Data Communications And

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Chapter 7

Local Area Networks : The Basics

Data Communications andComputer Networks: A Business User’s Approach

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Last time

• Types of errors and their prevention

• Error detection– Parity – 50%– CRC – can detect nearly all errors

• Error correction– 1. Do nothing

– 2. Return an error message to the transmitter

– 3. Fix the error with no further help from the transmitter

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Three Major Types of Networks Local Area Network (LAN)

Serves users within a confined geographical area (usually within a mile).

Metropolitan Area Network (MAN) Covers a geographic area the size of a city or

suburb. The purpose of a MAN is often to bypass local telephone companies when accessing long-distance service.

Wide Area Network (WAN) Covers a wide geographical area, such as a state or

a country. Examples: Tyment, Telenet, Uninet, and Accunet.

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Emphasis

Definition of a Local Area Network (LAN)

• Physical and Logical LAN Topologies

• Different Medium Access Control Protocols

• Common LAN Systems– Ethernet (1st commercially available LAN)

– Token ring

– FDDI (fiber data distributed interface) ring

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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

•very low error rates.

(WANs now do this too)

Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.

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Data Communications and Computer Networks Chapter 7

Functions of a Local Area NetworkFile server - A large storage disk drive that acts as a central storage repository.

Print server - Provides the authorization to access a particular printer, accept and queue print jobs, and provides a user access to the print queue to perform administrative duties.

Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks.

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Functions of a Local Area NetworkVideo transfers - High speed LANs are capable of supporting video image and live video transfers.

Manufacturing support - LANs can support manufacturing and industrial environments.

Distributed processing - LANs can support network operating systems which perform the operations of distributed processing.

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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.

Support for heterogeneous forms of hardware and software.

Access to other LANs and WANs (Figure 7-1).

Private ownership.

Secure transfers at high speeds with low error rates.

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Disadvantages of Local Area NetworksEquipment and support can be costly.

Level of maintenance continues to grow.

Private ownership?

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.

A LAN is only as strong as it weakest link, and there are many links.

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Basic Network Topologies

Topology: the geometric configuration

Separate from protocols!

Local 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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Basic Network Topologies

Logical vs physical design (topologies)

1. Logical design: how the data moves around the network from workstation to workstation

2. Physical design: how the network physically appears if drawn on a sheet of paper (laid out)

Names of topologies usually refer to the physical design.

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Bus/Tree TopologyThe original topology – 1970’s

Workstation has a network interface card (NIC) provides a physical connection to a network

Data can be transferred using either

1. baseband digital signals

2. broadband analog signals.

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Bus/Tree TopologyWorkstation has a network interface card (NIC) provides a physical connection to a network

Attaches to the bus (a coaxial cable) via a tap.

NIC is an electronic device that performs the necessary signal conversions and protocols operations so that the workstation can send and receive data on the network.

Tap is a passive device

Does not alter the signal

Does not require electricity to operate

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Bus/Tree TopologyBaseband signals

Digital signals – 10 Mbps

Bidirectional and more outward in both directions from the workstation transmitting.

Easy to install and maintain

Fewer than 100 workstations

Buses can be split and joined, creating trees.

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Bus/Tree TopologyBroadband signals

Usually uni-directional and transmit in only one direction.

Analogy and FDM for multiple channels (amplification necessary). Because of this, special wiring considerations are necessary.

100 to 1000 workstations over larger distances due to easy amplification

Buses can be split and joined, creating trees.

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Twice the propagationdelay of baseband

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Advantages/disadvantages of a busDifficult to add new devices if no tap exists.

No tap existing means cutting into the line

As such, this topology is loosing popularity

Plenty still around; Ethernet uses this.

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Star-wired Topologies

Stars versus a single line

Two types:

1. Star-wired bus

(often call the star topology)

2. Star-wired ring

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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 hub.

Hub is an unintelligent device that immediately transmits whatever data it receives to all connections

Unshielded twisted pair usually used to connect workstation to hub.

Hub takes incoming signal and immediately broadcasts it out all connected links.

Hubs can be interconnected to extend size of network.

Very popular!

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Star-wired bus physical topology for a LAN

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Interconnection of two hubs in a star-wired bus LAN

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Star-wired Bus TopologyModular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus.

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 in one direction.

Like hubs, MAUs can be interconnected to increase network size.

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Ring topology viewed logically

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NIC Repeaters for Star-wired RingAll data must pass through a NIC repeater

Performs basic functions:

Bypass: data does not copy to the workstation, used for inactive devices

Copy: data from ring copied to workstation

Write: data from the workstation copied to the ring

Regenerates: recopies data back onto ring

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Three possible operations of the workstation repeater on a ring topology

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Physical organization of a ring topologyLooks like a star!

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Multi-station access unit on a ring topology

The MAU replaces the hub for this topology

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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.

Range varies from 50 to 800 ft with speeds of 2 to 11 Mbps

Most wireless LANs include a wired LAN backbone

Usually requires a wireless NIC

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Stanford’s wireless network

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Wireless Topology – different formsNewer IEEE 802.11 and 802.11b (Wi-Fi) standard defines various forms of wireless LAN connections.

Speeds up to 11 Mbps with 802.11b standard.

Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

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Wireless TopologyAcceptable transmission ranges broken up into areas:

1. Basic service set – that surrounding an access point

2. Extended service set – collection of basic service sets

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, and the access point (AP), which is an Ethernet port plus a transceiver.

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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Single-cell wireless LAN configuration

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Multiple-cell wireless LAN configuration

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Ad-hoc configuration for a wireless LAN

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Wireless TopologyWith directional antennae designed for point-to-point transmission (rare), 802.11b can work for more than 10 miles.

With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet.

Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.

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Wireless TopologyIn actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps

To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. (how good?)

What will Bluetooth’s impact be on 802.11b (Wi-Fi)?

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Other Wireless Standards•IEEE 802.11 (older 2 Mbps)

•IEEE 802.11b (11 Mbps, 2.4 GHz)Also called Wi-Fi (wireless fidelity)

•IEEE 802.11a (54 Mbps, 5 GHz, in 2002)

•IEEE 802.11g (54 Mbps, 2.4 GHz, in 2002)

•HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)

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Summary of topologies

• Logical vs physical topologies

• Bus and star-rings - old technologies– Still some around

• You’ll probably use 1. Star-wired bus (star)

With bus or routers (now much easier to use)

2. Wireless network (Wi-fi)

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Protocols

• Last time topologies

• Now how the data actually flows

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Medium Access Control (MAC) 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.

Since a LAN is a broadcast network, it’s imperative that only oneone workstation at a time be allowed to transmit data onto the network. (for multiple channels, one channel per workstation)

Switches have changed that, but we’ll take about that in Ch8.

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Medium Access Control ProtocolsThree basic categories:

1. Contention-based protocols (1st come; 1st serve)

2. Round robin protocols (take your turn!!)

3. Reservation protocols (requests please!)

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Contention-Based ProtocolsEssentially 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.

Half duplex protocol.

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Contention-Based ProtocolsIf 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. Workstations use a persistence algorithm to decide when to resubmit.

Hopefully, both workstations do not try again at the exact same time.

CSMA/CD is an example of a non-deterministic protocol (cannot calculate time at which a workstation will transmit)

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CollisionsCollision window is the interval during which a workstation’s signal can propagate down the bus and back. During this period a workstation might falsely hear no one transmitting and retransmit a signal.

Changes for collisions go up when every workstation wants to send something – heavy traffic!

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Collisions

• Busy CSMA/CD networks can waste over half their time dealing with collisions.

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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? Hogged?

Token ring LANs are losing the battle with CSMA/CD LANs.

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Token Passing Protocols

• Before workstation can transmit it must possess the one and only token

• Two types:– Token ring– Token bus

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Data Transmission on a token ring LAN

Station A has just released the token

Station M copies the data meant for it from BB removes the data it sent and releases the token

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Why CSMA/CD more popular than Token Ring1st LAN MAC method, more installations and equipment.

Token rings have almost always lagged behind in transmission speed.

CSMA/CD less expensive to implement because of widespread marketing and acceptance.

CSMA/CD is a simpler protocol.

Wireless CSMA/CD now exists

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Token BusJust like the token ring, but a bus instead

Each workstation maintains a list of neighbors that one passes the token to (logical not physical) neighbor

Designed primarily for manufacturing plants since a non-deterministic protocol is not acceptable there. Why?

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Reservation ProtocolsWorkstation places a reservation with central server.

Workstation cannot transmit until reservation comes up.

Under light loads, this acts similar to CSMA/CD.

Under heavy loads, this acts similar to token ring.

Powerful access method but again losing out to CSMA/CD.

Most common example of reservation protocol is demand priority protocol.

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Problems with the OSI model for LANs

Problems with broadcast networks!

• Routing (no decisions have to be made)

• Data link and physical link tightly coupled– Need to specify hardware! Ugh!

• Broadcast networks need to be have a medium access control to decide who talks

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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 (MAC) 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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Modification of OSI model to split data link layer intotwo sublayers

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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.

Note how the two frames are different. 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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Frame format for IEEE 802.3 CSMA/CD Protocol

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Frame Format for IEEE 802.5 Token Ring Protocol

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Local Area Network SystemsThe entire system with protocol and topology

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Local Area Network SystemsEthernet or CSMA/CD

Most common form of LAN today.

Star-wired bus is most common topology but bus topology also around.

Ethernet comes in many forms depending upon medium used and transmission speed and technology.

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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 available.

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Ethernet1000 Mbps introduces a few interesting wrinkles:

Transmission is full duplex (separate transmit and receive), thus no collisions.

Prioritization is possible using 802.1p protocol.

Topology can be star or mesh (for trunks).

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EthernetCabling can be either UTP (unshielded twisted pair) or optical (but 10 Gbps Ethernet may not work over UTP due to radio frequency interference).

Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queueing.

Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps limited only by media.

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Ethernet StandardsKey

• XXXBase or XXX Broad

• XXX 10 means 10Mbps

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Local Area Network SystemsIBM Token Ring

Deterministic LAN offered at speeds of 4, 16 and 100 Mbps using star-ring topology.

Very good throughput under heavy loads.

More expensive components than CSMA/CD.

Losing ground quickly to CSMA/CD. May be extinct 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 rings - inner ring can 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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FDDI dual ring topology

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Local Area Network Systems100VG-AnyLAN

Deterministic LAN based on demand priority access method.

Similar to hub topology (star design).

Two levels of priority - normal and high.

Supports a wide-variety of media types.

Losing ground quickly to CSMA/CD. Will be extinct soon?

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LANs In Action : A Small Office SolutionWhat type of system will interconnect twenty workstations in one room and 15 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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LANs In Action : A Home Office Local Area Network SolutionWhat if you have two computers at home and want both to share a printer and a connection to the Internet.

Some type of “network-in-a-box” 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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What we covered

• Topologies– Star-bus– Wireless

• Protocols– CSMA/CD

• Systems– Ethernet

Next time: wireless networks?

Data Communications And

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