Local Area Networks

LAN Overview roadmap

• Why a LAN?• How does a computer attach to a LAN?• What’s the difference between switch and

a hub?• How is a host configured?• What’s an OS and a NOS?• Network Computing Models: Mainframe,

File Server, Client/Server, Peer to Peer.

Why a Local Area Network?

• Aggregate users of common resources• Share access to:

– Internet– file, database, application or compute servers– printers

• Share Resources - Share Costs - Save Money

• Share information - Save Time and Increase Productivity

Why a Local Area Network?Workstation

EthernetHub

Shared 10MbpsShared 10Mbps DedicatedDedicated 100 Mbps100 Mbps

Internet orCorporate Intranet

EthernetSwitch

Router

MultimediaWorkstation

Server

Printer

DedicatedDedicated 100 Mbps100 Mbps

Local Area Networks

“Just as computer networks have grown across continents and oceans to interconnect major computing facilities around the world, they are now growing down corridors and between buildings to interconnect minicomputers in offices and labs.”

Robert M. Metcalfe, Xerox, 1976

How does a computer attach to a LAN?

Networking OperatingSoftware/System (NOS)Coordinates access to LAN,provides software interface for PC applications

Wiring HubServes as center of network,contains multiple independentbut connected modules where network equipment can be connected

ServerLarge disk storesfiles and databases,Fast processor computes on demand

Network Interface Card (NIC)Amplifies electronic signals, packages data for transmission, and controls access to the network cable

WireCat 5 UTP

PersonalComputerAccesses LAN resources

How does a computer attach to a LAN?

• Computer with appropriate interface bus. • NIC (Network Interface Card)• NOS (Network Operating System)• Cable (normally Cat 5 UTP for 10BaseT)• Ethernet Hub or Switch• Server or peer

Your LAN connected computer

• Computer needs appropriate bus for NIC• for example, if it’s a PC:

– ISA (Industry Standard Architecture) is passe– PCI (Intel’s Peripheral Component Interconnect) is

modern– PC-Card (used to be called PCMCIA) for laptops.– Parallel printer port ( for old laptops).– Serial port (generally for external modem)– USB (Universal Serial Bus) is newest approach

The Network Interface Card (NIC)

Network Connector Port

PC or Workstationloaded with OS and NOS

Network Interface Card (NIC)

• NIC are the I/O device for NOS• Also known as LAN adapters• NICs Amplifies electronic signals • Packages data for transmission• Physically connects computer to transmission media (cable)• Associated with the NIC is a

unique address called the MAC (Media Access Control) address

• OSI Layers 1 and 2

The Network Interface Card

• Today, NIC generally means Ethernet NIC– Could be 10BaseT, 100BaseT if more modern, 10 or

100BaseF if fiber, or 1000BaseT - GigE

• may do speed/duplex autonegotiation– 10/100 Mbps, Full/Half Duplex

• may offload some upper layer protocol processing from CPU– checksumming and CRC (Cyclic Redundancy Check)

calculation

– automatic decision to accept (in promiscous or sniffing mode) or reject (normally) packets not addressed for this workstation

– multicast address recognition

The Wire

• Today, generally Category 5 UTP (Unshielded Twisted Pair)– 4 pairs of copper wire, twisted and bundled to tight

specification

– properly installed, provides 100-350 Mhz signalling bandwidth.

– Ethernet and Fast (100 Mbps) Ethernet use two of four pairs.

• Cat 5 Ethernet uses “RJ-45” connectors• In older installations, could also be Cat 3, 10Base2 (“thin-

net”) or even 10Base5 (“thicknet”)• In newer installations: Cat 5e, Cat 6, or even Category 7.

Ethernet hub or switch

• Switch is a multi-port bridge– provides dedicated bandwidth to each port– may provide VLANS, multicast services

• Hub– fine for small LANs (under about 6 people)– shared bandwidth

• Hubs normally require no configuration• Sophisticated switches need administration

Hub

HubHub: Device that serves as the center of a star-topology network, sometimes referred to as a multiport electrical repeater, or in Ethernet, a wire concentrator; not intelligent

Hubs: A Single Bus

1 single Segment

123

124

125

126

127

128

Hub

1 single Segment

Hub Summary

• OSI physical Layer• Amplifies signals• Propagates signals through the network

– Must observe 4 repeater diameter limit

• No filtering• No path determination or switching• Used as network concentration point• Single physical Collision Domain

• BridgeBridge: – Very simple device that connects and passes

packets between two or three network segments

– Bridge is intelligent -- analyzes incoming packets and forwards or drops based on MAC Layer 2 addressing information.

– Two kinds of bridging methods primarily--

• Transparent Bridging (Ethernet)

• Source-route Bridging (used in token-ring environments)

Bridge: The Switch’s Predecessor

Bridge Example

BRIDGE

Segment 1Segment 2

123

124

125

126

127

128

Learning

0260.8c01.1111

0260.8c01.2222 0260.8c01.3333

0260.8c01.4444

E0: 0260.8c01.1111

E0: 0260.8c01.2222

E1: 0260.8c01.3333

E1: 0260.8c01.4444

•Source MAC addresses are associated with Ethernet ports

Port Eth. 0 Port Eth.1

Bridge Learning

• The source MAC address of all frames received is compared against the information in the forwarding database.

• If the source MAC address is not found in the forwarding database, it is added along with the port identifier it received on.

• The age value of this database entry is reset to indicate that this is a "fresh" entry

• If the MAC address was last seen on a different bridge port, then the port identifier for the entry is changed to the new port identifier, and the age value of this entry is reset.

• If the frame is received on the same port as the port in the database entry, then only the age value of this entry is reset.

Forwarding

Port Eth. 0 Port Eth.1

0260.8c01.1111

0260.8c01.2222 0260.8c01.3333

0260.8c01.4444

E0: 0260.8c01.1111

E0: 0260.8c01.2222

E1: 0260.8c01.3333

E1: 0260.8c01.4444

•Traffic propagated to specific destinations

Filtering, Dropping

0260.8c01.1111

0260.8c01.2222 0260.8c01.3333

0260.8c01.4444

E0: 0260.8c01.1111

E0: 0260.8c01.2222

E1: 0260.8c01.3333

E1: 0260.8c01.4444

E0: 0260.8c01.2222

•Congestion control through network knowledge

XX

Port Eth. 0 Port Eth.1

Spanning Tree Removes Loops

B Root

B10

10

B10

10

B10

10

B10

9

• Problem: a frame gets flooded or forwarded in both directions around a loop

• Solution: makes one of the ports inactive until needed for redundancy

• 1) elect root• 2) cheapest “port to root”

for every bridge• 3) cheapest bridge (designated)

for every segment• 4) block more

expensive,unused port

Cost

Segment 1

Segment 2

Segment 3

Segment 4

- Denotes blocked port

Switch? Hub? What’s the difference?

Hub Gives Shared Bandwidth

Switch GivesDedicated Bandwidth

Switches

• Use bridging technology to forward traffic between ports;

• Provides full dedicated data transmission rate between two stations that are directly connected to the switch ports

• Builds and maintains address tables called content addressable memory (CAMs)

• CAMs match MAC addresses to make rapid forwarding decisions

Switches are better than Bridges

BRIDGE

Segment 1 Segment 2

10 MbpsCoaxial Cable

“shared”

Workstation

31

32

33

34

35

36

Switches are better than Bridges

SWITCH

N Segments N Segments

10 MbpsUTP Cable

“dedicated”

Workstation

31

32

33

34

35

36

Switch Advantages

• Packets switched over dedicated links • Users experience better performance• LAN switches permits users to transmit simultaneously. • Assigning users bandwidth based on need.

– Some users use 10 megabit ports and NICs

– Some users and servers get 100 megabit ports

• Full Duplex transmission possible

The Big Picture Workstation

EthernetHub

Shared 10MbpsShared 10Mbps DedicatedDedicated 100 Mbps100 Mbps

Internet orCorporate Intranet

EthernetSwitch

Router

MultimediaWorkstation

Server

Printer

DedicatedDedicated 100 Mbps100 Mbps

Operating Systems and Network Operating Systems

The Operating System ties together all the computer’s hardware

OS HW

HWHW

HW HWHW

HW

HW

HW

HW

HW

HW

OS Definition

• The central control program on a computer that manages all aspects of the computer's hardware and controls the execution of software operations

• Collection of programs which provide the computer with capabilities and functionality not specifically supported in the computer's hardware.

• Usually linked closely to a vendor-specific computer architecture, called a platform. Examples include DOS, OS/2, Apple Mac OS, UNIX and Microsoft Windows 95/98 and Windows NT.

DriverDriver

DriverDriver

Driver

The OS coordinates all this

Monitor

Mouse,Keyboard

OS

Scanner

NIC

Fax/Modem

OS Features

• An OS:– Makes devices available to applications via

software– Allows users to interact with applications by

using these devices

• Examples of devices:– Input: mouse, keyboard, tablets, scanners– Output: printer, plotter, fax modem, phone line,

monitor, HD, Backup Tapes, video camera.

Network Operating System (NOS)• If OS is the software that

manages the resources of the computer, then

• NOS is the software that manages the resources of the network

• NOS models:– Peer-to-peer model– Server Based model

PCs, hub/Switch, printerservers, routers

Server Based NOS

• MS Windows NT (TCP/IP mainly)• Novell’s Netware (IPX and TCP/IP)• Banyan Vines• IBM’s LAN Server

Peer Peer Peer

NOS Services

• Usually, NOS services are provided by one or more centralized servers

• Services often administered by the IS department

• File Service

• Data Base Service

• Print Service

• Messaging Service

• Directory Service

• Communication Service

O/S and NOS

• Modern Operating Systems have built in network drivers (OSI compliant stacks)

• Network Operating Systems (NOS) support some form of network-oriented client/server application: – Novell Netware for PC’s (Client/Server)– Appletalk for Apple Macintoshes– NFS for UNIX systems– TCP/IP, NetBEUI, DLC, etc.. Systems under MS-OS

OS and NOS Examples - UNIX

• First OS with integrated networking, based on IP

• Unix services were inspiration for most other NOS vendors– NFS (Network File System) for file serving– LPR (line printer) for printer sharing

• TCP/IP and UDP, etc. standard on all UNIX platforms• Examples of UNIX OS:

– LINUX, HP-UX, IBM AIX, DEC Ultrix, SunOS, Solaris, SCO UNIX, BSD4.3, System5.4

• Supported NICs: Ethernet, Token-Ring, Fast Ethernet, Gigabit-Ethernet, ATM

O/S and NOS Examples :

MS-DOS/Windows 95/98 • Most popular O/S for 32-bit Intel compatible PC’s • Market share over 90%• Initially without network capabilities

– Networking only possible with additional software: Novell Netware, Microsoft LAN Manager

• Enhanced version Windows95/98, integrate networking capabilities– NetBEUI, TCP/IP, Novell IPX ODI Protocol, IBM DLC,

Microsoft DLC, Banyan VINES protocol, ATM Call Manager, etc..

O/S and NOS Examples :Windows NT (Now improved into Windows 2000)• A 32-bit preemptive multitasking, multithreaded, multiprocessing

operating system• Developed by Microsoft with the same w95/98 GUI• Initially deployed on Intel PC architecture, then ported to Motorola

PowerPC, Digital Alpha AXP, and MIPS R4000 platforms • Brings Windows closer to UNIX performance• Two level approach: Windows NT Server 4.0 and Windows NT

Workstation• Extended HTML/Java Services• NOS: mainly TCP/IP based• All Kind of NIC cards and protocols supported

LAN software and client configuration

• Windows Networking, pre 2000, minimizes configuration, but can’t be routed globally.– Workstations learn about neighbors by

broadcasting identity

• TCP/IP networking must be explicitly configured– Allows global connectivity from PC Host– Dynamic Host Configuration Protocol (DHCP)

solves configuration problem

• Windows 2000 seeks best of both worlds

Windows Networking (before Windows 2000)

• Workstation can be locally administered– Don’t need (or get) globally recognized name or number

(like an IP address). – NetBIOS (used by Windows) is not a routable protocol

• Windows broadcasts your computer’s name so others can locate it.

• If necessary, locates a Domain Controller to authenticate username and password

• Uses WINS server (Windows Internet Name Service) to resolve other workstation names

TCP/IP host configuration• Need to configure computer with:

– IP address: 207.140.138.120– Subnet size or “mask”: 255.255.255.0– Default router IP address: 207.140.138.1

• use this gateway to access hosts outside this subnet

– DNS (Domain Name Server) IP address• can be located outside local subnet

– Domain name: lucent.com or sales.lucent.com• for locating local machines, domain name is added• for example, vail.lucent.com could be called “vail”

TCP/IP host configuration - DHCP

• Configuration is complicated, so DHCP (Dynamic Host Configuration Protocol) was invented– Client broadcasts DHCP requests

– DHCP server responds with:• Unique IP address for client to use• correct subnet mask• default router, DNS server, Domain name• Other services: WINS very common

– DHCP provides client with everything needed.

Windows 2000 Networking

• Uses TCP/IP, but with a twist– Tries to maintain Windows “ease of use”– DHCP to configure workstations– Dynamic DNS to name workstations

• Active Directory powerful way to replicate data between servers.

The path to Client/Server

Mainframe Architecture

File Sharing Architecture

Client Server Architecture

Mainframe architecture• Not a client/server architecture. • All intelligence is within the central host

computer• Users interact with the host through a

terminal • Terminals send keystrokes directly to the

host• User interaction can be done using PCs and

UNIX workstations• Older mainframe software architectures do

not easily support graphical user interfaces (GUI)

File sharing architecture • Not a client/server architecture• The original PC networks were based on file sharing

architectures• The server downloads files from the shared location

to the desktop environment• The requested user job is then run (including logic

and data) in the desktop environment• Limitations: works if shared usage is low, update

contention is low, and the volume of data to be transferred is low.

• In the 1990s, PC LAN computing changed because the capacity of the file sharing was strained as the number of online users grew and graphical user interfaces (GUIs) became popular

• PCs are now being used in client/server architectures

Client/Server architecture

• As a result of the limitations of file sharing architectures, the client/server architecture emerged

• Database server replaces the file server• Using a relational database management system

(DBMS), user queries could be answered directly• Reduces network traffic by providing a query

response rather than total file transfer • Improves multi-user updating through a GUI front

end to a shared database• Remote Procedure Calls (RPCs) or Structured

Query Language (SQL) statements are typically used to communicate between the client and server

Client/Server The future

• Two tier architectures– good solution 10 to 100 people interacting on a LAN

simultaneously by maintaining a connection via "keep-alive" messages with each client, even when no work is being done

• Three tier architectures (multi-tier architecture)– middle tier was added between the client and the database

management server environment. Good for large number of users (in the thousands)

• Distributed/Collaborative Enterprise Architectures– emerged in 1993 Based in Object Request Broker (ORB)

and Common Object Request Broker Architecture (CORBA)– Based on Object Oriented data bases (C++, SmallTalk,

JAVA)

The Client/Server Basics

The Client• Application sitting in a PC or workstation,

along with its associated software, actually operated by an end user.

• Some important attributes of the client include: – the applications software installed on the

computer; – the client's user interface, which provides a

method for interacting with the server (back office).

– the computer's processor, memory, and networking options.

The Server

• Back end Application (called daemon in Unix) installed in a central PC, workstation, minicomputer or large computing system

• Provides a central service to user stations on a Local Area Network or across the WAN

• Typical examples of these centralized services are:– Data bases– File storage (file servers), – Printer sharing (print servers), – Centralized application software storage/processing

(application servers), – and Wide Area Network access (communication

servers).

Compact Definition

• Client server is referring to a model which divides applications in two parts:– Client or front-end: the software process used

by the end-user– Server or back-end: the software process

created by the actual application running in the server

• On a machine you can find multiple clients and multiple servers at the same time

An Example: Checking Bank account• Authentication process for security

• The client program in your computer forwards your request to an intermediate server program at the bank

• That program may in turn forward the request to its own client program that sends a request to a database server at another bank computer to retrieve your account balance.

• The balance is returned back to the bank data client, which in turn serves it back to the client in your personal computer, which displays the information for you.

Client/Server and Internet

• Web browser is a client program that requests services (the sending of Web pages or files) from a Web server (which technically is called a Hypertext Transport Protocol or HTTP server) in another computer somewhere on the Internet.

• Your PC with TCP/IP installed allows you to make client requests for files from File Transfer Protocol (FTP) servers in other computers on the Internet.

Drivers To adopt Client/Server

• TCO Model - Total Cost of Ownership is lower• Central high performance is cheaper than

distributed performance.– One fast printer is cheaper than 10 slow– One large fast file server is cheaper than 100 medium

sized disks in PC’s– One fast computer server is cheaper than 100

medium-fast PC’s or workstations (TCO)

SAP PeopleSoftOracle

Financial

Server Based Model

• All communications centers around the server– Client-to-server– Server-to-server

• Client is a user of services, Server is a provider• Clients do not directly communicate with other

clients • 85% of the marketplace• Servers are a high-powered micro, mini or

mainframe computer

Server Based Pros and Cons

• Pros:– Scalable– Ease of management and security– Single point of administration and maintenance– Easy to to provide fault-tolerant services

• Redundant power supply, mirrored servers, back-up– Advanced services

• Cons:– Difficult to install– Requires staff expertise

Peer-to-Peer Model

• Allows communication between any two devices in the LAN

• Good for small networks environment• Pros:

– Flexible– Each workstation can provide services to other devices– Easy to install and run– Basic services: file sharing, printer sharing, e-mail

services

• Cons:– Difficult to administer when network begins to grow– Do not offer a central point of administration

Peer-to-Peer

• Apple Talk• Windows 95/98 using NetBEUI• Novell Personal Netware• Napster!

Peer Peer Peer

Napster is a big Peer-to-Peer network

• Other Internet-wide file sharing services are coming too– Freenet, Gnutella, FileFury

• All of Internet becomes as easy to access as the LAN.

• Directory Servers automate administration• Capitalizes and insists on high speed

Internet and WAN connections.• Servers on WAN seem as “local” as the

LAN.

The Future of Local Area Networks

• Today, LAN is gateway to Internet and WAN

• Wireless, IP telephony latest in a series of important LAN drivers

• Ethernet (and soon wireless) only way to go

• Fast Ethernet and Gigabit Ethernet are easy upgrade paths for older networks.

• Client/Server will continue to evolve– Agents, Java, B2B, B2C.

• Web-based computing paradigm keeps older networks, computers viable while driving new application and software innovation.

Food for Thought

• What are the advantages or disadvantages of a Web server/client architecture?

• What’s the difference between the mainframe/terminal relationship, the client/server relationship, and the web server/web browser relationship?