Local Area Network Technologies

Local Area NetworkTechnologies

INFO 341

A Definition

! A LAN is computer network in which devices (computers, printers) are connect to a shared medium (wire or cable) to exchange data or share resources within a small geographic area" An office, the floor of a building, a building itself, or

maybe a small campus.

" Not always clear where a “local area” ends and a “wide area” begins

" Some say when you need to involve a 3rd party such as a phone company or other telecom provider that’s where the “local area” ends

LANs are Popular

! LANs are the most popular form of computer network

! They are relatively cheap" Much of the hardware has become commodity

hardware with consumer versions available

! They are typically very fast - often faster than much more expensive WAN technologies

! Deployed today in not just workplaces and offices, but also in the home

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Sunday, April 16, 2006

Underlying Principle

! Locality of Reference" An organizing principle for many phenomenon

! Consider ‘client’ files in a physical filing cabinet

" Two key types! Physical locality of reference

! Temporal locality of reference

LAN Technologies

! Most popular are:" Ethernet – far and away the most widely deployed

" Token Ring – tends to be in niche markets today

" FDDI (Fiber Distributed Data Interconnect)! formerly used on the “backbone” in large university or

corporate networks to interconnect building-wide Ethernets. The same fiber now carries gigabit Ethernet

" ATM – Asynchronous Transfer Mode! Five years ago thought by many to be the future as it was

faster than the Ethernet of the day. Today used in niche areas as Ethernet has improved and still dominates

" “Wireless LAN” - for example, Mary Gates Hall

Ethernet

! Development roots in the late 1960’s at the University of Hawaii

! CSMA/CD used in the ALOHA wireless network

! Xerox began its research into Ethernet in 1972! Digital, Intel and Xerox devised a standard called DIX

Ethernet (Digital, Intel, Xerox) Blue Book spec.! IEEE 802 committee began its standards work soon

thereafter, changed the frame format, called it 802.3! By 1982 Ethernet was established

! It is now the dominant network standard for new LAN installations, i.e., it’s entrenched

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Ethernet Architecture

! Key Characteristics" Manchester encoding

! Change in voltage from positive to zero is a “0”, change from zero to positive encodes a “1”

" CSMA/CD to control access to the medium

" Originally a 10 megabit/sec architecture! Today 10, 100, 1000 megabit (Gb), 10 Gb options

" Originally wired using a Bus topology! Today usually wired as a star

! Cheap and pervasive

CSMA/CD

Carrier Sense with Multiple Access & Collision Detection (CSMA/CD) provides contention based control

" While one computer transmits to another, all the other computers must wait

" Listen to the medium to see whether a message is being transmitted

" If the medium is quiet, transmit message. If the medium is busy, wait for the signal to clear and then transmit.

" If a collision occurs, wait for the signal to clear, wait a random interval, and then retransmit. ! Uses binary exponential backoff mechanism to guarantee

contention will be reduced quickly

" Old telephone “party line” analogy

The Polite Cocktail Party

CSMA/CD Characteristics

! Equal access for all computers

! Access times are unpredictable but short

! Highly susceptible to network congestion" > 40% utilization is often considered saturated

" but it’s not, as shown by empirical studies

! As network grows performance can become unpredictable, especially when frames are lost and upper layer protocols time out

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Ethernet Hardware

! NICs

! Hubs

! Repeaters

! Bridges

! Switches

Hardware Variations

! Hardware can be different depending on the type of Ethernet you are using" 10Mb/sec NICs, 100MB NICs, GB Ethernet NICs,

10Mb/sec Hubs, 100Mb/sec Hubs, GB Hubs

" Combo devices such as10/100 NICs, 10/100 Hubs, 100/GB switches – many possible variations

" All the variation is there because Ethernet is an evolving technology that has changed over time

" The frame format stays the same, while the physical, electrical, and timings specs evolve

Let’s start at the beginning…

! Thick Ethernet – 10Base5" 10 Mb/sec network, max length of a

baseband “segment” is 500 meters, hence “10Base5”

! Wired in a bus topology using a “thick” very rigid coax cable

! Each end of the bus requires a 50-ohm termination resistor

! Max of 100 devices per segment

! Length can be extended using a “repeater”

! Entire buildings can be covered

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Ethernet Repeaters

! Do exactly what their name implies

! Regenerate the signal voltage and timing so that it can travel a longer distance" For example you could extend the maximum

length of a 10Base5 thicknet network from 500 meters to 1000 meters by installing a repeater at the end of one segment

! Thick Ethernet can have a maximum of 5 segments, 2500 meters total cable length

! 5-4-3-2-1 rule

! Repeaters (and hubs) work at OSI Layer I

Repeater example

Ethernet Bridges

! Similar to a repeater but operate at OSI Layer II

! Also regenerates the signal so that it can travel a longer distance

! A bridge keeps track of which devices are on different segments (by their MAC addresses).

! A bridge only retransmits signals when the sender and receiver are on different segments

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Bridges

" Bridges maintain a “bridge table” in memory, essentially keeping track of what addresses are on each side of the bridge.

" Only frames that need to pass through the bridge get through, other don’t. ! Note: Bridges do pass all “broadcast” traffic which are pieces

of data that need to go to all machines on a network. We will discuss broadcast packets in the future.

" Bridges help control congestion and can improve performance on a network

Thick Ethernet connections

! Each device that connects to the network requires a “NIC” (Network Interface card).

! That NIC is connected to a “transceiver” using an “AUI” cable

! The transceiver is connected to the thick coax cable using things like “vampire” taps

Thick Ethernet connections

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NIC – Network Interface Card

! Handles the transmission and reception of bits, offloading this work from the CPU

! Installs in an expansion slot! May also be built onto the motherboard as a

single chip (very common today)! May be an external unit that connects through a

USB port or a Firewire port but … this isn’t necessarily a good idea!

! Often built into network printers, other network devices

NIC functions

! Understands the electrical signals on the network, the rate data will be sent or received, all the physical layer details for that network type.

! Provides a connector so a physical connection can be made to the cable" With Ethernet NICs you may have an RJ-45 jack,

an AUI (attachment unit interface) connector, a BNC connector or all three on the same card

Motherboard, NIC, and external connections

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NIC costs

! NICs are manufactured by a variety of companies and costs vary tremendously" $10 for no-name 10/100BaseT NICs to several

hundred dollars for brand name Gb Ethernet NICs over copper

" Much more for Gb Ethernet over fiber! Name brands like 3Com, and Intel are widely pervasive in

business. ! Usually best to use a name brand card because

remember….in order to work a NIC will need a software driver for your operating system.

! If you use a no-name card, finding a driver can be difficult, and if that vendor goes out of business you may not get an update when a new OS comes along

Each NIC is Unique

! Each NIC has a “MAC” (Media Access Control) Address that is completely unique

! This MAC Address is necessary so that pieces of data can be delivered to the correct location, without a unique address for each card we couldn’t do that

! To find a MAC address on a Windows PC …

Start, Run, type: cmd (Click OK) and then at the command prompt type:

ipconfig -all

!on a Macintosh, Unix, or Linux box type: ifconfig

MAC Addresses

! Notice that the MAC Address is called the “Physical Address” by Windows

! Notice that it is displayed as a hexadecimal value

! What is hex and why is it displayed like that?! MAC = LLC = Physical = Ethernet = Layer II

address, they are all synonyms

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Hex – Base 16

! Number system that is Base 16 rather than base 10.

! Count as follows:" 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F" 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B" 1C, 1D, 1E, 1F, 20, 21, 22….

! Why do people write values this way?" Just a shorthand for the binary values." Each 2 digit base 16 number represents 8 bits in

binary

Vendor Assigned Codes

! Notice that every NIC in the lab starts out with the same three hex values

! Each vendor is assigned a code and every card they manufacture has a MAC address that begins with that code

! 00:0a:95:99:b1:32 = Apple Computer

Thick-net problems

! Thick net is no longer widely deployed today." Too slow (10 megabits/second)

" Awkward cabling and connections" Better options started to appear

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Thin Ethernet

! Often called 10Base2" 10Mb/sec network, max length of a “segment” is

200 meters

" Bus topology like thick Ethernet

" Used a coax cable like thick Ethernet but the size of the cable is much smaller – hence “thinnet”

" Often used BNC connectors with T Connectors rather than vampire taps

Thinnet Characteristics

! Multiple trunk segments connected by repeaters, bridges or routers

! Each end of the bus must be terminated

! Maximum of 5 trunk segments

! Limit of 30 devices total, including computers, repeaters etc. per segment

! Total Thinnet Network distance 925 meters

Thinnet Cable and Connectors

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Thicknet and thinnet bus topology

! Cabling" Thicknet Coaxial

" Thinnet Coaxial

! 802.3 Standards" 10base5 - Thicknet 500 meter

distance max. per segment" 10base2 - Thinnet 200 meter

distance max per segment

Typical Ethernet (thin or thick) Segment

Advantages of Thin net

! Cabling and connections much simpler to manage and deploy than thick net

! Transceiver was typically built into the NIC so you removed another “black box” from the picture

! Fewer components meant less cost

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Thin net issues

! Maintained the bus topology along with the potential wiring problems that a bus topology brings

" BNC connectors easy to connect but also easy to disconnect and then…no network!

! Speed still the same, no improvement

10BaseT Ethernet

! Wired in a Star topology" Even though wired like a star, works the same

as a bus. All data appears on every wire and devices contend for access using CSMA/CD

! UTP (unshielded twisted pair) cabling! Requires a hub! Maximum cable length 100 meters from

hub to device! Maximum of 1024 devices per network

Small 10BaseT Net with Uplink

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10BaseT Hub

! Essentially a “multi-port repeater”" Propagates all the data on one cable to all the

others connected there

! Comes in a variety of shapes and sizes with a varying number of ports. Each device plugs into a port using an RJ-45 connector.

! One port is often designated as an uplink port for connecting to another hub or networking device

“Daisy Chaining” Hubs

! 10Mb hubs can be connected to other 10Mb hubs in a “daisy chain” to make more ports available

! Should not chain more than 3 hubs in a row

! Use an “uplink port” if available, or a “crossover cable”" Normally hubs are expecting that you will be connecting a

device like a computer or printer to each port.

" If however you want to connect to another network device like a hub instead, you need to use a “crossover” cable to cross over the send and receive pairs of the wire

" Some hubs have a switch on an “uplink” port that allows you to switch it and it will do the crossover for you on that port so you can use a “straight-through” cable to connect to another hub

Crossover Cable

! Used when connecting from hub to hub, or if you want to connect two 10BaseT Ethernet computers together directly without using a hub

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Cascading Hubs

Some (usually more expensive) hubs use proprietary cables to stack multiple hubs from that same vendor together so they act like a single hub. This allows you to add more ports without violating the 3 hub rule.

10BaseT Hub Capabilities

! While all 10BaseT hubs do essentially the same thing, there are differences" How many ports does it support?

" Does it have an expandable type of chassis?

" Can you stack them?

" Is the hub “manageable” in that it supports SNMP (Simple Network Management Protocol)

" Can you telnet to the hub to look at its setup or configuration?

Hub Variations

Hubs can range from small inexpensive 4 port versions like above ($35) to high-density stackable and manageable products that cost many times more

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100Base-T Ethernet

! Very much like 10BaseT but faster! Also a star topology! UTP cabling, but must be Category 5

" 10BaseT can use Category 3

! Also requires a hub" But the hub must be a 100Mb/sec hub!

! Same maximum cable length 100 meters from hub to device

! Same maximum of 1024 devices per network! Each device requires a NIC, but again a

100Mb/sec NIC or a combo 10/100 NIC

100BaseT Costs

! 4-5 Years ago there was a huge premium for 100MB. Today it is very close to 10BaseT.

" A small 10BaseT hub may cost $35, a 100MB hub $37

" Most NICs sold today are 10/100 combo cards and are sold for $25-$50 each

" If it weren’t for cable infrastructure limitations present in many organizations (they have existing cat 3 wire installed) 10Mb Ethernet would probably be dead today and would no longer be manufactured

Gigabit Ethernet

! Two options: " 1000BaseT - Gigabit Ethernet over copper twisted pair

" 1000BaseFX – Gigabit over Fiber

! Same 100 meters from hub to device over Category 5 cable

! Requires Gigabit Hubs and Gigabit NICs

! Often used as an uplink connection to a campus or organization level network or used with servers

! Today due to added expense of hardware typically not used with desktop machines" Very few desktop boxes can handle processing that much data

that fast anyway, so you don’t get the performance increase you might expect

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Switches

! Switches physically resemble hubs, they have multiple ports and each device attaches to a port in a star topology

! However switches do NOT act like hubs" Switches do not repeat all

the data to every port

! Instead switches act like a “multi-port bridge”

Switch operation

! Switches “learn” what MAC addresses are connected to each port

! Switches only send data to ports that should receive that data, not to all

" Rather than multiple devices contending for access to a single 100MB Ethernet, in essence each device has it’s own 100Mb Ethernet all to itself

" Like bridges however, broadcasts are propagated to all ports by definition

! Congestion is reduced! Network performance improved

Switch types! 10Mb Switches allow you to connect 10Mb devices to any port

! 100Mb Switches allow you to connect 100Mb devices

! 1000Mb Switches let you connect Gb devices

! 10/100 Switches let you mix and match

! 10/100/1000 Switches also available for under $100US

! Most switches have an uplink port or two that are faster than the other ports. Example: 10Mb switch might have (2) 100Mb uplink ports, or a 100Mn switch might have (2) Gb uplinks

! Why? Usually there will be particular ports that will receive a lot of traffic, like the port that has you connection to the Internet on it, or where your file server is

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Here!….10 Gb/sec Ethernet

! New standard for 10 Gb/sec IEEE 802.3ae

! Doesn’t use CSMD/CD any longer – instead uses separate channels for sending and receiving

! Fiber based at present

! So is it still Ethernet?! " Sometimes people like to hold on to a name even though the

technology is different just for marketing and comfort reasons

! Initial use expected to be in “MANs” – Metropolitan Area Networks" Most desktops can’t even handle Gb Ethernet today due to

CPU and PCI bus limitations

FDDI

! Fiber Distributed Data Interface" Developed in the mid-80’s

" Designed for high performance networks

" Used for LANs & WANs

" Often used as backbone connection between LANs

" 100Mb/s

" Token passing/Ring topology

" Expensive! But getting cheaper.

" Standard for Storage Area Networks (SAN)

FDDI Characteristics

! Supports UTP, STP & Optical Cable

! Supports Dual-Ring Redundancy

! Supports Bypass switches for fault tolerance

! 500 Computers per fiber segment

! Max distance between computers = 2km

! Max total network distance = 200km

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Summary

! Key principle behind LANs, locality of reference

! LAN components, media, repeaters, bridges, hubs, switches, NIC, MAC Addresses

! Media Access strategy, CSMA/CD

! The most popular LAN technology today is Ethernet

! Ethernet has been improved many times and continues to change as technology changes

! Other LAN technologies like Token Ring and FDDI exist, but they often have niche uses at present

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Local Area Network Technologies

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