1

Introduction

CSC474: Computer Networks Design & Configuration

A. Supanit Angsirikul

Rangsit University

1

Agenda

• Type of Networks
• OSI Model
• Network Devices
• Internet Model

2

Type of Networks

3

WAN

LAN

LAN

LAN

• Local-area networks (LANs) evolved around the PC revolution.
• Wide-area networks (WANs) interconnect LANs or PCs with �geographically dispersed �users to create connectivity

LAN: Local Area Network

4

hub

hub

hub

hub

1st floor

2nd floor

3rd floor

4th floor

router

• Under single management
• Share resources, e.g., printer, file server within organization
• Confined geographical area
• typically 2 km
• Involves
• Hub, Switch, Router
• High data rate
• 10 Mbps or more
• Example
• Ethernet
• Token Ring
• Wi-Fi (IEEE802.11b)

server

stations

stations

station

WAN: Wide Area Network

• Wide Area Networks (WANs)
• Over long distances
• Cross public area
• Lower speeds
• speeds up to a few Mbps possible, but around 50-100 Kbps more typical
• interconnect LANs or PCs with geographically dispersed �involves routers

5

WAN

LAN

LAN

WAN: Wide Area Network

• Example
• Leased lines
• Public Switched Telephone Network (PSTN)
• Public Switched Data Network (PSDN)
• X.25
• Frame Relay
• ATM
• Integrated Services Digital Network (ISDN)
• Wireless WAN
• GSM
• CDMA
• 3G

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WAN

LAN

LAN

Performance of Networks

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Range

Bandwidth (Mbps)

Latency (ms)

LAN

1-2 kms

10-1000

1-10

WAN

2-50 kms

1-600

10

Wireless LAN

0.15-1.5 km

2-11

5-20

Wireless WAN

worldwide

0.010-2

100-500

Networks and Components

• A physical network is a collection of communication devices: e.g.,
• Cables
• Routers
• Switches
• etc.
• Protocol Stacks
• Define functions must be performed for each type of network device
• Define common rules and format of data used to support each function

8

Communication functions

Functions required to achieve computer communications.

• User Interface
• Message formatting
• Addressing
• Error Control
• Medium Access Control
• Routing
• etc.

9

Network

OSI Model

• Researched and developed by the ISO - International Organization for Standardizations.
• 1977: establish a subcommittee to develop a communications architecture.
• 1984: publish ISO-7498, the Open System Interconnection (OSI) reference model
• The OSI Model define a set of layers and the services performed by each layer

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Transport

Data-Link

Network

Physical

Application

(Upper)

Layers

Session

Presentation

Application

Data Flow

Layers

Role of Application Layers

11

• Application Level request/reply protocol

• User interface

• How data is presented
• Special processing such as encryption

Telnet

FTP,HTTP,WWW

ASCII

EBCDIC

JPEG

RPC, RMI

Message Queuing

Transport

Data-Link

Network

Physical

Examples

Session

Presentation

Application

Role of Data Flow Layers

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TCP

UDP

SPX

802.3/802.2

HDLC

EIA/TIA-232�V.35

IP

IPX

Presentation

Application

Session

Examples

• Mux/Demux application data to available network
• Reliable or unreliable delivery

• Combines bits into bytes and �bytes into frames
• Access to media using MAC address
• Error detection, not correction

• Move bits between devices
• Specifies voltage, wire speed, and �pinout cables

Transport

Data-Link

Physical

Network

• Provide logical addressing that routers use for path determination

Network Devices

• Cables & Connectors (L1)
• Repeaters (L1)
• Hubs (L1)
• Network Interface Cards (L1,L2)
• Bridges (L1,L2)
• Switching Hubs (L1,L2)
• Routers (L1,L2,L3)
• Multi Layer Switches (L1,L2,L3)

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Layers 1: Physical Layer

Information Transmission

• Signal
• Coding
• Medium
• Mechanical (Connector)

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Jack

Plug

Repeaters

• A repeater:
• Amplifies and retiming a signal
• Extends the distance a signal may be run reliably over a cable
• Makes no decisions based on signal content

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Hubs

• Used as a concentrator to join multiple workstations with a single link to the rest of the LAN
• Functions as a multi-port repeater
• Signals received on any port are immediately retransmitted to all other ports on the hub.
• Increases the number of connectable devices
• Operates at the OSI Physical Layer

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Layers 2: Data Link Layer

Control the Information transfer over Physical Link

• Framing
• Synchronization
• Addressing
• Error Control
• Medium Access Control
• Etc.

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Framing, Addressing and Synchronization

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To identify

• Source End
• Destination End

To determine the starting of a frame

To determine the end of a frame

• Cyclic Redundancy Check (CRC)
• Parity Check

Medium Access Control

• How to share a common physical channel
• Time Division, e.g., PSTN, ISDN
• Packet Switching
• Contention, e.g. Ethernet
• Round Robin, e.g. Token Ring
• Reservation, e.g. WAN packet switching Network

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MAC: CSMA/CD (Ethernet/802.3)

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• Every node can receive a transmission by all other nodes
• need MAC address to identify a destination
• only destination copies frame to it, all other nodes have to discarded the frame

A finds the bus is free, send a frame

B find the bus is free, send its frame collide

A detect collision, A retransmit.

B detect collision, B retransmit.

Network Interface Cards (NICs)

• Each computer connected to a network needs at least one Network Interface Card (NIC)
• Operates at the Physical and Data Link�Layers of the ISO Reference Model
• Physical Layer:
• Cable, Connector Signal STD
• Data Link Layer:
• Frame Format, MAC STD
• Media-dependent (physical connector)
• Media Access Method-dependent (Ethernet vs Token Ring, etc.)
• L3 and upper protocols -independent

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NICs/HUBs Network

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A

D

D

10 BASE 2

A

D

D

D

Amplify &

Retime

B

D

B

D

X

X

L2 Bridges

• Operates at the L2 of the ISO Reference Model
• Filter/Route incoming L2 frame base on L2 address, e.g., MAC address
• For LAN, use to reduce the number of user that share a common medium (Collision Domain)

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Layer 2 Switching

• Usually, used to interconnect the network of the same Layer 2 protocol.
• Filtering and Routing frame based on Layer 2 address

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Network

Network

A

B

C

D

E

Switch

To A, B 🡪 Filter (Blocked)

To C 🡪 Switch to port 2

To D, E 🡪 Switch port 3

1

2

3

Different Network Standard

• Different Physical Layer
• Voltage/Current
• Signal/Coding
• Connector
• Etc.

• Different Data-Link Layer
• Framing
• Addressing
• MAC
• Etc.

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A

A

B

1

2

2

?

B

Routers

• Creates a single virtual network on top of different kinds of data link Network.

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Layer 3: Network Layer Protocol

• Creates a single virtual network on top of different kinds of physical and data link layers.

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Token Ring

Frame Relay

Ethernet

LAN

Virtual network

Routing

• Route decision based on Layer 3 (Logical) address.
• When there are more than one path to the destination

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Virtual network

Layer 4: Transport Protocol

• Interface with the underlying network
• if application messages are too big for network layer, segment them and reassemble at the receiving end
• multiplex multiple application connections over one network connection, if possible, to efficiently use network bandwidth
• provide multiple network connections for one application connection (if higher bandwidth needed than what one network connection can deliver)
• Example
• TCP Connection Oriented
• UDP Connectionless

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Transport

User

User

User

User

Network

mux

Inv mux

Layer 5: Session Protocol

• Establishes, manages and terminate sessions between application
• Support session oriented traffic, e.g.,
• Request/Reply Protocol
• Synchronicity
• Re-sync. after failures
• Examples
• Connection Oriented
• RPC, RMI
• Connectionless
• Message Queuing

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Transport

Request

Reply

Application

Presentation

Session

Application

Presentation

Session

Layer 6: Presentation Protocol

• Ensure the data is readable by receiving systems
• Problem: different computers represent data in different formats
• Example: representation of unsigned short integer 1 in 2 bytes
• “big-endian” (e.g., Motorola 680x0) 0000000000000001
• “little-endian” (e.g., Intel 80x86) 1000000000000000

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Transport

Request

Reply

Application

Presentation

Session

Application

Presentation

Session

Layer 7: Application Layer

• Provide services that support the various types of distributed applications
• OSI protocols
• electronic mail (X.400, almost entirely extinct these days)
• name/directory services (X.500, some residual interest and some implementations)
• Internet protocols
• Include Presentation & Session Layers
• SMTP (simple mail transfer protocol)
• FTP (file transfer)
• telnet (remote login)
• http (hypertext transfer protocol)

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Transport

Application

Presentation

Session

Application

Presentation

Session

Peer-to-peer communications

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Encapsulation

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TCP/IP model development

• The late-60s The Defense Advance Research Projects Agency (DARPA) originally developed Transmission Control Protocol/Internet Protocol (TCP/IP) to interconnect various defense department computer networks.
• The Internet, an International Wide Area Network, uses TCP/IP to connect networks across the world.
• Layer 4: Application
• Layer 3: Transport
• Layer 2: Internet
• Layer 1: Network access

​

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A comparison of Internet and OSI protocol

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Physical

Data Link

Network

Transport

Session

Presentation

Application

Network

Access

Internet

Transport

Application

• SMTP
• FTP
• TELNET
• etc.

OSI

Internet

hardware

firmware

User

space

software

OS

space