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Standards

Chapter 2 (Revised August 2002)

Copyright 2003 Prentice-HallPanko’s Business Data Networks and Telecommunications, 4th edition

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Warning: Difficult Material

The most difficult chapter in the book

Abstract and unfamiliar concepts

Concepts are highly interrelated

Will require especially diligent study

Must be mastered for you to do well in the rest of the course

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Figure 2.1: Standards Govern Communication

MessageMessageClient PC Server

MessageMessage MessageMessage

Standards Typically Focus on Message Exchanges:

Message Format (Syntax)

Message Sequencing (Responses follow Requests)

Message Semantics: Meanings of Values in Specific Fields

Definition: Standards are rules of operation that govern communication between two (or more) hardware or software processes on different machines.

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Figure 2.2: Internet Protocol (IP) Packet

Version(4 bits)

HeaderLength(4 bits)

Diff-Serv (8 bits)

Total Length(16 bits)

Identification (16 bits)Flags

(3 bits) Fragment Offset

(13 bits)

Protocol (8 bits)1=ICMP, 6=TCP,

17=UDP

Time to Live(8 bits)

Header Checksum (16 bits)

Bit 0 Bit 31Figure Shows 32 Bits on Each Line

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Figure 2.2: Internet Protocol (IP) Packet

 Figure 2.2: Internet Protocol (IP) Packet

 

Source IP Address (32 bits)

Data Field (dozens, hundreds, or thousands of bits)

Destination IP Address (32 bits)

PaddingOptions (if any)

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Horizontal Layered Message Communication

MessageMessageClient PC Server

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Figure 2.3: Message Communication in TCP/IP

ApplicationLayer Proc Proc = Process

HTTP Msg ApplicationLayer Proc

TransportLayer Proc

TCP Msg TransportLayer Proc

InternetLayer Proc

IP Packet InternetLayer Proc

InternetLayer Proc

Client PC ServerEthernet Switch Router

Data LinkLayer Proc

EthFrame

Data LinkLayer Proc

Data LinkLayer Proc

Data LinkLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

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Figure 2.3: Message Communication in TCP/IP

ApplicationLayer Proc

Proc = ProcessHTTP Msg Application

Layer Proc

Client PC ServerEthernet Switch Router

Browser WebserverApplicationProgram

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Figure 2.4: Layer Purposes

The Application Layer

The purpose of the application layer is to allow two application programs on different hosts to work together.

When a browser talks to a webserver application program, the standard for communication is the Hypertext Transfer Protocol (HTTP). This is why website URLs begin with HTTP://.

Other application layer services use different application layer standards

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TransportLayer Proc

TransportLayer Proc TCP MsgTCP Msg Transport

Layer Proc

Host-to-Host CommunicationHTTP Requires TCP

At the Transport Layer

Client PC ServerEthernet Switch Router

Figure 2.3: Message Communication in TCP/IP

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Figure 2.4: Layer Purposes

The Transport Layer

The purpose of the transport layer is to allow two host computers to talk to one another even if they have very different internal designs, such as a PC and a workstation server.

If you use HTTP at the application layer, you are required to use the Transmission Control Protocol (TCP) at the transport layer.

Other applications require different transport layer standards

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Figure 2.6: Physical, Data Link, and Internet Layer Transmission

Network X

Network Z Network Y

Switches

Routers

Switches

RouteNetwork Y

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Figure 2.3: Message Communication in TCP/IP

InternetLayer Proc

IP Packet InternetLayer Proc

InternetLayer Proc

Hop-by-Hop Communication Across an InternetHost-Router-Router-…Router-Host

Client PC ServerEthernet Switch Router

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Figure 2.4: Layer Purposes

The Internet Layer

The purpose of the internet layer is to route packets from the source host to the destination host across one or more networks connected by routers.

TCP requires the use of the Internet Protocol (IP) at the internet layer.

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Figure 2.3: Message Communication in TCP/IP

Data LinkLayer Proc

EthFrame

Data LinkLayer Proc

Data LinkLayer Proc

Data LinkLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

PhysicalLayer Proc

Hop-by-Hop Transmission Across One NetworkStation-Switch-Switch-…-Switch-Station

Propagation Across a Single Wire, Optical Fiber, or Radio Connection

Client PC ServerEthernet Switch Router

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Figure 2.6: Physical, Data Link, and Internet Layer Transmission

Network X

Network Z Network Y

Switches

Routers

Switches

Data Link

Note: There are 3 Data Links. One for each network traversed

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Figure 2.4: Layer Purposes

The Data Link Layer

The purpose of the data link layer is to govern the movement of messages from a source station to a destination station or router across a single network containing switches.

If the client station is located on an Ethernet LAN, the Ethernet data link layer standard is used.

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Figure 2.6: Physical, Data Link, and Internet Layer Transmission

Network X

Network Z Network Y

Switches

Routers

Switches

Physical Links

Note: There are 7 Physical Links

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Figure 2.4: Layer Purposes

The Physical Layer

The purpose of the physical layer is to govern the transmission of bits one at a time over a wire, radio, or other connection between a station and a switch, between pairs of switches, or between a switch and a router.

For a station on an Ethernet LAN, an Ethernet physical layer standard will be used. (Ethernet offers multiple physical layer standards.)

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Figure 2.4: Layer Purposes

The Physical Layer

TransmissionMedia

Connectors

Voltage levelsTo represent 1s and 0s

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Figure 2.5: Protocols in Two Examples

HTTP POP

Physical

Layer

Data Link

Internet

Transport

Application

TCP

IP

Ethernet

Ethernet

Example 1: Internet WebAccess from a LAN

Modem (V.92)

PPP (Point-to-Point Protocol)

IP

TCP

Example 2: DownloadingInternet E-Mail over a Telephone Line and Modem

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Layers 1-3: Closer Look

Internet Layer

Data Link Layer

Physical Layer

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Figure 2.7: Comparing the Physical, Data Link, and Internet Layers

Layer

Messagesare Called

Physical Data Link Internet

None: Bit-by-BitTransmission

Frame Packet

Switch RouterRepeater (Hub)ConnectingDevice

2 31Device*Layer

*Devices are defined by their highest layer of operation. They also operate on lower layers.

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Each station on an Ethernet network

has a 48-bit network address

Frame carries IP packet in its data field

like a truck carrying a package

Figure 2.8: Ethernet MAC Layer Frame

 

Field

Preamble (56 bits)10101010 …

Start of Frame Delimiter (8 bits)10101011

Destination MAC Address (48 bits)

Source MAC Address (48 bits)

PAD (if needed)

Data Field (variable) Contains IP Packet

Length (16 bits)

Frame Check Sequence (32 bits)

LLC Header

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Figure 2.7: Comparing the Physical, Data Link, and Internet Layers

Data Link InternetPhysicalLayer

Format Conversion None

Switches convertbetween differentphysical layerconnections fordifferent ports

UTP Optical Fiber

SwitchClient PC Server

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Figure 2.7: Comparing the Physical, Data Link, and Internet Layers

Data Link InternetPhysicalLayer

Format Conversion None

Routers convertbetween differentnetworks—different physical and data link layer standards

EthernetNetwork

ATMNetwork

Router

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Figure 2.10: All Switches in a Network and All Routers in an Internet Must Follow the Same Standard

Network 1 (Ethernet)

Network 3 (ATM)

Client PC Ethernet Switch

Ethernet Switch Ethernet Switch

Router (IP)

Router (IP)

ATM SwitchATM SwitchServer

Network 2

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Vertical Layered Communicationin a Single Host

Internet Layer

Data Link Layer

Physical Layer

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Figure 2.11: Vertical Communication on the Source Host

IP PacketData Link Process

Host A

Internet Process

Physical Process

IP Packet

DL-T DL-HIP Packet

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Figure 2.11: Vertical Communication on the Source Host

Internet Layer Process Creates an IP packet

Passes the packet down to the data link layer process

Data Link Layer Process Creates a new frame

Places (encapsulates) the IP packet in the data field of the frame, adding a frame header and perhaps a trailer

Passes frame down to the physical layer process

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Encapsulation

Encapsulation is placing a message in the data field of another message.

Data LinkLayerTrailer

IP Packet in Data FieldOf the Frame

Frame

Data LinkLayer

Header

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Figure 2.14: Vertical Communication on the Destination Host (Host B)

IP Packet

IP Packet

DL-T IP Packet DL-H

Internet Process

Data Link Process

Physical Process

Host B

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Figure 2.14: Vertical Communication on the Destination Host (Host B)

Physical Layer Process Converts the signal into bits of the frame Passes the frame up to the data link layer process

Data Link Layer Process Checks the data link layer header (and, if present,

trailer)

Decapsulates the IP packet

Passes the packet up to the internet layer process

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Figure 2.12: Vertical Communication on Switch X1

Host A Switch X2

A B

Frame Frame

Switch X1

Port 1PHY

Port 2PHY

Port 3PHY

Port 4PHY

Data Link Layer Process

1 2 3 4

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Figure 2.13: Vertical Communication on Router R1

Port 1 DL

PHY

Internet Layer Process

Port 2DL

Port 3DL

Port 4DL

PHY PHY PHY

Router R1

Switch X2

Router R1 receives frame from Switch X2 in Port 1.Port 1 DL Process decapsulates packet.Port 1 DL passes packet to internet process.

IP Packet

IP Packet

DL-T IP Packet DL-H

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Figure 2.13: Vertical Communication on Router R1

PHY

Port 4DL

Port 1DL

PHY

Internet Layer Process

Port 2DL

Port 3DL

PHY PHY

Router R1

Router 2

Internet process sends packet out on Port 4.DL Process on Port 4 encapsulates packet in frame.DL Process passes frame to Port 4 PHY.PHY Process sends the bits out as signals to Router 2

IP Packet

IP Packet

DL-T IP Packet DL-H

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Figure 2.15: Transport and Application Layer Standards

Notes: Transport standard can connect computers of different types.Transport standard often is reliable (corrects errors)

Transport Layer

App 1 App 2 App 3 App 4

Client PC

Network or Internet

Server

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Figure 2.15: Transport and Application Layer Standards

Notes: Application standard links specific pairs of applications on different multitasking hosts.

Application LayerApp 1 App 2 App 3 App 4

Client PC

Network or Internet

Server

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Figure 2.16: Communication at All Layers on the Source Host

HTTPMsg

ApplicationProcess

TCPHdr

HTTPMsg

TransportProcess

Application process creates an HTTP message.

Passes HTTP Msg down to next-lower layer (Transport).

Transport process encapsulates HTTP message within a TCP message (called a segment) by adding a TCP header.

TCP Message(TCP Segment)

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Figure 2.16: Communication at All Layers on the Source Host

HTTPMsg

ApplicationProcess

TCPHdr

HTTPMsg

TransportProcess

TCPHdr

IPHdr

HTTPMsg

InternetProcess

Passes HTTP message (Msg) down to next-lower layer (Internet)

Internet process encapsulates TCP segment within an IP packet by adding an IP header

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Figure 2.16: Communication at All Layers on the Source Host

HTTPMsg

ApplicationProcess

TCPHdr

HTTPMsg

TransportProcess

TCPHdr

IPHdr

HTTPMsg

InternetProcess

DLTrlr

TCPHdr

IPHdr

DLHdr

HTTPMsg

Data LinkProcess

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Figure 2.16: Communication at All Layers on the Source Host

Test Your Understanding

You are on an Ethernet LAN.

Your computer wishes to send an SNMP message (Simple Network Management Protocol).

SNMP requires the use of UDP at the transport layer

UDP uses IP for delivery

From beginning to end, name the headers, messages, and trailers you will see in the final frame

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Figure 2.16: Communication at All Layers on the Source Host

Test Your Understanding

Your host’s internet layer process sends an ICMP message to another host.

ICMP is an internet layer standard. ICMP messages are encapsulated in IP packets

You are using a V.90 telephone modem for Internet access. Modems use PPP at the data link layer.

From beginning to end, name the headers, messages, and trailers you will see in the final frame

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Figure 2.16: Communication at All Layers on the Source Host

HTTPMsg

ApplicationProcess

TCPHdr

HTTPMsg

TransportProcess

TCPHdr

IPHdr

HTTPMsg

InternetProcess

DLTrlr

TCPHdr

IPHdr

DLHdr

HTTPMsg

Data LinkProcess

PhysicalProcess

Physical layer process converts the frame’s bits into signals and sends them out.

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Figure 2.16: Communication at All Layers on the Source Host

HTTPMsg

ApplicationProcess

TCPHdr

HTTPMsg

TransportProcess

TCPHdr

IPHdr

HTTPMsg

InternetProcess

DLTrlr

TCPHdr

IPHdr

DLHdr

HTTPMsg

Data LinkProcess

Final FrameFor HTTP Msg

Delivery

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Figure 2.16: Communication at All Layers on the Source Host

TCPHdr

TransportProcess

TCPHdr

IPHdr

InternetProcess

DLTrlr

TCPHdr

IPHdr

DLHdr

Data LinkProcess

Layered Communication for TCP Supervisory message Delivery

Final MessageFor TCP

SupervisoryMessage

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Figure 2.17: TCP/IP, OSI, and TCP/IP-OSI Hybrid Architecture

Presentation

TCP/IP OSI Hybrid TCP/IP-OSI

Application Application

Application

Session

Transport

Internet

Transport

Network

Transport

Internet

Data Link

Physical

Data Link

Physical

Use OSI StandardsHere

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Figure 2.17: TCP/IP, OSI, and TCP/IP-OSI Hybrid Architecture

The Hybrid TCP/IP-OSI Architecture is used on the Internet and dominates internal corporate networks.

The standards agencies for the OSI architecture are ISO and ITU-T.

The standards agency for TCP/IP is the IETF. Most IETF documents are called requests for comments (RFCs).

Some RFCs—but not all—are Internet Official Protocol Standards.

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Key Point

The most common standards pattern in organizations is to use OSI standards at the physical and data link layers and TCP/IP standards at the internet, transport, and application layers.

This is very important for you to keep in mind because this hybrid TCP/IP–OSI standards architecture will form the basis for most of this book.

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Figure 2.18: OSI Session Layer (5)

Transport Layer

Client PC

Network or Internet

Server

App 1 App 2 App 3 App 4Session Layer (Layer 5)

Manages Series of TransactionsBetween Applications

Over a Transport Connection

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Figure 2.18: OSI Session Layer (5)

OSI Very useful for applications that need to manage

exchanges of application messages closely

Few applications need this, however

TCP/IP Applications must manage application message

exchanges by themselves.

No general support in the architecture

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Figure 2.19: OSI Presentation Layer (6)

Presentation Layer(Transfer Syntax C)App 2

Internal Syntax AApp 3

Internal Syntax B

The presentation layer governs theSyntax of messages

Hosts have different data representations, etc.Agree upon a transfer syntax for messages going between them

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Figure 2.18: OSI Presentation Layer (6)

OSI Very useful because it handles differences in data

formatting at a general level. Frees application programs from handling data formatting differences

TCP/IP Each application must manage presentation

differences by themselves

MIME message description standards help by letting receiver know the type of file contained in a message

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Other Standards Architectures

IPX/SPX Novell NetWare file servers

NetBEUI Small LANs with older Microsoft servers

SNA Mainframe computers

AppleTalk AppleTalk

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Mixing Packets from Different Architectures

All architectures produce packets that can be carried in frames. Frames do not care what packets they carry, and frames containing packets from different standards architectures can mix freely on a network.

IP Packet IPX Packet

Ethernet Frames in an Ethernet Network

New: Not in Book

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Figure 2.20: Other Standards Architecture

TCP/IP IPX/SPX NetBEUI

Application NetBIOSVariousNCP

Transport

Internet

Uses OSIStandards Here

SPX

IPX

Uses OSIStandards Here

Uses OSIStandards Here

No Internet Layer

NetBEUI

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Figure 2.20: Other Standards Architecture

OSI SNA AppleTalk

ApplicationNo Application

Layer**Uses OSI

Layering butproprietary

protocols at eachlayer above the

physical and datalink layers

NetworkAddressableUnit (NAU)***

Services

Presentation

Session Data Flow Control

TransportTransmission

Control

Network Path Control

Uses OSIStandards Here

Uses OSIStandards Here

Data Link

Physical

TCP/IP

Application

Transport

Internet

Uses OSIStandards Here

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Key Point

Although this book will focus on OSI at the lower layers and TCP/IP at the upper layer because of the dominance of this combination, real organizations use multiple standards architectures at higher layers (TCP/IP, IPX/SPX, SNA, AppleTalk, Net BEUI, etc).

Physical and Data Link Layers: OSINearly Absolute Dominance

Upper Layers: TCP/IPDominant but Not Absolute

Upper:IPX/SPX

Upper:SNA