12-12-2012, 05:10 PM
Introduction to LAN Protocols
LAN Protocols.pdf (Size: 63.6 KB / Downloads: 39)
Introduction
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). Subsequent
chapters in Part II, “LAN Protocols,” address specific protocols in more detail. Figure 2-1 illustrates the
basic layout of these three implementations.
What Is a LAN?
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects
workstations, personal computers, printers, servers, 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 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
Media contention occurs when two or more network devices have data to send at the same time. 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 packet 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.
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 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.