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Chapter 7
Introduction to Local Area Networks
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IntroductionA local area network is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates.
Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.
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Primary Function of a LANTo provide access to hardware and software resources that will allow users to perform one or more of the following activities:
File serving - A large storage disk drive acts as a central storage repository.
For example, suppose two or more users wish to share a data set. In
this case, the data set, like the application software, would be stored
on the file server, while the network provided access to those users
who had the appropriate permissions..
Print serving - Providing the authorization to access a particular printer, accept and queue print jobs, and providing a user access to the print queue to perform administrative duties.
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Primary Function of a LAN continuedVideo transfers - High speed LANs are capable of supporting video image and live video transfers.
Manufacturing support - LANs can support manufacturing and industrial environments.
Academic support – In classrooms, labs, and wireless
E-mail support
Interconnection between multiple systems
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Advantages of Local Area NetworksAbility to share hardware and software resources.
Individual workstation might survive network failure.
Component and system evolution are possible (both evolutions can be
independent of each other).• For example, if new workstations are desired, it is possible to replace older
workstations with newer ones with few, if any, changes required to the network itself.
Support for heterogeneous forms of hardware and software.• Under some conditions, local area networks allow equipment from different
manufacturers to be mixed on the same network. For example, IBM personal computers with Apple microcomputers.
Private ownership.
Secure transfers at high speeds with low error rates.
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Access to other LANs and WANs (Figure 7-1).
Chapter Seven - Introduction to Local Area NetworksAdvantages of Local Area Networks
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Disadvantages of Local Area NetworksEquipment and support can be costly.
Level of maintenance continues to grow.
Some types of hardware may not interoperate.
Just because a LAN can support two different kinds of packages does not mean their data can interchange easily.
• For example, even if a local area network supports two different types of database systems, users may not be able to share data between the two database systems.
A LAN is only as strong as it weakest link, and there are many links.• For example, a network may suffer if the file server cannot adequately serve all
the requests from users of the network.
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Basic Network TopologiesLocal area networks are interconnected using one of four basic configurations:
1. Bus/tree
2. Star-wired bus
3. Star-wired ring
4. Wireless
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Bus/Tree TopologyThe original topology
Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap.
Data can be transferred using either baseband digital signals or broadband analog signals.
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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Bus/Tree TopologyBaseband signals are bidirectional and more outward in both directions from the workstation transmitting.
Broadband signals are usually uni-directional and transmit in only one direction. Because of this, special wiring considerations are necessary.
Buses can be split and joined, creating trees.
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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Star-wired Bus TopologyLogically operates as a bus, but physically looks like a star.Star design is based on hub. All workstations attach to a central device such as hub.
Unshielded twisted pair usually used to connect workstation to hub.Hub takes incoming signal and immediately broadcasts it out to all other workstations (or devices) connected to thehub.
Hubs can be interconnected to extend size of network.
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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Star-wired Bus TopologyModular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus.
• simple installation and maintenance, low-cost components
Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable.
Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium.
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Star-wired Ring TopologyLogically operates as a ring but physically appears as a star.
Star-wired ring topology is based on MAU (multi-station access unit) which functions similarly to a hub.
Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion.
Like hubs, MAUs can be interconnected to increase network size.
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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Wireless TopologyNot really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point.
Several versions of IEEE 802.11 standard defines various forms of wireless LAN connections.
Workstations reside within a basic service set, while multiple basic service sets create an extended service set.
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Wireless TopologyTwo basic components necessary: the client radio, usually a PC card with an integrated antenna installed in a laptop or workstation, and the access point (AP), which is an Ethernet port plus a transceiver( واالستقبال اإلرسال .( جهاز
The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions.
Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set.
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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Wireless TopologyIEEE 802.11 – The original wireless standard, capable of transmitting data at 2 Mbps
IEEE 802.11b – The second wireless standard, capable of transmitting data at 11 Mbps
In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps (from a July 2000 test by Network Computing).
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Wireless TopologyWith directional antennae designed for point-to-point transmission, 802.11b can transmit for more than 10 miles.
With an omni-directional( االتجاهات antenna on a ( متعددةtypical AP, range may drop to as little as 100 feet.
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Wireless Topology
IEEE 802.11a – One of the more recent standards, capable of transmitting data at 54 Mbps (theoretical) using the 5 GHz frequency range.
IEEE 802.11g – The other recent standard, also capable of transmitting data at 54 Mbps (theoretical) but using the same frequencies as 802.11b (2.4 GHz) and is backwards compatible with 802.11b.
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Wireless Topology
HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)
To provide security, most systems use either Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection, or a more advanced standard such as WPA ( Wi-Fi Protected Access ).
Wireless LANs may also be configured without an access point. These configurations are called “ad-hoc”.
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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Medium Access Control ProtocolsHow does a workstation get its data onto the LAN medium?
A medium access control protocol is the software that allows workstations to “take turns” at transmitting data.
Two basic categories:
1. Contention-based protocols
2. Round robin protocols
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Contention-Based Protocols - CSMA/CDEssentially first come first served.
Most common example is carrier sense multiple access with collision detection (CSMA/CD).
If no one is transmitting, a workstation can transmit.
If someone else is transmitting, the workstation “backs off” and waits.
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Contention-Based Protocols - CSMA/CDIf two workstations transmit at the same time, a collision occurs.
When the two workstations hear the collision, they stop transmitting immediately.
Each workstation backs off a random amount of time and tries again.
Hopefully, both workstations do not try again at the exact same time.
CSMA/CD is half-duplex protocol.
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Chapter Seven - Introduction to Local Area Networks
Because the number of times a workstation will have to wait is unknown, it is not possible to determine exactly when a workstation will be allowed to transmit its data without collision. Thus, CSMA/CD is a nondeterministic protocol.
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Contention-Based Protocols - CSMA/CACA stands for Collision Avoidance - protocol does not listen and detect collisions. Instead, it tries to avoid collisions before they happen
How does CSMA/CA do this?
All devices, before they transmit, must wait an amount of time called an intra-frame space (IFS)
Some applications have a short IFS, while others have a long IFS. If two applications want to transmit at the same time, the application with the shorter IFS will go first.
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Round Robin ProtocolsEach workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation.
Most common example is token ring LAN in which a software token is passed from workstation to workstation.
Token ring is an example of a deterministic protocol.
Token ring more complex than CSMA/CD. What happens if token is lost? Duplicated?
Token ring LANs are losing the battle with CSMA/CD LANs.
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Chapter Seven - Introduction to Local Area Networks
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Medium Access Control SublayerTo better support local area networks, the data link layer of the OSI model was broken into two sublayers:
1. Logical link control sublayer
2. Medium access control sublayer
Medium access control sublayer defines the frame layout and is more closely tied to a specific medium at the physical layer.
Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT.
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Chapter Seven - Introduction to Local Area Networks
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IEEE 802 Frame FormatsThe IEEE 802 suite of protocols defines the frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5).
Each frame format describes how the data package is formed.
The two frames do not have the same layout. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another.
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Chapter Seven - Introduction to Local Area Networks
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Local Area Network SystemsWired Ethernet or CSMA/CD
Most common form of LAN today.
Star-wired bus is most common topology but bus topology still not totally dead yet.
Wired Ethernet comes in many forms depending upon medium used and transmission speed and technology.
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Wired EthernetOriginally, CSMA/CD was 10 Mbps
Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps
Then 1000 Mbps (1 Gbps) was introduced
10 Gbps is now being installed in high-end applications
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Wired Ethernet1000 Mbps introduces a few interesting things:
Transmission is full duplex (separate transmit and receive), thus no collisions.
Prioritization is possible using 802.1p protocol.
Topology can be star or mesh.
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Wired EthernetOne of the latest features: Power over Ethernet (PoE)
What if you have a remote device that has an Ethernet connection? It will require a power connection.
What if you don’t have an electrical outlet nearby?
Use PoE. The power to drive the Ethernet NIC is sent over the wiring along with the usual Ethernet signals.
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Wireless EthernetAs we have already seen, IEEE has created the 802.11b, 802.11a, and 802.11g wireless standards
IEEE 802.11n (100 Mbps) will be ratified soon and should start appearing in product form in 2007 (maybe?)
Latest wireless Ethernet is using MIMO technology – multiple input multiple output – sender and receiver have multiple antennas for optimum reception
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Local Area Network SystemsIBM Token Ring
Deterministic LAN offered at speeds of 4, 16 and 100 Mbps.
Very good throughput under heavy loads.
More expensive components than CSMA/CD.
Losing ground quickly to CSMA/CD. May be extinct(تنقرض) soon.
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Local Area Network SystemsFDDI (Fiber Distributed Data Interface)
Based on the token ring design using 100 Mbps fiber connections.
Allows for two concentric ( المركز rings - inner ring (متحدcan support data travel in opposite direction or work as backup.
Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.
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LANs In Action : A Small Office SolutionWhat type of system will interconnect twenty workstations in one room and fifteen workstations in another room to a central server, which offers:
• Internal e-mail
• A database that contains all customer information
• High quality printer access
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Chapter Seven - Introduction to Local Area Networks
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Chapter Seven - Introduction to Local Area Networks
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LANs In Action : A Small Office Home Office (SOHO) LAN SolutionWhat if you have two computers at home and want both to share a printer and a connection to the Internet.
Some type of SOHO solution might solve this problem.
Essentially a LAN with a 2- or 3-port hub, connecting cables, and software.
In some models the hub also acts as a router to the Internet.
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Chapter Seven - Introduction to Local Area Networks
