What Is a LAN?
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects
workstations,personalcomputers,printers,servers,andotherdevices.LANsoffercomputerusersmany
advantages,includingsharedaccesstodevicesandapplications,fileexchangebetweenconnectedusers,
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 Chapter 1,
“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.
LAN Media-Access Methods
Mediacontentionoccurswhentwoormorenetworkdeviceshavedatatosendatthesametime.Because
multiple devices cannot talk on the network simultaneously, some type of method must be used to allow
one device access to the network media at a time. This is done in two main ways: carrier sense multiple
access collision detect (CSMA/CD) and token passing.
In networks using CSMA/CD technology such as Ethernet, network devices contend for the network
media. When a device has data to send, it first listens to see if any other device is currently using the
network. If not, it starts sending its data. After finishing its transmission, it listens again to see if a
collision occurred. A collision occurs when two devices send data simultaneously. When a collision
happens, each device waits a random length of time before resending its data. In most cases, a collision
will not occur again between the two devices. Because of this type of network contention, the busier a
network becomes, the more collisions occur. This is why performance of Ethernet degrades rapidly as
the number of devices on a single network increases.
In token-passing networks such as Token Ring and FDDI, a special network frame called a token is
passed around the network from device to device. When a device has data to send, it must wait until it
has the token and then sends its data. When the data transmission is complete, the token is released so
that other devices may use the network media. The main advantage of token-passing networks is that
they are deterministic. In other words, it is easy to calculate the maximum time that will pass before a
device has the opportunity to send data. This explains the popularity of token-passing networks in some
real-time environments such as factories, where machinery must be capable of communicating at a
determinable interval.
For CSMA/CD networks, switches segment the network into multiple collision domains. This reduces
the number of devices per network segment that must contend for the media. By creating smaller
collision domains, the performance of a network can be increased significantly without requiring
addressing changes.
NormallyCSMA/CDnetworksarehalf-duplex,meaningthatwhileadevicesendsinformation,itcannot
receive at the time. While that device is talking, it is incapable of also listening for other traffic. This is
much like a walkie-talkie. When one person wants to talk, he presses the transmit button and begins
speaking. While he is talking, no one else on the same frequency can talk. When the sending person is
finished, he releases the transmit button and the frequency is available to others.
When switches are introduced, full-duplex operation is possible. Full-duplex works much like a
telephone—you can listen as well as talk at the same time. When a network device is attached directly
to the port of a network switch, the two devices may be capable of operating in full-duplex mode. In
full-duplex mode, performance can be increased, but
not quite as much as some like to claim. A 100-Mbps Ethernet segment is capable of transmitting 200
Mbps of data, but only 100 Mbps can travel in one direction at a time. Because most data connections
areasymmetric(withmoredatatravelinginonedirectionthantheother),thegainisnotasgreatasmany
claim. However, full-duplex operation does increase the throughput of most applications because the
network media is no longer shared. Two devices on a full-duplex connection can send data as soon as it
is ready.
Token-passing networks such as Token Ring can also benefit from network switches. In large networks,
the delay between turns to transmit may be significant because the token is passed around the network.
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.
Inaunicasttransmission,asinglepacketissentfromthesourcetoadestinationonanetwork.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 on to 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.
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.
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.
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.
LAN Devices
Devices commonly used in LANs include repeaters, hubs, LAN extenders, bridges, LAN switches, and
routers.
A repeater is a physical layer device used to interconnect the media segments of an extended network.
Arepeateressentiallyenablesaseriesofcablesegmentstobetreatedasasinglecable.Repeatersreceive
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
connecteddevices.Repeatersareincapableofperformingcomplexfilteringandothertrafficprocessing.
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.
A hub is a physical layer device that connects multiple user stations, each via a dedicated cable.
Electricalinterconnectionsareestablishedinsidethehub.Hubsareusedtocreateaphysicalstarnetwork
whilemaintainingthelogicalbusorringconfigurationoftheLAN.Insomerespects,ahubfunctionsas
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. However, LAN extenders 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.
Introduction to LAN Protocols
