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This chapter introduces the various
media-access methods, transmission methods, topologies, and devices used
in a local area network (LAN). Topics addressed focus on the methods and
devices used in Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber
Distributed Data Interface (FDDI). . Figure 2-1 illustrates the basic
layout of these three implementations.
Figure 2-1: Three LAN implementations are used most commonly.
What is a LAN?
A LAN is
a high-speed, fault-tolerant data network that covers a relatively small
geographic area. It typically connects workstations, personal computers,
printers, and other devices. LANs offer computer users many advantages,
including shared access to devices and applications, file exchange
between connected users, and communication between users via electronic
mail and other applications.
LAN Protocols and the OSI Reference
Model
LAN protocols function at the lowest two
layers of the OSI reference model, as discussed in "Internetworking
Basics," between the physical layer and the data link layer. Figure
2-2 illustrates how several popular LAN protocols map to the OSI
reference model.
Figure 2-2: Popular LAN protocols mapped to the OSI reference
model.
LAN Media-Access Methods
LAN protocols typically use one of two
methods to access the physical network medium: carrier sense
multiple access collision detect (CSMA/CD) and token passing.
In the CSMA/CD media-access scheme,
network devices contend for use of the physical network medium. CSMA/CD
is therefore sometimes called contention access. Examples of
LANs that use the CSMA/CD media-access scheme are Ethernet/IEEE 802.3
networks, including 100BaseT.
In the token-passing media-access scheme,
network devices access the physical medium based on possession of a
token. Examples of LANs that use the token-passing media-access scheme
are Token Ring/IEEE 802.5 and FDDI.
LAN Transmission Methods
LAN data transmissions fall into three
classifications: unicast, multicast, and broadcast.
In each type of transmission, a single packet is sent to one or more
nodes.
In a unicast transmission, a single
packet is sent from the source to a destination on a network. First, the
source node addresses the packet by using the address of the destination
node. The package is then sent onto the network, and finally, the
network passes the packet to its destination.
A multicast transmission consists of a
single data packet that is copied and sent to a specific subset of nodes
on the network. First, the source node addresses the packet by using a
multicast address. The packet is then sent into the network, which makes
copies of the packet and sends a copy to each node that is part of the
multicast address.
A broadcast transmission consists of a
single data packet that is copied and sent to all nodes on the network.
In these types of transmissions, the source node addresses the packet by
using the broadcast address. The packet is then sent into the network,
which makes copies of the packet and sends a copy to every node on the
network.
LAN Topologies
LAN topologies define the manner in which
network devices are organized. Four common LAN topologies exist: bus,
ring, star, and tree. These topologies are logical architectures, but
the actual devices need not be physically organized in these
configurations. Logical bus and ring topologies, for example, are
commonly organized physically as a star. A bus topology is a linear LAN
architecture in which transmissions from network stations propagate the
length of the medium and are received by all other stations. Of the
three most widely used LAN implementations, Ethernet/IEEE 802.3
networks--- , including 100BaseT---, implement a bus topology, which is
illustrated in Figure 2-3.
Figure 2-3: Some networks
implement a local bus topology.
A ring topology is a LAN architecture
that consists of a series of devices connected to one another by
unidirectional transmission links to form a single closed loop. Both
Token Ring/IEEE 802.5 and FDDI networks implement a ring topology. Figure
2-4 depicts a logical ring topology.
A star
topology is a LAN architecture in which the endpoints on a network are
connected to a common central hub, or switch, by dedicated links.
Logical bus and ring topologies are often implemented physically in a
star topology, which is illustrated in Figure 2-5.
A tree topology is a LAN architecture
that is identical to the bus topology, except that branches with
multiple nodes are possible in this case. Figure
2-5 illustrates a logical tree topology.
Figure 2-4: Some networks implement a logical
ring topology.
Figure 2-5:
A logical tree topology can contain multiple
nodes.
Devices
commonly used in LANs include repeaters, hubs, LAN
extenders, bridges, LAN switches, and routers.
Note Repeaters, hubs, and LAN
extenders are discussed briefly in this section. The function and
operation of bridges, switches, and routers are discussed generally in "Bridging
and Switching Basics," and "Routing Basics."
A repeater
is a physical layer device used to interconnect the media segments of an
extended network. A repeater essentially enables a series of cable
segments to be treated as a single cable. Repeaters receive signals from
one network segment and amplify, retime, and retransmit those signals to
another network segment. These actions prevent signal deterioration
caused by long cable lengths and large numbers of connected devices.
Repeaters are incapable of performing complex filtering and other
traffic processing. In addition, all electrical signals, including
electrical disturbances and other errors, are repeated and amplified.
The total number of repeaters and network segments that can be connected
is limited due to timing and other issues. Figure
2-6 illustrates a repeater connecting two network segments.
Figure 2-6: A
repeater connects two network segments.
A hub is a physical-layer device
that connects
multiple user stations, each via a dedicated cable. Electrical
interconnections are established inside the hub. Hubs are used to create
a physical star network while maintaining the logical bus or ring
configuration of the LAN. In some respects, a hub functions as a
multiport repeater.
A LAN extender
is a remote-access multilayer switch that connects to a host router. LAN
extenders forward traffic from all the standard network-layer protocols
(such as IP, IPX, and AppleTalk), and filter traffic based on the MAC
address or network-layer protocol type. LAN extenders scale well because
the host router filters out unwanted broadcasts and multicasts. LAN
extenders, however, are not capable of segmenting traffic or creating
security firewalls. Figure 2-7 illustrates
multiple LAN extenders connected to the host router through a WAN.
Figure 2-7: Multiple LAN extenders can connect to the host
router through a WAN.
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