Ethernet Part I: Networking Basics
Computer networking has become an integral part of business today.
Individuals, professionals and academics have also learned to rely on computer
networks for capabilities such as electronic mail and access to remote
databases for research and communication purposes. Networking has thus become
an increasingly pervasive, worldwide reality because it is fast, efficient,
reliable and effective. Just how all this information is transmitted, stored,
categorized and accessed remains a mystery to the average computer user.
This tutorial will explain the basics of some of the most popular
technologies used in networking, and will include the following:
Types of LAN Technology – including
Ethernet, Fast Ethernet, Gigabit Ethernet, 10 Gigabit Ethernet,
ATM, PoE and Token Ring
Ethernet Products – including transceivers,
network interface cards, hubs and repeaters
Types of Networks
In describing the basics of networking technology, it will be helpful to
explain the different types of networks in use.
Local Area Networks (LANs)
A network is any collection of independent computers that exchange
information with each other over a shared communication medium. Local Area
Networks or LANs are usually confined to a limited geographic area, such as a
single building or a college campus. LANs can be small, linking as few as three
computers, but can often link hundreds of computers used by thousands of
people. The development of standard networking protocols and media has resulted
in worldwide proliferation of LANs throughout business and educational
organizations.
Wide Area Networks (WANs)
Often elements of a network are widely separated physically. Wide area
networking combines multiple LANs that are geographically separate. This is
accomplished by connecting the several LANs with dedicated leased lines such as
a T1 or a T3, by dial-up phone lines (both synchronous and asynchronous), by
satellite links and by data packet carrier services. WANs can be as simple as a
modem and a remote access server for employees to dial into, or it can be as
complex as hundreds of branch offices globally linked. Special routing
protocols and filters minimize the expense of sending data over vast distances.
Wireless Local Area Networks (WLANs)
Wireless LANs, or WLANs, use radio frequency (RF) technology to transmit
and receive data over the air. This minimizes the need for wired connections.
WLANs give users mobility as they allow connection to a local area network
without having to be physically connected by a cable. This freedom means users
can access shared resources without looking for a place to plug in cables,
provided that their terminals are mobile and within the designated network coverage
area. With mobility, WLANs give flexibility and increased productivity,
appealing to both entrepreneurs and to home users. WLANs may also enable
network administrators to connect devices that may be physically difficult to
reach with a cable.
The Institute for
Electrical and Electronic Engineers (IEEE) developed the 802.11 specification
for wireless LAN technology. 802.11 specifies over-the-air interface between a
wireless client and a base station, or between two wireless clients. WLAN
802.11 standards also have security protocols that were developed to provide
the same level of security as that of a wired LAN.
The first of these protocols is Wired Equivalent Privacy (WEP). WEP provides
security by encrypting data sent over radio waves from end point to end point.
The second WLAN security protocol is Wi-Fi Protected Access (WPA). WPA
was developed as an upgrade to the security features of WEP. It works with
existing products that are WEP-enabled but provides two key improvements:
improved data encryption through the temporal key integrity protocol (TKIP)
which scrambles the keys using a hashing algorithm. It has means for
integrity-checking to ensure that keys have not been tampered with. WPA also
provides user authentication with the extensible authentication protocol (EAP).
Wireless Protocols
Specification
|
Data Rate
|
Modulation Scheme
|
Security
|
802.11
|
1 or 2 Mbps in the 2.4 GHz band
|
FHSS, DSSS
|
WEP & WPA
|
802.11a
|
54 Mbps in the 5 GHz band
|
OFDM
|
WEP & WPA
|
802.11b/High Rate/Wi-Fi
|
11 Mbps (with a fallback to
5.5, 2, and 1 Mbps) in the 2.4 GHz band
|
DSSS with CCK
|
WEP & WPA
|
802.11g/Wi-Fi
|
54 Mbps in the 2.4 GHz band
|
OFDM when above 20Mbps, DSSS
with CCK when below 20Mbps
|
WEP & WPA
|
More than just a technology, the Internet has become a way of life for
many people, and it has spurred a revolution of sorts for both public and
private sharing of information. The most popular source of information about
almost anything, the Internet is used daily by technical and non-technical
users alike.
The Internet: The Largest Network of All
With the meteoric rise in demand for connectivity, the Internet has
become a major communications highway for millions of users. It is a
decentralized system of linked networks that are worldwide in scope. It
facilitates data communication services such as remote log-in, file transfer,
electronic mail, the World Wide Web and newsgroups. It consists of independent
hosts of computers that can designate which Internet services to use and which
of their local services to make available to the global community.
Initially restricted to military and academic institutions, the Internet
now operates on a three-level hierarchy composed of backbone networks,
mid-level networks and stub networks. It is a full-fledged conduit for any and
all forms of information and commerce. Internet websites now provide personal,
educational, political and economic resources to virtually any point on the
planet.
Intranet: A Secure Internet-like Network for Organizations
With advancements in browser-based software for the Internet, many
private organizations have implemented intranets. An intranet is a
private network utilizing Internet-type tools, but available only within that
organization. For large organizations, an intranet provides easy access to
corporate information for designated employees.
Extranet: A Secure Means for Sharing Information with Partners
While an intranet is used to disseminate confidential information within a
corporation, an extranet is commonly used by companies to share
data in a secure fashion with their business partners. Internet-type tools are
used by content providers to update the extranet. Encryption and user
authentication means are provided to protect the information, and to ensure
that designated people with the proper access privileges are allowed to view
it.
Types of LAN Technology
Ethernet
Ethernet is the most popular physical layer LAN technology in use today.
It defines the number of conductors that are required for a connection, the
performance thresholds that can be expected, and provides the framework for
data transmission. A standard Ethernet network can transmit data at a rate up
to 10 Megabits per second (10 Mbps). Other LAN types include Token Ring, Fast
Ethernet, Gigabit Ethernet, 10 Gigabit Ethernet, Fiber Distributed Data
Interface (FDDI), Asynchronous Transfer Mode (ATM) and Local Talk.
Ethernet is popular because it strikes a good balance between speed,
cost and ease of installation. These benefits, combined with wide acceptance in
the computer marketplace and the ability to support virtually all popular
network protocols, make Ethernet an ideal networking technology for most
computer users today.
The Institute for Electrical and Electronic Engineers developed an
Ethernet standard known as IEEE Standard 802.3. This standard defines rules for
configuring an Ethernet network and also specifies how the elements in an
Ethernet network interact with one another. By adhering to the IEEE standard,
network equipment and network protocols can communicate efficiently.
Fast Ethernet
The Fast Ethernet standard (IEEE 802.3u) has been established for
Ethernet networks that need higher transmission speeds. This standard raises
the Ethernet speed limit from 10 Mbps to 100 Mbps with only minimal changes to
the existing cable structure. Fast Ethernet provides faster throughput for
video, multimedia, graphics, Internet surfing and stronger error detection and
correction.
There are three types of Fast Ethernet: 100BASE-TX for use with level 5
UTP cable; 100BASE-FX for use with fiber-optic cable; and 100BASE-T4 which
utilizes an extra two wires for use with level 3 UTP cable. The 100BASE-TX
standard has become the most popular due to its close compatibility with the
10BASE-T Ethernet standard.
Network managers who want to incorporate Fast Ethernet into an existing
configuration are required to make many decisions. The number of users in each
site on the network that need the higher throughput must be determined; which
segments of the backbone need to be reconfigured specifically for 100BASE-T;
plus what hardware is necessary in order to connect the 100BASE-T segments with
existing 10BASE-T segments. Gigabit Ethernet is a future technology that
promises a migration path beyond Fast Ethernet so the next generation of
networks will support even higher data transfer speeds.
Gigabit Ethernet
Gigabit Ethernet was developed to meet the need for faster communication
networks with applications such as multimedia and Voice over IP (VoIP). Also
known as “gigabit-Ethernet-over-copper” or 1000Base-T, GigE is a version of
Ethernet that runs at speeds 10 times faster than 100Base-T. It is defined in
the IEEE 802.3 standard and is currently used as an enterprise backbone.
Existing Ethernet LANs with 10 and 100 Mbps cards can feed into a Gigabit
Ethernet backbone to interconnect high performance switches, routers and servers.
From the data link layer of the OSI model upward, the look and
implementation of Gigabit Ethernet is identical to that of Ethernet. The most
important differences between Gigabit Ethernet and Fast Ethernet include the
additional support of full duplex operation in the MAC layer and the data
rates.
10 Gigabit Ethernet
10 Gigabit Ethernet is the fastest and most recent of the Ethernet
standards. IEEE 802.3ae defines a version of Ethernet with a nominal rate of
10Gbits/s that makes it 10 times faster than Gigabit Ethernet.
Unlike other Ethernet systems, 10 Gigabit Ethernet is based entirely on
the use of optical fiber connections. This developing standard is moving away
from a LAN design that broadcasts to all nodes, toward a system which includes
some elements of wide area routing. As it is still very new, which of the
standards will gain commercial acceptance has yet to be determined.
Asynchronous Transfer Mode (ATM)
ATM is a cell-based fast-packet communication technique that can support
data-transfer rates from sub-T1 speeds to 10 Gbps. ATM achieves its high speeds
in part by transmitting data in fixed-size cells and dispensing with
error-correction protocols. It relies on the inherent integrity of digital
lines to ensure data integrity.
ATM can be integrated into an existing network as needed without having
to update the entire network. Its fixed-length cell-relay operation is the
signaling technology of the future and offers more predictable performance than
variable length frames. Networks are extremely versatile and an ATM network can
connect points in a building, or across the country, and still be treated as a
single network.
Power over Ethernet (PoE)
PoE is a solution in which an electrical current is run to networking
hardware over the Ethernet Category 5 cable or higher. This solution does not
require an extra AC power cord at the product location. This minimizes the
amount of cable needed as well as eliminates the difficulties and cost of
installing extra outlets.
LAN Technology Specifications
Name
|
IEEE Standard
|
Data Rate
|
Media Type
|
Maximum Distance
|
Ethernet
|
802.3
|
10 Mbps
|
10Base-T
|
100 meters
|
Fast Ethernet/
100Base-T
|
|
100 Mbps
|
100Base-TX
100Base-FX
|
100 meters
2000 meters
|
Gigabit Ethernet/
GigE
|
802.3z
|
1000 Mbps
|
1000Base-T
1000Base-SX
1000Base-LX
|
100 meters
275/550 meters
550/5000 meters
|
10 Gigabit Ethernet
|
|
10 Gbps
|
10GBase-SR
10GBase-LX4
10GBase-LR/ER
10GBase-SW/LW/EW
|
300 meters
300m MMF/ 10km SMF
10km/40km
300m/10km/40km
|
Token Ring
Token Ring is another form of network configuration. It differs from
Ethernet in that all messages are transferred in one direction along the ring
at all times. Token Ring networks sequentially pass a “token” to each connected
device. When the token arrives at a particular computer (or device), the
recipient is allowed to transmit data onto the network. Since only one device
may be transmitting at any given time, no data collisions occur. Access to the
network is guaranteed, and time-sensitive applications can be supported.
However, these benefits come at a price. Component costs are usually higher,
and the networks themselves are considered to be more complex and difficult to
implement. Various PC vendors have been proponents of Token Ring networks.
Networking and Ethernet Basics
Protocols
After a physical connection has been established, network protocols
define the standards that allow computers to communicate. A protocol
establishes the rules and encoding specifications for sending data. This
defines how computers identify one another on a network, the form that the data
should take in transit, and how this information is processed once it reaches
its final destination. Protocols also define procedures for determining the
type of error checking that will be used, the data compression method, if one
is needed, how the sending device will indicate that it has finished sending a
message, how the receiving device will indicate that it has received a message,
and the handling of lost or damaged transmissions or “packets”.
The main types of network protocols in use today are: TCP/IP (for UNIX,
Windows NT, Windows 95 and other platforms); IPX (for Novell NetWare); DECnet
(for networking Digital Equipment Corp. computers); AppleTalk (for Macintosh
computers), and NetBIOS/NetBEUI (for LAN Manager and Windows NT networks).
Although each network protocol is different, they all share the same
physical cabling. This common method of accessing the physical network allows
multiple protocols to peacefully coexist over the network media, and allows the
builder of a network to use common hardware for a variety of protocols. This
concept is known as “protocol independence,” which means that devices which are
compatible at the physical and data link layers allow the user to run many
different protocols over the same medium.
The Open System Interconnection Model
The Open System Interconnection (OSI) model specifies how dissimilar
computing devices such as Network Interface Cards (NICs), bridges and routers
exchange data over a network by offering a networking framework for
implementing protocols in seven layers. Beginning at the application layer,
control is passed from one layer to the next. The following describes the seven
layers as defined by the OSI model, shown in the order they occur whenever a
user transmits information.
Layer 7: Application
This layer supports the application and
end-user processes. Within this layer, user privacy is considered and
communication partners, service and constraints are all identified. File
transfers, email, Telnet and FTP applications are all provided within this
layer.
Layer 6: Presentation (Syntax)
Within this layer, information is
translated back and forth between application and network formats. This
translation transforms the information into data the application layer and
network recognize regardless of encryption and formatting.
Layer 5: Session
Within this layer, connections between
applications are made, managed and terminated as needed to allow for data
exchanges between applications at each end of a dialogue.
Layer 4: Transport
Complete data transfer is ensured as information
is transferred transparently between systems in this layer. The transport layer
also assures appropriate flow control and end-to-end error recovery.
Layer 3: Network
Using switching and routing
technologies, this layer is responsible for creating virtual circuits to
transmit information from node to node. Other functions include routing,
forwarding, addressing, internet working, error and congestion control, and
packet sequencing.
Layer 2: Data Link
Information in data packets are encoded
and decoded into bits within this layer. Errors from the physical layer flow
control and frame synchronization are corrected here utilizing transmission
protocol knowledge and management. This layer consists of two sub layers: the
Media Access Control (MAC) layer, which controls the way networked computers
gain access to data and transmit it, and the Logical Link Control (LLC) layer,
which controls frame synchronization, flow control and error checking.
Layer 1: Physical
This layer enables hardware to send and
receive data over a carrier such as cabling, a card or other physical means. It
conveys the bitstream through the network at the electrical and mechanical
level. Fast Ethernet, RS232, and ATM are all protocols with physical layer
components.
This order is then reversed as information is received, so that the
physical layer is the first and application layer is the final layer that
information passes through.
Standard Ethernet Code
In order to understand standard Ethernet code, one must understand what
each digit means. Following is a guide:
Guide to Ethernet Coding
10
|
at the beginning means the
network operates at 10Mbps.
|
BASE
|
means the type of signalling
used is baseband.
|
2 or 5
|
at the end indicates the
maximum cable length in meters.
|
T
|
the end stands for twisted-pair
cable.
|
X
|
at the end stands for full
duplex-capable cable.
|
FL
|
at the end stands for fiber
optic cable.
|
For example: 100BASE-TX indicates a Fast Ethernet
connection (100 Mbps) that uses a
twisted pair cable capable of full-duplex transmissions.
Media
An important part of designing and installing an Ethernet is selecting
the appropriate Ethernet medium. There are four major types of media in use
today: Thick wire for 10BASE5 networks; thin coax for 10BASE2 networks;
unshielded twisted pair (UTP) for 10BASE-T networks; and fiber optic for
10BASE-FL or Fiber-Optic Inter-Repeater Link (FOIRL) networks. This wide
variety of media reflects the evolution of Ethernet and also points to the
technology’s flexibility. Thick wire was one of the first cabling systems used
in Ethernet, but it was expensive and difficult to use. This evolved to thin
coax, which is easier to work with and less expensive. It is important to note
that each type of Ethernet, Fast Ethernet, Gigabit Ethernet, 10 Gigabit
Ethernet, has its own preferred media types.
The most popular wiring schemes are 10BASE-T and 100BASE-TX, which use
unshielded twisted pair (UTP) cable. This is similar to telephone cable and
comes in a variety of grades, with each higher grade offering better
performance. Level 5 cable is the highest, most expensive grade, offering
support for transmission rates of up to 100 Mbps. Level 4 and level 3 cable are
less expensive, but cannot support the same data throughput speeds; level 4
cable can support speeds of up to 20 Mbps; level 3 up to 16 Mbps. The
100BASE-T4 standard allows for support of 100 Mbps Ethernet over level 3
cables, but at the expense of adding another pair of wires (4 pair instead of
the 2 pair used for 10BASE-T). For most users, this is an awkward scheme and
therefore 100BASE-T4 has seen little popularity. Level 2 and level 1 cables are
not used in the design of 10BASE-T networks.
For specialized applications, fiber-optic, or 10BASE-FL, Ethernet
segments are popular. Fiber-optic cable is more expensive, but it is invaluable
in situations where electronic emissions and environmental hazards are a
concern. Fiber-optic cable is often used in inter-building applications to
insulate networking equipment from electrical damage caused by lightning.
Because it does not conduct electricity, fiber-optic cable can also be useful
in areas where heavy electromagnetic interference is present, such as on a
factory floor. The Ethernet standard allows for fiber-optic cable segments up
to two kilometres long, making fiber-optic Ethernet perfect for connecting
nodes and buildings that are otherwise not reachable with copper media.
Topologies
Network topology is the geometric arrangement of nodes and cable links
in a LAN. Two general configurations are used, bus and star. These two
topologies define how nodes are connected to one another in a communication
network. A node is an active device connected to the network, such as a
computer or a printer. A node can also be a piece of networking equipment such
as a hub, switch or a router.
A bus topology consists of nodes linked together in a series with each
node connected to a long cable or bus. Many nodes can tap into the bus and
begin communication with all other nodes on that cable segment. A break
anywhere in the cable will usually cause the entire segment to be inoperable
until the break is repaired. Examples of bus topology include 10BASE2 and 10BASE5.
General Topology Configurations
10BASE-T Ethernet and Fast Ethernet use a star topology where access is
controlled by a central computer. Generally a computer is located at one end of
the segment, and the other end is terminated in central location with a hub or
a switch. Because UTP is often run in conjunction with telephone cabling, this
central location can be a telephone closet or other area where it is convenient
to connect the UTP segment to a backbone. The primary advantage of this type of
network is reliability, for if one of these ‘point-to-point’ segments has a
break; it will only affect the two nodes on that link. Other computer users on
the network continue to operate as if that segment were non-existent.
Collisions
Ethernet is a shared medium, so there are rules for sending packets of
data to avoid conflicts and to protect data integrity. Nodes determine when the
network is available for sending packets. It is possible that two or more nodes
at different locations will attempt to send data at the same time. When this
happens, a packet collision occurs.
Minimizing collisions is a crucial element in the design and operation
of networks. Increased collisions are often the result of too many users on the
network. This leads to competition for network bandwidth and can slow the
performance of the network from the user’s point of view. Segmenting the
network is one way of reducing an overcrowded network, i.e., by dividing it
into different pieces logically joined together with a bridge or switch.
CSMA/CD
In order to manage collisions Ethernet uses a protocol called Carrier
Sense Multiple Access/Collision Detection (CSMA/CD). CSMA/CD is a type of
contention protocol that defines how to respond when a collision is detected,
or when two devices attempt to transmit packages simultaneously. Ethernet
allows each device to send messages at any time without having to wait for
network permission; thus, there is a high possibility that devices may try to
send messages at the same time.
After detecting a collision, each device that was transmitting a packet
delays a random amount of time before re-transmitting the packet. If another
collision occurs, the device waits twice as long before trying to re-transmit.
Ethernet Products
The standards and technology just discussed will help define the
specific products that network managers use to build Ethernet networks. The
following presents the key products needed to build an Ethernet LAN.
Transceivers
Transceivers are also referred to as Medium Access Units (MAUs). They
are used to connect nodes to the various Ethernet media. Most computers and
network interface cards contain a built-in 10BASE-T or 10BASE2 transceiver
which allows them to be connected directly to Ethernet without the need for an
external transceiver.
Many Ethernet devices provide an attachment unit interface (AUI)
connector to allow the user to connect to any type of medium via an external
transceiver. The AUI connector consists of a 15-pin D-shell type connector,
female on the computer side, male on the transceiver side.
For Fast Ethernet networks, a new interface called the MII (Media
Independent Interface) was developed to offer a flexible way to support 100
Mbps connections. The MII is a popular way to connect 100BASE-FX links to
copper-based Fast Ethernet devices.
Network Interface Cards
Network Interface Cards, commonly referred to as NICs, are used to
connect a PC to a network. The NIC provides a physical connection between the
networking cable and the computer’s internal bus. Different computers have
different bus architectures. PCI bus slots are most commonly found on
486/Pentium PCs and ISA expansion slots are commonly found on 386 and older
PCs. NICs come in three basic varieties: 8-bit, 16-bit, and 32-bit. The larger
the number of bits that can be transferred to the NIC, the faster the NIC can
transfer data to the network cable. Most NICs are designed for a particular
type of network, protocol, and medium, though some can serve multiple networks.
Many NIC adapters comply with plug-and-play specifications. On these
systems, NICs are automatically configured without user intervention, while on
non-plug-and-play systems, configuration is done manually through a set-up
program and/or DIP switches.
Cards are available to support almost all networking standards. Fast
Ethernet NICs are often 10/100 capable, and will automatically set to the
appropriate speed. Gigabit Ethernet NICs are 10/100/1000 capable with auto
negotiation depending on the user’s Ethernet speed. Full duplex networking is
another option where a dedicated connection to a switch allows a NIC to operate
at twice the speed.
Hubs/Repeaters
Hubs/repeaters are used to connect together two or more Ethernet
segments of any type of medium. In larger designs, signal quality begins to
deteriorate as segments exceed their maximum length. Hubs provide the signal
amplification required to allow a segment to be extended a greater distance. A
hub repeats any incoming signal to all ports.
Ethernet hubs are necessary in star topologies such as 10BASE-T. A
multi-port twisted pair hub allows several point-to-point segments to be joined
into one network. One end of the point-to-point link is attached to the hub and
the other is attached to the computer. If the hub is attached to a backbone,
then all computers at the end of the twisted pair segments can communicate with
all the hosts on the backbone. The number and type of hubs in any one-collision
domain is limited by the Ethernet rules. These repeater rules are discussed in
more detail later.
A very important fact to note about hubs is that they only allow users
to share Ethernet. A network of hubs/repeaters is termed a “shared Ethernet,”
meaning that all members of the network are contending for transmission of data
onto a single network (collision domain). A hub/repeater propagates all
electrical signals including the invalid ones. Therefore, if a collision or
electrical interference occurs on one segment, repeaters make it appear on all
others as well. This means that individual members of a shared network will
only get a percentage of the available network bandwidth.
Basically, the number and type of hubs in any one collision domain for
10Mbps Ethernet is limited by the following rules:
Network Type
|
Max Nodes Per Segment
|
Max Distance Per Segment
|
10BASE-T
|
2
|
100m
|
10BASE-FL
|
2
|
2000m
|
