LAN Protocols
This page introduces the various media-access methods, transmission methods,
topologies, and devices used in a local area network (LAN); to include methods
and devices used in Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber
Distributed Data Interface (FDDI).
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
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.
- A multicast transmission consists of a single data packet that is
copied and sent to a specific subset of nodes on the network.
- A broadcast transmission consists of a single data packet that is
copied and sent to all nodes 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.
- 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 tree topology is a LAN architecture that is identical to the bus
topology, except that branches with multiple nodes are possible in this case.
- 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.
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. 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.
- 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.
- Bridges analyze incoming frames, make forwarding decisions based on
information contained in the frames, and forward the frames toward the
destination. In some cases, such as source-route bridging, the entire path to
the destination is contained in each frame. In other cases, such as transparent
bridging, frames are forwarded one hop at a time toward the destination.
- Switches are data link layer devices that, like bridges, enable
multiple physical LAN segments to be interconnected into a single larger
network. Similar to bridges, switches forward and flood traffic based on MAC
addresses. Because switching is performed in hardware instead of in software,
however, it is significantly faster. Switches use either store-and-forward
switching or cut-through switching when forwarding traffic. Many types of
switches exist, including ATM switches, LAN switches, and various types of WAN
switches.
- Routers perform two basic activities: determining optimal routing
paths and transporting information groups (typically called packets) through an
internetwork. In the context of the routing process, the latter of these is
referred to as switching. Although switching is relatively straightforward, path
determination can be very complex
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