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Chapter 5 LOCAL AREA NETWORK CONCEPTS AND ARCHITECTURES. Objectives. Introduce LAN Study OSI model Look at LAN Media Investigate LAN Architecture and Components Study standard LAN Architectures. What is a Local Area Network?.

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objectives
Objectives
  • Introduce LAN
  • Study OSI model
  • Look at LAN Media
  • Investigate LAN Architecture and Components
  • Study standard LAN Architectures
what is a local area network
What is a Local Area Network?
  • LAN is a combination of hardware & software tech. that allows computers to share a variety of resources e.g. printers, storage devices, Data, Applications, etc.
  • It allows messages to be sent between attached computers  Enable users to work together electronically = “Collaborative computing”
what is a local area network1
What is a Local Area Network?
  • Generally, LANs are confined to an area no larger than a single building or a small group of buildings
  • It can be extended by connecting to other similar or dissimilar LANs, to remote users, or to mainframes computers = “LAN Connectivity” or “Internetworking”
  • Can be connected to other LANs of trading partners = “Enterprise Networking”
  • The computers themselves are not part of the LAN !!!
categorizing lan architecture osi model
Categorizing LAN Architecture:OSI Model
  • Consists of 7 layers that loosely group the functional requirements for communication between two computing devices.
  • Each layer relies on lower layers to perform more elementary functions and to offer total transparency to the intricacies of those functions. At the same time, each layer provides the same transparent service to upper layers.
osi model
OSI Model
  • Physical Layer: responsible for the establishment, maintenance, & termination of physical connection between communicating devices “Point-to-Point data link”.
  • Data-Link Layer: responsible for the providing protocols that deliver reliability to upper layers for Point-to-Point connections established by physical layer protocols. To allow the OSI model to closely adhere to the protocol structure, & operation of a LAN, Data-Link layer was splitted into two sublayers.
data link sublayers
Data-Link Sublayers
  • Media Access Control (MAC): interfaces with the physical layer & is represented by protocols that define how the shared LAN media is to be accessed by the many connected computers.
  • Logical Link Control (LLC): interfaces to the network layer.
  • The advantage of splitting the Data-Link layer & of having a single common LLC protocol is that it offers transparency to the upper layers while allowing the MAC sublayers protocols to vary independently.
osi model cont d
OSI Model cont’d
  • Network Layer: responsible for the establishment, maintenance, & termination of end-to-end network links. Network layer protocols are required when computers that aren’t physically connected to the same LAN must communicate.
  • Transport Layer: responsible for providing reliability for the end-to-end network layer connections. It provide end-to-end recovery & flow control. It also, provide mechanisms for sequentially organizing network layer packets into a coherent message.
osi model cont d1
OSI Model cont’d
  • Session Layer: responsible for establishing, maintaining, & terminating sessions between user application programs.
  • Presentation Layer: provide an interface between user applications & various presentation-related services required by those applications. An example is data encryption/decryption protocols.
  • Application Layer: it includes utilities that support end-user application programs but it does not include end-user application programs.
encapsulation de encapsulation
Encapsulation/De-encapsulation
  • Encapsulation: in this process, each successive layer of the OSI model adds a header according to the syntax of the protocol that occupies that layer.
  • De-encapsulation: in this process, each successive layer of the OSI model removes headers &/or trailers & processes the data that was passed to it from the corresponding layer protocol on the source client.
  • These two processes describe how the various protocol layers interact with each other to enable an end-to-end communications session.
lan media
LAN Media
  • Not Twisted Pair
  • Unshielded Twisted Pair (UTP)
  • Shielded Twisted Pair (STP)
  • Coaxial Cable (Coax)
  • Fiber Optic
not twisted pair
Not Twisted Pair
  • Phone wire
  • RYBG
  • Flat Gray Modular Wiring
  • 4, 6 and 8 wires
unshielded twisted pair
Unshielded Twisted Pair
  • No Shielding
  • EIA Cat (1 – 5)
  • AWG
  • Attenuation: Loss of signal volume and power over a long distance
  • NeXT: a strong signal overpowering a weaker signal on an adjacent pair
shielded twisted pair
Shielded Twisted Pair
  • Shielding is metallic foil or copper braid
  • Shielded from EMI and RFI
coaxial cable coax
Coaxial Cable (coax)
  • Reliable High speed data transmission over relatively long distance
  • Used in Ethernet and comes in different thickness
fiber optic
Fiber Optic
  • Untappable and Immune to EMI and RFI
  • Glass Vs. Plastic
  • Multimode Step Index: 200Mbps < 1Km
  • Multimode Graded Index: 3Gbps several Kms
  • Single mode: light rays are more focused only one wavelength can pass at a time. (most expensive)
how is a lan implemented
How is a LAN Implemented ?
  • Appropriate networking hardware & software must be added to every computer or shared peripheral device that is to communicate via the LAN.
  • Some type of network media must physically connect the various networked computers and peripheral devices to converse with each other.
the lan architecture model
The LAN Architecture Model
  • All network architecture are made up of the same logical components.
  • To accurately describe a given network architecture, one needs to know the following:
    • Access methodology.
    • Logical topology.
    • Physical topology
access methodology
Access Methodology
  • Since many users is to send requests onto the shared LAN media at the same time, there must be some way to control access by multiple users to that media. These media-sharing methods are named “Access methodologies”.
  • Sharing the media is an important concept in LANs, which are sometimes called “media-sharing LANs”.
  • There is two access controlling methods:

1- CSMA/CD 2-Token Passing

csma cd
CSMA/CD
  • It’s based on the philosophy: “Let’s just let everyone onto the media whenever they want & if two users access the media at the same second, we’ll work it out somehow.”
  • Carrier sense multiple access with collision detection
  • Carrier sense:the PC wishing to put data onto the shared media listens to the network to see if any other users are “on line” by trying to sense a neutral electrical signal known as the carrier.
  • If no transmission is detected, multiple access allows anyone onto the media.
csma cd1
CSMA/CD
  • If two user PCs should access the same media in the same time, a collision occurs & collision detectionlets the user PCs to know that their data wasn’t delivered & controls retransmission is such a way to avoid collisions.
  • Another factor of collisions is propagation delaying, which is the time it takes to a signal from a source PC to reach a destination PC.
  • Because of this delay, it’s possible for a workstation to sense if there is no signal on the shared media, when in fact another distant workstation has transmitted a signal that hasn’t yet reached the carrier sensing PC.
token passing
Token Passing
  • “Don’t you dare access the media until it’s your turn. You must first ask permission, & only if I give you the magic token may you put your data on the shared media”.
  • It ensures that each PC user has 100% of the network channel available for data requests & transfers by insisting that no PC accesses the network without processing a specific packet of data (Token).
  • The token is first generated by a specified PC known as active monitorand passed among PCs until one PC would like to access the network.
token passing1
Token Passing
  • The requesting PC seizes the token, changes the token status from free to busy, puts its data frame onto the network, & doesn’t release the token until it’s assured that its data was delivered.
  • Successful data delivery is confirmed by the destination workstation setting frame status flagsto indicate a successful receipt of the frame.
  • Upon receipt of the original frame with frame status flag set to “destination address recognized, frame copied successfully” the sending PC rests the token status from busy to free & release it.
  • The token is passed along the next PC.
logical topology
Logical Topology
  • After the data message ha reached the shared-media LAN, the next step is to determine how that message will be passed from workstation to workstation until the message reaches its intended destination.
  • This passing technology is known as “Logical Topology”.
  • There are two known logical topologies:

1- Sequential 2- Broadcast

sequential topology
Sequential Topology
  • Also known as “ring logical topology”.
  • The data is passed from one PC (or node) to another.
  • Each node examines the destination address of the data packet to determine if this packet is meant for it
  • If the data was not meant to be delivered at this node, the data packet is passed along to another node in the logical ring.
broadcast topology
Broadcast Topology
  • A data message is sent simultaneously to all nodes on the network.
  • Each node decides individually if the data message was directed toward it. If not, the message is ignored.
  • No need to pass the message to a neighboring node.
physical topology
Physical Topology
  • The clients & servers must be physically connected to each other according to some configuration & be linked by the shared media of choice.
  • The physical layout configuration can have a significant impact on LAN performance & reliability.
  • There are three physical topologies:

1- Bus 2- Ring 3-Star

bus topology
Bus Topology
  • A linear arrangement with terminators on either end & devices connected to the “Bus” via connectors &/or transceivers.
  • A break or loose connection anywhere along the entire bus will bring the whole network down.
ring topology
Ring Topology
  • Each PC connected via a ring topology is actually an active part of the ring, passing data packets in a sequential pattern around the ring.
  • If one of the PCs dies or a network adapter card malfunctions, the “sequence” is broken, the token is lost, & the network is down !
star topology
Star Topology
  • It avoids the drawbacks of both Bus & Ring topologies by employing some type of central management device. This central device may called a Hub, a wiring center, a concentrator, a MAU (multistation access unit), a repeater, or a switching hub.
  • By isolating each PC or node on its own leg or segment of the network, any node failure only affects that leg.
  • If this central device goes down, the whole network goes down too.
slide37

NETWORK ARCHITECTURES

• Classic Architectures:

    • Ethernet
    • Token Ring
    • FDDI
  • High Speed Architectures
    • Family of Fast Ethernet
      • 100BaseT
      • 100VG-AnyLAN
      • Gigabit Ethernet (1000BaseT)
      • 10 Gigabit Ethernet
    • HSTR (High Speed Token Ring)
    • Fibre Channel
    • iSCSI
    • LAN-Based ATM
  • Home Network Architectures:
    • HPNA.
    • Bluetooth and PAN
    • Wireless spread spectrum technologies.
slide38

Ethernet

• Origins:

– Invented by Robert Metcalfe (founder of 3Com CO.).

– Although Ethernet &IEEE 802.3 are different standards.

– Ethernet is used to refer to IEEE 802.3 compliant network.

• Functionality:

– Access methodology: CSMA/CD.

– Logical topology: broadcast.

– Physical topology: traditionally, bus; currently, star.

ethernet
Ethernet

Figure 5-8 Ethernet and IEEE 802.3 Standards

slide40

Ethernet

• Media related Ethernet standards:

slide41

Token Ring

• Origin:

– Olaf Soderbulm in 1969.

– IBM standardized it as IEEE 802.5.

• Functionality:

– Access methodology: token passing.

– Logical topology: sequential.

– Physical topology: before, ring; now, star.

slide42

Token Ring

• Standards:

– IEEE 802.5 no speed specification.

– Operate at speed of 4 &16 Mbps.

– 24-bit data packet

– The starting delimiter field alert the token ring card installed in workstation that a frame is approaching.

– receive access control field.

– Workstation distinguish btw tokens &MAC sub layer frames.

– If token bit =0 then frame represents free token.

– If token bit =1 then frame represents busy token.

– Routing info used with source routing bridges that link multiple token ring LANs (LAN-to-LAN).

– Sequential logical topology =message passing form neighbor to neighbor.

– Token ring architecture = logical ring, physical star.

active monitor
Active Monitor
  • Removes Dead frames
  • Replace lost or damaged token
  • Responsible for master clock
  • Makes sure there is only one active monitor
  • Provide buffer for token in small networks
slide44

FDDI

• Origins:

– Fiber Distributed Date Interface.

– 100 Mbps network architecture.

– Specified 1984 by ANSI(X3T9.5).

– No IEEE standard.

–supports IEEE802.2 protocol. It is most popular.

• Functionality:

– Access methodology:Modified token passing.

– Logical topology:Sequential.

– Physical topology:Dual counter-rotating rings.

slide45

FDDI

• Built-in reliability &Longer distance:

– Support 100Mbps of bandwidth.

– High degree of reliability &security.

– Reliability comes from fiber +EMI +RFI +design of physical topology of FDDI.

– EMI (Electromagnetic Interference).

– RFI (Radio Frequency Interference).

– FDDI physical topology compromised of two separate rings in which data moves simultaneously in opposite directions.

– 1st ring: Primary data ring.

– 2nd ring: Secondary or backup data ring used in failure of primary ring or an attached workstation.

slide46

FDDI

Figure 5-13 FDDI Network Architecture and Technology

slide47

FDDI

– Both rings attached to a single hub or a concentrator.

– Distance: FDDI LAN cover 500 nodes at 2km apart.

– If repeaters used every 2km media can stretch up to 200km.

– Interoperate with IEEE 802.3 10-Mbps Ethernet.

– Interoperation needs FDDI-to-Ethernet bridge.

– Bridge can connect many Ethernets.

– PCs, and mainframes etc. must be equipped with either FDDI NIC or external FDDI controllers if they wish to access the FDDI LAN.

slide48

FDDI

– To cut down costs &benefit of 100 Mbps bandwidth managers only connect one of the 2 FDDI fiber rings.

– This is known as SAS (Single Attachment Stations).

– Else if both fiber rings connected it is called DAS (Dual Attachment Stations).

– The heart of the FDDI LAN is the FDDI concentrator or hub.

– The design of the hubs is modular with backbone connections to both FDDI rings.

– The dual counter rotating rings network architecture of FDDI has a self-healing capabilities.

slide49

FDDI

Figure 5-14 FDDI’s Self-Healing Ability

slide50

FDDI

• Standards:

Two ways of modification to the token passing access methodology.

– FDDI removes the token from the ring &transmit a full data frame. If the transmition is complete it releases a new token. Collision is avoided as only one station can have the free token at a time, and a station cannot put a data message onto the Network without a token.

– A Station can send more than one message per token.

slide51

FDDI

• FDDI could be run under copper wires, shielded or unshielded twisted pair (UTP) {CDDI} copper distributed data interface.It still support 100Mbps but limit distance to 100m/segment.ANSI standard for CDDI is TP-PMD (Twisted Pair Physical Media dependant).

Figure 5-15 FDDI Token and Data Frame Layouts

slide52
FDDI

•Applications:

–Network architecture trends.

–Campus backbone:

–attach multiple devices to FDDI rings (dual ring of trees).

–A server may be attached to more than one FDDI concentrator to provide redundant

connections and avoid fault tolerance (dual homing).

–High bandwidth work groups:

– used when connecting less than 20 PCs for high bandwidth communications.

–E.g. Multimedia workstations, engineering workstations, CAD/CAM workstations.

–As a “power user” require GUIs like windows.

–High bandwidth sub workgroup connections:

–Only 2 or 3 servers

slide53
FDDI

Figure 5-16 Alternative Applications of the FDDI Network Architecture

high speed network archtectures
HIGH-SPEED NETWORK ARCHTECTURES
  • 100BaseT:
    • Represents a family of fast Ethernet standards offering 100 Mbps performance and adhering to the CSMA/CD access methodology.
    • The three media-specific physical layer standards of 100BaseT are:

1-100BaseTX: the most common of the three & the one for which the most technology is available. It specifies 100-Mbps performance over two pair of category 5 UTP (Unshielded twisted pair) or two pair of type 1 STP (Shielded twisted pair).

100baset
100BaseT

2- 100BaseT4: Physical layer standard for 100-Mbps transmission over four pairs of Category 3,4,or 5 UTP.

3- 100BaseFX: Physical layer standard for 100-Mbps transmission over fiber optic cable.

Network Architecture: it use the same IEEE802.3 MAC sublayer frame layout & yet transmit it at 10 times faster than 10BaseT. There must be a trade-off that comes in the maximum network diameter:

  • 10BaseT’s maximum network diameter is 2500 m with up to 4 repeaters between any two nodes. >>
100baset1
100BaseT
      • 100BaseT’s maximum network diameter is 210 m with up to only 2 repeaters between end nodes.
  • Technology:
    • Most of the 100BaseT NICs are called 10/100 NICs which means that they are able to support either 10BaseT or 100BaseT but not simultaneously.
    • 10BaseT & 100BaseT networks can only interoperate with the help of internetworking devices such as 10/100 bridges & routers.
    • Some Ethernet switches can support 100BaseT connection & can auto-sense, or distinguish between 10BaseT & 100BaseT traffic.
slide58
HSTR
  • High Speed Token Ring
  • 100Mbps Token Ring
  • No IEEE Standard
  • Supports IEEE 802.1q which allows Ethernet frames to be encapsulated within Token Ring frames
  • Important for organizations that must support both network architectures
gigabit ethernet
Gigabit Ethernet

From the family of Fast Ethernet, IEEE 802.3z standard.

– Known as (1000Base-X)

  • 1000BaseSX: Multimode Fiber Optic, horizontal floorplanning
  • 1000BaseLX: Singlemode Fiber Optic, vertical backbone
  • 1000BaseCX: Copper Wire (Dead)
  • 1000BaseTX: 4 pairs Cat 5 UTP, max. 100m.

- The final standard retains Ethernet’s CSMA/CD access methodology.

- Ethernet frame size did not change.

gigabit ethernet1
Gigabit Ethernet
  • Gigabit Ethernet combined Speed with Maximum Transmission distance by using single mode fibers that can run up to 5Km, This reflects on its applications:
    • Resolving Server bandwidth constraints
    • Removing bottlenecks from backbone.

Beyond Gigabit Ethernet: 10 Gigabit Ethernet

fiber channel
Fiber Channel

•Alternative Gigabyte Ethernet NT Architecture.

•ANSI standard X3T9.3.

•Speed of 133 to 1.062 Gbps.

•Uses optical fiber and copper cables.

•Used to connect high-performance storage devices and RAID (Redundant Arrays of Independence/Inexpensive Disks) subsystems to computers.

•Fiber channel switches and Network Interface Cards are also available.

lan based atm
LAN-Based ATM

•ATM (Asynchronous Transfer Mode).

•It is a switched NT technology.

•Speed range from 25Mbps to several Gbps.

•NICs are available for servers &work stations.

•For ATM based to communicate with non-ATM based computers a process known as LAN emulation must be implemented.

•For applications to take advantage of ATM’s speed and features they must be “ATM aware”.

•ATM has been implemented in animation and stock-trading industries.

home networks
Home Networks
  • HPNA (Home Phone Line Networking Alliance).
    • Runs Ethernet over the RGYB Telephone line by using available bandwidth (10Mbps).
  • Wireless: like BlueTooth.
    • CSMA\CA, more overhead, only 1.6Mbps, support up to 16 nodes.
    • Operate in the range of 2.4GHz.
    • They jump frequencies to avoid conflict and interference.
    • Known also as Spread Spectrum Technologies
guided transmission media
Guided Transmission Media
  • Transmission capacity depends on distance and type of network (point-to-point or multipoint)
  • Twisted Pair
  • Coaxial cable
  • Optical fiber
twisted pair
Twisted Pair
  • Least expensive and most widely used
  • Two insulated copper wires arranged in regular spiral pattern
  • Number of pairs bundled together in a cable
  • Twisting decreases crosstalk interference between adjacent pairs in cables
    • Using different twist length for neighboring pairs
twisted pair applications
Twisted Pair - Applications
  • Most common transmission medium for both analog & digital signals
  • Telephone network
    • Between house and local exchange (subscriber loop)
  • Within buildings
    • Telephones connected to private branch exchange (PBX) for voice traffic
    • Connections to digital switch or digital PBX (64kbps)
  • For local area networks (LAN)
    • 10Mbps or 100Mbps
twisted pair pros and cons
Twisted Pair - Pros and Cons
  • Cheap
  • Easy to work with
  • Low data rate
  • Short range
twisted pair transmission characteristics
Twisted Pair - Transmission Characteristics
  • Analog
    • Amplifiers every 5km to 6km
  • Digital
    • Use either analog or digital signals
    • repeater every 2km or 3km
  • Attenuation is strong function of frequency
  • Susceptible to interference and noise
    • Easy coupling with electromagnetic fields
    • A wire run parallel to power line picks up 60-Hz energy
    • Impulse noise easily intrudes into twisted pairs
twisted pair transmission characteristics1
Twisted Pair - Transmission Characteristics
  • Measures to reduce impairments
    • Shielding with metallic braids or sheathing reduces interference
    • Twisting reduces low frequency interference
    • Different twist length in adjacent pairs reduces crosstalk
  • Limited distance
  • Limited bandwidth
    • For point-to-point analog signaling, 1MHz
  • Limited data rate
    • For long distance digital point-to-point signaling, 4 Mbps
    • For very short distances, 100Mbps-1Gbps
unshielded and shielded tp
Unshielded and Shielded TP
  • Unshielded Twisted Pair (UTP)
    • Ordinary telephone wire
    • Cheapest
    • Easiest to install
    • Suffers from external EM interference
  • Shielded Twisted Pair (STP)
    • Metal braid or sheathing that reduces interference
    • Better performance at higher data rates
    • More expensive
    • Harder to handle (thick, heavy)
unshielded twisted pair utp
Unshielded Twisted-Pair (UTP)
  • Quality of UTP vary from telephone-grade wire to extremely high-speed cable
  • Cable has four pairs of wires inside the jacket
  • Each pair is twisted with a different number of twists per inch to help eliminate interference
    • The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot
utp categories
UTP Categories
  • Cat 3
    • up to 16MHz
    • Voice grade found in most offices
    • Twist length of 7.5 cm to 10 cm
  • Cat 4
    • up to 20 MHz
utp category 5
UTP Category 5
  • Up to 100MHz
  • Commonly pre-installed in new office buildings
  • Twist length 0.6 cm to 0.85 cm
  • Current standard for data
  • 100 meter maximum segment length
  • 100 mb/s available
  • GB/s over short distances
  • Inexpensive
  • Can be pulled in existing conduit
unshielded twisted pair connector
Unshielded Twisted Pair Connector
  • The standard connector for unshielded twisted pair cabling is an RJ-45 connector.
    • A plastic connector that looks like a large telephone-style connector
    • RJ stands for Registered Jack; connector follows a standard borrowed from telephone industry.
    • Standard designates which wire goes with each pin inside the connector.
cat 5 network cables
Cat 5 Network Cables

Category 5 Cable composed

of 4 twisted pairs

Cat 5Cable RJ45

composed of 4 twisted pairs

Shielded Cat 5 Network

Cable RJ45

comparison of shielded unshielded twisted pair
Comparison of Shielded & Unshielded Twisted Pair

Attenuation (dB per 100m)

Near-End Crosstalk (dB)

coaxial cable
Coaxial Cable
  • Hollow outer cylindrical conductor surrounding a single inner conductor
  • Inner conductor held by regularly spaced insulating rings or solid dielectric material
  • Operates at higher frequencies than twisted pair
coaxial cable layers
Coaxial Cable: Layers

outer jacket

(polyethylene)

shield(braided wire)

insulating material

copper or aluminum

conductor

optical fiber

The optical fiber cable in the foreground has the equivalent information-carrying capacity of the copper cable in the background.

Optical Fiber
optical fiber1
Optical Fiber
  • Thin, flexible material to guide optical rays
  • Cylindrical cross-section with three concentric links
  • Core:
    • Innermost section of fiber
    • One or more very thin (diameter 8-100 mm) strands or fibers.
  • Cladding:
    • Surrounds each strand
    • Plastic or glass coating with optical properties different from core
optical fiber2
Optical Fiber
  • Jacket
    • Outermost layer, surrounding one or more claddings
    • Made of plastic and other materials
    • Protects from environmental elements like moisture, abrasions and crushing
optical fiber single fiber

plastic jacket

glass or plastic

cladding

fiber core

Optical Fiber: Single fiber
optical fiber benefits
Optical Fiber - Benefits
  • Greater capacity
    • Data rates of hundreds of Gbps over tens of Kms
  • Smaller size & weight
  • Significantly lower attenuation
  • Electromagnetic isolation
    • Not affected by external EM fields
    • Not vulnerable to interference, impulse noise, or crosstalk
    • No energy radiation; little interference with other devices; security from eavesdropping
  • Greater repeater spacing
    • 10s of km at least
    • Lower cost and fewer error sources
optical fiber applications
Optical Fiber - Applications
  • Long-haul trunks:
    • Increasingly common in telephone networks
    • About 1500 km in length with high capacity (20,000-60,000 voice channels)
  • Metropolitan trunks:
    • Average length of about 12 km with capacity of 100,000 voice channels
    • Mostly, repeaters not required
  • Rural exchange trunks: Lengths from 40 to 160 km with fewer than 5000 voice channels
  • Subscriber loops:Handles image, video, voice, data
  • LANs: 100Mbps to 1 Gbps, support hundreds of stations on campus
transmission characteristics
Transmission Characteristics
  • Single-encoded beam of light transmitted by total internal reflection.
  • Fiber has two basic types:
    • Single mode
    • Multimode:
      • Graded index
      • Step index.
transmission modes
Transmission Modes:
  • Single mode fiber
    • the light is guided down the center of an extremely narrow core
  • Multimode step-index fiber
    • the reflective walls of the fiber move the light pulses to the receiver
  • Multimode graded-index fiber
    • acts to refract the light toward the center of the fiber by variations in the density
step index multimode
Step-index multimode:
  • Core made of one type of glass.
  • Light traveling in fiber travels in straight lines, reflecting off the core/cladding interface
  • Rays at shallow edges reflected and propagated along fiber
  • Other rays absorbed by surrounding material
  • Allows for multiple propagation paths with different path lengths and time to traverse fiber
  • A pulse of light is dispersed while traveling through the fiber
  • Limits rate at which data can be accurately received
  • Best suited for transmission over very short distances
graded multimode fiber
Graded Multimode Fiber
  • Core is composed of many different layers of glass, with indices of refraction producing a parabola index profile
  • A properly constructed index profile will compensate for the different path lengths of each mode
  • Bandwidth capacity of graded fiber 100 times larger than step index fiber
  • Normally uses inexpensive LED laser transmitter and receivers
  • Maximum distance up to 2 km
  • Most common type is 62.5/125mm
  • Uses wavelengths of 850nm and 1300nm
  • Often used for building backbones and short inter-building communications
graded multimode fiber1
Graded Multimode Fiber
  • Higher refractive index at center makes rays close to axis advance slower than rays close to cladding
  • Light in core curves helically reducing traveling distance (does not zigzag off cladding)
  • Shorter path & higher speed makes light at periphery as well as axis travel at same speed
single mode fiber
Single-Mode Fiber
  • Shrinks core size to a dimension about 6 times the wavelength of the fiber, causing all the light to travel in only one mode
  • Modal dispersion disappears and bandwidth of the fiber increases by at least a factor of 100 over graded index fiber
  • Can be used for distances of 30 km or when high data rates are required
optical fiber light sources
Optical Fiber – Light Sources
  • Semiconductor devices that emit light when voltage applied
  • Light Emitting Diode (LED)
    • Cheaper
    • Wider operating temp range
    • Longer operational life
  • Injection Laser Diode (ILD)
    • More efficient
    • Greater data rate
  • Wavelength Division Multiplexing (WDM)
    • Multiple beams of light at different frequencies transmitted simultaneously
    • 100 beams operation at 10 Gbps, for a total of 1 trillion bps
fiber optic attenuation
Fiber Optic Attenuation
  • Attenuation of optical fiber is a result of two factors, absorption and scattering
  • Absorption is caused by absorption of light and conversion to heat by molecules in the glass.
  • absorption occurs at discrete wavelengths, and occurs most strongly around 1000 nm, 1400 nm and above1600 nm.
  • Scattering occurs when light collides with individual atoms in the glass
  • Light scattered at angles outside the numerical aperture of fiber will be absorbed into the cladding or transmitted back toward the source.
fiber optic attenuation1
Fiber Optic Attenuation
  • Scattering is a function of wavelength, proportional to inverse fourth power of wavelength of light
    • doubling wavelength of light, reduces scattering losses 16 times
  • For long distance transmission, use longest practical wavelength for minimal attenuation and maximum distance between repeaters
  • Fiber optic systems transmit in the "windows" created between the absorption bands at 850 nm, 1300 nm and 1550 nm
  • Plastic fiber has a more limited wavelength band, that limits practical use to 660 nm LED sources
fiber types and typical specifications

Fiber Type

Core/

Cladding

Diameter

Attenuation Coefficient (dB km)

Bandwidth

@ 1300 nm

.

(microns)

850 nm

1300 nm

1550 nm

(MHz-km)

Step Index

 200/240

 6

 NA

 NA

 50@850

Multimode

50/125

3

1

NA

600

Graded Index

62.5/125

3

1

NA

500

.

85/125*

3

1

NA

500

.

100/140*

3

1

NA

300

Singlemode

8-9/125

NA

0.5

0.3

high

Plastic

1 mm

(1 dB/m @665 nm)

Low

Fiber Types and Typical Specifications
fiber optic cables
Fiber Optic Cables

Duplex Multimode

62.5/125 mm

Duplex Single-mode

9/125 mm

Fiber optic cable

coaxial cable2
Coaxial Cable
  • Outer conductor covered with a jacket or shield
  • Diameter from 1 to 2.5 cm
  • Shielded concentric construction reduces interference & crosstalk
  • Can be used over longer distances & supports more stations on a shard line that twisted pair
coaxial cable applications
Coaxial Cable Applications
  • Most versatile medium
  • Television distribution
    • Ariel to TV, Cable TV
    • Can carry hundreds of TV channels for tens of kms.
  • Long distance telephone transmission
    • Can carry 10,000 voice channels simultaneously
    • Being replaced by fiber optic
  • Short distance computer systems links
  • Local area networks
coaxial cable transmission characteristics
Coaxial Cable - Transmission Characteristics
  • Used to transmit both analog & digital signals
  • Superior frequency characteristics compared to twisted pair (1KHz-1GHz)
  • Less susceptible to interference & crosstalk
  • Constraints on performance are attenuation, thermal noise, intermodulation noise
  • Analog:
    • Amplifiers every few km
    • Closer spacing if higher frequency (up to 0.5 GHz)
  • Digital:
    • Repeater every 1km
    • Closer spacing for higher data rates
lan media1
LAN Media
  • Unshielded Twisted Pair (UTP) is currently the most popular.
    • There are different grades of UTP
      • Category 1, 2, 3, 4, 5
  • Shielded Twisted Pair (STP) is similar, except:
    • it has a foil shielding or copper braid to reduce EMI
    • costs considerably more
lan media2
LAN Media
  • Coaxial cable
    • features a solid metal core surrounded by a plastic insulator, then a foil shield & braid, and finally a plastic or vinyl protective jacket
    • cable-tv uses coaxial cable
    • there are different grades of coax cable
lan media3
LAN Media
  • Fiber Optics
    • glass core surrounded by glass cladding, and protected by a plastic or vinyl jacket
    • very secure
    • unaffected by EMI
    • typically capable of 200 Mbps - 3 Gbps
    • different grades are available
      • multimode step index, multimode graded index, single mode
lan switching vs classical segmentation

Bridge/

Routers

Client

A

Client

B

Server

X

Client

C

Server

Y

Client

D

Server

Z

100 Mbps

10 Mbps

10 Mbps

10 Mbps

10 Mbps

Client

A

Client

B

Server

Y

Client

C

Server

Y

10

Mbps

10

Mbps

10

Mbps

10

Mbps

Client

C

Client

D

Server

Z

LAN Switching vs. Classical Segmentation
  • Classical Segmentation:
  • works best when most traffic local
  • max. throughput = backbone speed
  • Switching:
  • provides high degree of segmentation
  • max. throughput = switch speed