
bandwidth applications
• More expensive
Shared Topology
(Hubs)
Switched Topology
(Switches)
10Mbps 100Mbps
Standard Ethernet Fast Ethernet
What is Ethernet?
In the early 1980s, Digital Equipment Corporation,Intel and Xerox developed the Ethernet Local Area Networking format.This technology was
soon accepted by the IEEE Committee, creating the 802.3 standard.This standard dictates the use of CSMA/CD (Carrier Sense Multiple Access with
Collision Detection) as its accessing scheme.Networks use a variety of NICs (Network Interface Cards), hubs,transceivers, converters,repeaters &
switches,as well as different types of transmission media for carrying signals.
❒ Accessing Scheme:CSMA/CD - Carrier sense multiple access with Collision Detection.
❒ Speed:10Mbps/100Mbps/1000Mbps (1 GBps).
❒ Network Architecture:Coax Bus,UTP Star, Fiber Star
10BASE-T RJ45
100BASE-TX RJ45
JACK
10BASE-2 BNC
PLUG
AUI DB15 MALE
(used on transceivers)
10BASE-FL
100BASE-FX
Dual Fiber SC
PLUG
Connectors Commonly Used in Ethernet Environments
10BASE-FL
100BASE-FX
Dual Fiber ST
PLUG
Basic Ethernet
Theory
HALLWAY
OFFICE CUBICLES
WALL WITH DOOR
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