The Building Blocks: LAN

Shared and Switched EthernetConnecting Devices

Ethernet Access MethodsNetwork Operating SystemsBest Practice LAN Designs

Review

Layered communication: OSI model

Ethernet Frame Layout

Shared Ethernet Topology

Shared Ethernet’s logical topology is a bus topology. This means all computers on the network receive messages from all other computers, whether the message is intended for those computers or not.When a frame is received by a computer, the first task is to read the frame’s destination address to see if the message is meant for it or not.

Shared Ethernet Topology

Shared Ethernet Topology

If the message is not intended for a computer, the computer discards the message.If the message is intended for a computer, the computer processes the message and move it up to the next layer of the OSI model.Ethernets today use a physical star topology, with the network’s computers linked into hubs. It is also common to link use multiple hubs to form more complex physical topologies, enabling the networks to span longer distances.

Switched Ethernet Topology

Switched Ethernet uses switches instead of hubs. Switches that make switching decisions based on data link layer addresses are called workgroup or layer-2 switches.While a hub broadcasts frames to all ports, the switch reads the destination address of the frame and only sends it to the corresponding port.The effect is to turn the network into a group of point-to-point circuits and to change the logical topology of the network from a bus to a star.

Ethernet Physical Layer Standards

The three most commonly used forms of Ethernet today are:10BaseT (10 = 10Mbps, T = twisted pair and base = baseband, meaning one channel)Fast Ethernet which includes 100BaseT and 100BaseF (F = fiber)Gigabit Ethernet which includes 1 Gigabit Ethernet (1 GbE) and 10 GbE (10 Gbps). Even faster versions are being developed.

The 10BaseT Ethernet Standard

10BaseT uses twisted-pair cable. By far the most common form today is using 8-wire, category 5 cables with RJ-45 connectors (similar to phone jacks but larger).Twisted pair cables can be either unshielded (UTP) or shielded (STP). Shielding reduces interference from outside sources that may cause transmission errors.The maximum possible cable length is 100 meters.

Fast Ethernet

There are two forms of fast Ethernet, 100BaseT and 100BaseF. 100BaseT uses twisted-pair cable. 100BaseF uses fiber optic cable. Light

created by an LED (light-emitting diode) or laser is sent down a thin glass or plastic fiber.

Gigabit Ethernet (1GbE)

Gigabit Ethernet is the newest family of Ethernet protocols running at speeds of 1 Gbps and above. Can use either twisted pair or fiber. 1000BaseT uses CAT-5e UTP, using all four wire

pairs operating in parallel. Two types of Gigabit Ethernet use fiber optic

cables: 1000BaseSX uses MMF and has a maximum

segment length of 220 or 550 meters depending on the cable

1000BaseLX which uses MMF or SMF which can have a segment length of up to 5 kilometers for SMF.

10GbE

A 10 gigabit version of Ethernet is now being developed. 10GbE now has two forms, one for LANs and one for WANs. Interconnect LAN/WAN networks will be possible using 10GbE.The LAN form of 10GbE runs over four parallel MMF or SMF fibers.

Connecting Devices

HubsSwitchesRoutersRepeatersBridges

Hubs

Physical layer (Layer 1) devicesWhen hubs receive a signal from one device, it broadcasts the signal out all portsAll devices attached to the hub receive the signalCollisions need to be handledDisadvantage Shared bandwidth Latency grows as the number of station grows

Hub Operation

D4-47-55-C4-B6-9F

A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65

C3-2D-55-3B-A9-4F

Ethernet Hub

Station A1-44-D5-1F-AA-4C transmits a bit.

Hub Operation

D4-47-55-C4-B6-9F

A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65

C3-2D-55-3B-A9-4F

Ethernet Hub

Hub broadcasts the bitout all ports.

Switches

Data link layer (Layer 2) devicesProvide a set of separate point-to-point circuitsSend received frame out a single portSmall latencyCan use the maximum bandwidthPerform format convention between different physical layer standards, i.e., copper and optical fiberMaintain a switching table

Switch Operation

D4-47-55-C4-B6-9FOn Switch Port 13

A1-44-D5-1F-AA-4COn Switch Port 10

B2-CD-13-5B-E4-65On Switch Port 11

C3-2D-55-3B-A9-4FOn Switch Port 13

Ethernet Switch

Station A1-44-D5-1F-AA-4C transmits a bit.

Switch Operation

D4-47-55-C4-B6-9FOn Switch Port 13

A1-44-D5-1F-AA-4COn Switch Port 10

B2-CD-13-5B-E4-65On Switch Port 11

C3-2D-55-3B-A9-4FOn Switch Port 13

Ethernet Switch

A switch sends a frameout a single port—

the one to the receiver

Other Switches

Layer 3 switches Work at the network layer Forward packets based upon IP addresses Usually only support Ethernet at Layer 2 and

limited Layer 3 protocols Dominant within the site

Layer 4 switches Examine the port number field of a packet;

therefore switch packets based on the application they contain

Give priority to or deny forwarding to IP packets from certain applications

Routers

Connect different types of networks that have different physical and data link layer standards (Layer 3 device) i.e., internal Ethernet LAN, internal Token-

ring network, a leased T1 lineUsually are organized into a mesh topologyIdentify all possible routes and then select the best routeSlower and more expensive than switches

EthernetLAN 3

Internetworking with Routers

Ethernet LAN 1

EthernetLAN 2

Token-RingNetwork

Router W

Router Y

RouterX

RouterZ

T1Leased

Line

T1Leased

Line

Frame RelayNetwork

Site C Site A

Site BRouters Connect Different Types of Networks

Router Operation

When a frame arrives at a router The data link layer of the interface of that port

removes the frame of the packet and passes the packet up to the router’s network layer

The network layer makes a routing decision using routing tables and selects a interface

The data link layer of the selected interface put the packet in a frame suitable for that network and then passes it down to the physical layer

The physical layer of the interface translate the frame to bits and transmit

Format Conversion

IPPacket

IPPacket

PPPT

PPPH

1. Frame arriving at a router through Physical Layer

User PC Router

3. The data link layer process the PPP frame and passes the IP packet to the router’s network layer

2. The Physical passes it on to the Data Link layer

Format Conversion1. The router decides the route (and port) and then passes the IP packet

to the port attached to the selected network.

User PC Router

IPPacket

IPPacket

PPPT

PPPH

3. The data link places the packet in a frame suitable for that network and passes down to the physical layer.

IPPacket

IPPacket

DLT

DLH

4. The physical layer of the port convert the frame to signals and send them out.

2. The network layer passes it down to the data link layer of the port.

Layer 3 Switches and Routers in Site Internets

Layer 3 switchesswitch IP packetsrather than Ethernetframes.

Based on IP addresses instead of MAC addresses

Router

To OtherSites

Layer 3Switch

EthernetSwitch

EthernetSwitch

Layer 3 Switches and Routers in Site Internets

Router

To OtherSites

Layer 3Switch

EthernetSwitch

EthernetSwitch

However, they areusually limited to IP and perhaps IPX routing.

Also, they rarely have WAN interfaces ornon-Ethernet LAN ports.

Other Connecting Devices

Repeaters Layer 1 devices Used to extend the distance of a physical

link Regenerate and transmit signals

Bridges Data link (Layer 2) devices Connect two or more network segments

that use the same data link layer protocols and addresses

Slower than switches Not commonly used now

Media Access Control

Collisions occur when two computers on the same circuit transmit at the same timeMedia access control prevents or recover from collisions

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

1. Carrier Sense Multiple Access (CSMA) If a NIC wishes to transmit, it must

listen for traffic If there is no traffic, the NIC may transmit If there is traffic, the NIC must wait until

the transmission is finished When the transmitting station finishes

transmitting, the NIC listens for other stations transmitting before it transmits; if others are transmitting, it must wait until they are finished too

CSMA/CD

2. Collision Detection (CD) If there is a collision (by two or more

stations transmitting at the same time), All NICs stop transmitting and wait for a

random amount of time

The first NIC that finishes its wait my transmit

but only if there is no traffic!

If there is traffic, the NIC must wait until there is no traffic

CSMA/CD

3. Collision Detection (CD)

If there are multiple collisions,

The random wait is increased each time

After 16 collisions, the sending NIC discards the frame

Network Operating Systems

Connects all machines and peripherals on a network into an interactive wholeCoordinates and controls the functions of machines and peripherals across the networkSupports security and privacy for both the network and the individual usersControls access to resources on a user authentication basisAdvertises and manages resources from a centralized directoryGives the ability to share resources such as printers, files, and Internet access

Popular Network Operating Systems

Microsoft Windows Server NT server 2000 server, 2000 Advanced Server, 2000

Datacenter Server .NET server

Linux Many distributions Free, open-source

Novell NetWare Once dominated the NOS market IPX/SPX protocol

Popular Network Operating Systems

MicrosoftWindows

LINUX Novell NetWare

Ease of Learning

Very Good Poor Good

Ease of Use Very Good Poor Good

ReliabilityVery Good for recent versions

Excellent Very Good

Standardization

Availability ofDevice Drivers

Purchase Price

Excellent

Excellent

Moderate

Poor (ManyDistributions)

Poor

Low or Free

Excellent

Very Good

Higher thanWindows

ManagementLabor

Moderate HighHigher than

Windows

The Best Practice LAN Design

Effective Data Rates

The effective data rate is the maximum practical speed hardware layers can expect to provide. It depends on 4 factors: nominal data rate (e.g., 10Mbps for 10BaseT) error rate, since this determines the frame

retransmission rate efficiency of the data link protocol which

depends on the percentage of the transmission devoted to overhead

efficiency of the media access control protocol meaning how effective is the protocol at making use of the nominal data rate.

Media Access Control Protocol Efficiency

Shared and switched Ethernet differ in their MAC protocols. Shared Ethernet is very sensitive to network traffic levelsShared Ethernet network capacity is effectively limited to just under 50% of capacity At 97% data link protocol efficiency, 10BaseT can carry 0.97 x 0.5 x 10mbps = 4.85Mbps In a low traffic environment with only two active

users on a network, this would correspond to 2.5 Mbps/user

In moderate traffic, with 10 users, this would mean only 500 kbps/user

Eff. Data Rates for Switched Ethernet

Switched Ethernet dramatically improves performance since each computer appears to have its own dedicated circuit and collisions and congestion are no longer a problem.Experts believe switched Ethernet users can effective utilize 95% of network capacity.For a 10BaseT switched LAN, each computer would have an effective capacity = 0.97 x 0.95 x 10Mbps ~ 9 Mbps.For 100BaseT switched LAN, each computer would have an effective capacity = 0.97 x 0.95 x 100Mbps ~ 92Mbps.

Eff. Data Rates for Gigabit Ethernet

Gigabit Ethernet is typically implemented in a full-duplex switched environment.This means it provides 1 Gbps in both directions at once.This makes the effective data rate for sending data in one direction is 0.97 x 0.95 x 1 Gbps ~ 900 MbpsSince 1GbE is implemented as a full-duplex network, traffic can be sent simultaneously in both directions, so the effective capacity is really double this value or 1.8 Gbps.

1.       

Technology Low Traffic

Moderate Traffic

High Traffic

Shared 10BaseT 2.5 Mbps 1 Mbps 500 kbps

Shared 100BaseT 37.5 Mbps 15 Mbps 7.5 Mbps

Switched 10BaseT 9 Mbps 9 Mbps 9 Mbps

Switched 100BaseT 90 Mbps 90 Mbps 90 Mbps

Full Duplex 1 GbE 1.8 Gbps 1.8 Gbps 1.8 Gbps

Full Duplex 10 GbE 18 Gbps 18 Gbps 18 Gbps

Effective Data Rate per User

Assumptions: 1. Most frames are 1500 bytes or larger2. No transmission errors occur3. Low traffic means 2 active users, moderate traffic means 5 active users, high traffic means 10 active users

LAN Recommendations

Since network traffic almost always increase, the best practice design is for the worst case. For most networks, switched 10/100BaseT

is the best option For networks with high traffic levels,

switched 100BaseT or 1 GbE over MMF is best.

In most LANs, the circuit to and from the server is the network bottleneck. The solution for this is to use a 10/100 switch and then connect the server using a 100Mbps or higher circuit.