TechNote: LAN Technologies
Ethernet was developed by DIX (Digital, Intel and
Xerox) in the 1970s. In 1980 the IEEE 802.3 standard was released.
Two years later version 2 was introduced, which is the basis
for today's Ethernet networks. The access method (how the
wire is accessed) is Carrier Sense Multiple Access/Collision
Detection (CSMA/CD). In a CSMA/CD network stations listen
to check if the network is busy, if the network is free the
station transmits data. When two stations listen, and both
determine the network is available, they will start sending
the data simultaneously and a collision occurs. When
the collision is detected both stations will retransmit the
data after a random wait time created by a backoff algorithm.
In today's large-fast-growing-bandwidth-eating network environments
this will soon become a problem, stations will have to wait
more often before they can transmit data and more collisions
will occur. The solution to this is to separate the network
in multiple collisions domains, which devices can be used
for this purpose will be explained using a network diagram
for each of the following relevant network components.
An Ethernet network is a broadcast system, this means that
when a station transmits data every other station receives
the data. The frames contain an address in the frame header,
only the station with that address will pick up the frame
and pass it on to upper-layer protocols to be processed.
All devices in this domain will receive broadcast frames originating
from any other device within the domain. Broadcast domains are
typically bounded by routers because routers do not forward
broadcast frames. Broadcast frames are frames explicitly directed
to all nodes on the LAN, as networks grow this will become a
problem as well.
A repeaters is a simple device that is used to expand LANs
over larger distances by connecting segments. They do not
control broadcast or collision domains, they are not aware
of upper-layer protocols and frame formats, they merely regenerate/amplify
Repeaters operate at the Physical layer of the OSI model.
An important rule when using repeaters to expand a network
is the 5-4-3 rule, which defines that the maximum
distance between two hosts on the same network can be 5 segments,
4 repeaters, and only 3 of the segments can be populated,
as illustrated in the following logical network diagram:
Hubs, also known as concentrators or multiport repeaters,
are used in star/hierarchical networks to connect multiple
stations/cable segments. There are two main types of hubs: passive and active. An active hub takes
the incoming frames, amplifies the signal, and forwards it
to all other ports, a passive hub simply splits the signal
and forwards it. Another type of hubs can be managed allowing
individual port configuration and traffic monitoring, these
are know as intelligent- or managed hubs.
Hubs operate on the physical layer of the OSI model and they
are protocol transparent, that means they are not
aware of the upper-layer protocols and such as IP, IPX nor
MAC addressing. Hence they do not control broadcast or collision
domains, but they extend them as illustrated below:
Bridges are more intelligent than hubs; they operate on the
Data Link layer of the OSI model.
They are used to increase network performance by segmenting networks in separate collision domains. Bridges are also protocol
transparent, they are not aware of the upper-layer protocols.
They keep a table with MAC addresses of all nodes, and on
which segment they are located.
A bridge takes an incoming frame, reads its destination MAC
address and consults the database to decide what should be
done with the frame; if the location of the destination MAC
address is listed in the database, the frame is forwarded
to the corresponding port. If the destination port is the
same as the port where the frame arrived it will be discarded.
If the location is not known the frame will be flooded through all outgoing ports/segments.
As illustrated below, bridges control collision domains, they
do not control broadcast domains:
To improve network performance even more switches were developed,
switches are very similar to bridges; they also keep a table
with MAC addresses per port to make switching decisions, operate
in the OSI model and are protocol transparent.
Some of the main differences are:
- a switch has more ports than a bridge
- bridges switch in software whereas switches switch in hardware
- switches offer more variance in speed, an individual port
can be assigned 10 Mb/s or 100 Mb/s or even more.
As illustrated below, switches control collision domains,
they do not control broadcast domains*:
* Do not control broadcast domains unless Virtual Local Area
Networks (VLANs) are being used, and most modern switches
do support VLANs. The following diagram represents a router
configured with two VLANs. Like in the previous diagram each
port forms an collision domain, but as you can see in this
diagram the network is separated in two broadcast domains
using VLANs. If the network protocol used in this network
would be TCP/IP the VLANs would each have its own (sub-)network
address, for example VLAN 1 could be Class C 192.168.110.x
and VLAN 2 192.168.220.x.
Switches are able to use software to create Virtual LANs;
a logical grouping of network devices where the members can
be on different physical segments. A VLAN can be based on
Port IDs, MAC addresses, protocols or applications. For example
in the network diagram above port 1 to 12 on the switch could
be assigned to VLAN 1, and port 13 to 24 to VLAN 2, resulting
in two different broadcast domains, or station 1, 2 and 3
could be using IPX/SPX while station 4, 5 and 6 could be using
An example of a large network with VLANs could be an office
building with a switch on each of the three floors and a main
switch connecting them all together. An administrator would
be able to keep a list of MAC addresses and assign stations
from different floors to a single VLAN and for example create
a VLAN (broadcast domain) for each department in the company.
Switches share their MAC address table information with other
switches so the path to a destination can be found quickly.
Routers are used to interconnect multiple (sub-)networks and
route information between these networks by choosing an optimal
path ("route") to the destination. They operate on
the Network layer (Layer 3) of the OSI model and in contradiction
to hubs, bridges and switches, routers are protocol-aware. Examples
of these protocols are: IP, IPX, and AppleTalk. Routers make
forwarding decisions based on a table with network addresses
and there corresponding ports, this table is known as the route
table. Common use of routers is to connect two different
type of networks (for example Ethernet and Token ring) or to
interconnect LANs into a WAN. The concept of routing will be
covered in more detail in the Routing Protocols TechNote.
As illustrated below, routers control collision domains AND
A gateway (as a network component) is a device that connects
networks with dissimilar network protocols or architectures
and translates between the networks. Gateways are very intelligent
devices, generally they operate on the Transport layer and
on those above it (Session, Presentation, Application). A
gateway could be used to allow IPX/SPX clients to use a gateway
with a TCP/IP uplink to an internet connection. TCP/IP would
be converted to IPX/SPX. Another common use of a gateway is
to connect an Ethernet network to an IBM SNA mainframe environment.
A NIC (Network Interface Card) is an expansion cards for a
computer used to connect a to the physical network. The NIC's
interface itself is defined at the Physical layer (Layer 1)
of the OSI model, the physical address (also known
as Burned-In Address and commonly: MAC address) of the adapter
as well as the drivers to control the NIC are located at the
Data Link layer's MAC sub-layer. The reason the physical
address is defined at the Data Link layer is that the
Physical layer only handles bits.
Half-duplex means that only one host can communicate at a
given time, two hosts communicating with each other will take
turns transmitting. This is the default on non-switched LANs.
In full-duplex communication both hosts can transmit at the
same time, theoretical allowing twice as much data to be transmitted
over the same connection.
In order for full-duplex to work, some requirements must be
- The NICs, hubs etc. must support it,
- Collision Detection and Loopback functions must be disabled.
In reality the connections able to run at full-duplex are
cross-cable connections and connection to a port on a switch,
where collisions cannot occur because each end has it's own
wire pair (segment).
exam objectives for the CCNA exam:
Determine the appropriate uses for full- and half-duplex
Describe the causes and effects of network congestion in Ethernet
Describe the benefits of network segmentation with various networking
Identify the cause(s) of LAN connectivity problem
Describe the function, operation, and primary components on
here for the complete list of exam objectives.
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