1

Basic Communications Basic Communications ModelModelStandards are needed at all Standards are needed at all LayersLayers

User Layer

Application Layer

Computer (Transport) Layer

Transmission Layer

2

Subnet Subnet LayerLayer

1

2

3

Station A Station B

Transmission of Messages1. Within a Single Subnet, or

2. in Point-to-Point Links Between Subnets

4

1 2

3

Internet LayerInternet LayerStation A Station B

Routing of MessagesAcross multiple subnets in an internet

1

2

34

4

InternettingInternettingStation A Station B

internet: collection of subnets such that any station on any subnet can communicate with any other station on any other subnet simply by giving the

internet address of the other station.

1

2

34

5

Layering in Major Layering in Major ArchitecturesArchitectures

Basic Model

Application

Computer(Transport)

Internet

TCP/IP

Internet

IPX/SPX

Internet

Subnet(OSI)

OSI

Application (7)

Presentation (6)

Session (5)

Transport (4)

Network (3):Internet &

Subnet

Data Link (2)

Application Application

Transport:TCP, etc.

Complex:SPX, etc.

Physical (1)

Subnet(OSI)

Subnet(OSI)

6

OSI OSI ArchitectureArchitecture

Reference Model for Open Reference Model for Open Systems Interconnection (OSI)Systems Interconnection (OSI)

Standards AgenciesStandards Agencies• ISO (computers)ISO (computers)• Don’t confuse OSI and ISO!Don’t confuse OSI and ISO!• ITU-T (telecommunications)ITU-T (telecommunications)

De jure standards De jure standards (official/open)(official/open)

7

OSI OSI ArchitectureArchitecture

Standards are sophisticatedStandards are sophisticated Sophistication brings complexitySophistication brings complexity

• Slow standards developmentSlow standards development

• Products are late to market and Products are late to market and expensiveexpensive

• The result: low acceptance of OSI The result: low acceptance of OSI standards in generalstandards in general

But OSI dominates at the single network But OSI dominates at the single network (subnet) transmission layer(subnet) transmission layer

8

Physical Layer (OSI Physical Layer (OSI Layer 1)Layer 1)

Physical transmission standardsPhysical transmission standards PhysicalPhysical

• ConnectorConnector• Transmission mediaTransmission media

ElectricalElectrical• Voltage levelsVoltage levels• Meanings of various voltage levelsMeanings of various voltage levels

9

The Data Link Layer (OSI The Data Link Layer (OSI Layer 2)Layer 2)

10

Data Link Layer (OSI Data Link Layer (OSI Layer 2)Layer 2)

Organize data for transmission over a data linkOrganize data for transmission over a data link• Data packages at various layers are called protocol Data packages at various layers are called protocol

data units (PDUs)data units (PDUs)• The Data Link Layer PDU is called a frameThe Data Link Layer PDU is called a frame• Organization allows addressing on shared data linksOrganization allows addressing on shared data links• Organization allows error checking and other meta Organization allows error checking and other meta

delivery processesdelivery processes Media Access ControlMedia Access Control

• Controls when stations may transmit in shared Controls when stations may transmit in shared media LANs and other shared data linksmedia LANs and other shared data links

11

TCP/IP ArchitectureTCP/IP Architecture

Internet Engineering Task Force Internet Engineering Task Force (IETF)(IETF)• Of the Internet SocietyOf the Internet Society

De jureDe jure (official/open) (official/open)

12

TCP/IP ArchitectureTCP/IP Architecture

Standards are simpleStandards are simple• Often have “Simple” in the nameOften have “Simple” in the name• Standards and products are fast to marketStandards and products are fast to market• Products are inexpensive and fast to marketProducts are inexpensive and fast to market

Widely UsedWidely Used• On the InternetOn the Internet• In many corporate networksIn many corporate networks

• Dominates at internet and transport Dominates at internet and transport layerslayers

13

IPX/SPXIPX/SPX

Novell CorporationNovell Corporation De facto (proprietary/closed)De facto (proprietary/closed) Used only in Novell NetWare PC Used only in Novell NetWare PC

networksnetworks• But NetWare dominates PC But NetWare dominates PC

networkingnetworking• Second most widely used architecture Second most widely used architecture

at transport layerat transport layer

14

SNASNA

Systems Network ArchitectureSystems Network Architecture IBM CorporationIBM Corporation De facto (proprietary/closed)De facto (proprietary/closed) Dominates in mainframe Dominates in mainframe

communicationcommunication• On IBM mainframe systemsOn IBM mainframe systems• On mainframe systems of other On mainframe systems of other

vendorsvendors

15

Transport-Layer Transport-Layer TrafficTraffic

TCP/IPTCP/IP 38%38% RisingRising IPX/SPX (NetWare)IPX/SPX (NetWare) 34%34% FallingFalling

SNA (Mainframes)SNA (Mainframes) 18%18% Other (including OSI)Other (including OSI) 10% 10% TotalTotal 100%100%

Source: Molloy (1994), 1993 data

16

Transmission Transmission SpeedsSpeeds

Bit: single 1 or 0Bit: single 1 or 0 Transmission speed is measured in Transmission speed is measured in

bits per second (bps)bits per second (bps)• Duration of a single bitDuration of a single bit• Not velocity of propagationNot velocity of propagation

101101000011101010100101111010101

17

Notations for Transmission Notations for Transmission SpeedSpeed

Prefix Use Metric Meaning Example Interpretation

kilo kbps 1,000 64 kbps 64,000 bps

Mega Mbps 1 million 1.544 Mbps 1,544,000 bps

Giga Gbps 1billion 2.4 Gbps 2,400,000,000 bps

Tera Tbps 1 trillion 2 Tbps 2,000,000,000,000

Note upper case T

18

LANs, MANs, & LANs, MANs, & WANsWANs

LANLAN• Local area networkLocal area network• Single office, building, campusSingle office, building, campus• 10 Mbps to 100 Mbps to the desktop 10 Mbps to 100 Mbps to the desktop

commoncommon• 1 Gbps coming1 Gbps coming

Will carry most traffic, because Will carry most traffic, because most traffic is localmost traffic is local

19

PC Networking on a small PC Networking on a small LANLAN

Network InterfaceCard (NIC)

In each PC

Each client and server needs a NIC rather than a modem

20

PC Networking on a small PC Networking on a small LANLAN

Wiring Hub15 to 50 cm (6 to 19 inches)

Usually, a wiring hub connects the computers together

Telephone wiring

and RJ-45 plugs

21

LANs, MANs, & LANs, MANs, & WANsWANs

WANWAN• Wide area networkWide area network• Intercity, internationalIntercity, international• 9,600 bps to 1 Mbps common to the desktop9,600 bps to 1 Mbps common to the desktop• Links with higher speed are usually shared Links with higher speed are usually shared

(multiplexed) by several desktops(multiplexed) by several desktops Emerged before LANs, due to high cost Emerged before LANs, due to high cost

of long-distance telephone chargesof long-distance telephone charges