MD02 Network Models Network Concepts

1.1

CNT 3004

Module 2

Chapter 2

Network Models

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 2

NetworkModels

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.4

Note on Chapter 2

Chapter 2 (Network Models) in the 5th Edition has been reduced significantly and many details given in the 4th

Edition have been deleted and migrated to other chapters. Some of these chapters are not covered in CNT 3004 but the information, diagrams and examples on Network Models migrated to these chapters are needed in CNT 3004.

The slides for Chapter 2 presented in CNT 3004 reflect the new presentation approach of the 5th Edition but also retain the essential information given in the 4th

Edition.

We use the concept oflayers in our daily life.

As an example, let usconsider two friends whocommunicate throughpostal mail.

The process of sending aletter to a friend would becomplex if there were noservices available from thepost office.

LAYERED TASKSA three-layer protocol (4th Ed Textbook)Sending postal mail from Maria to Ann

Maria Ann

This slide is for information only.

2.6

A three-layer protocol (5th Ed textbook) Sending mail from Maria to Ann

Postal carrier facility

This slide is for information only.

THE OSI MODEL

Established in 1947, the International StandardsOrganization (ISO) is a multinational body dedicated toworldwide agreement on international standards.

An ISO standard that covers all aspects of networkcommunications is the Open Systems Interconnection(OSI) model.

OSI was first introduced in the late 1970s.

ISO is the organization.OSI is the model.

2.8

Seven layers of the OSI model

The OSI model is composed of seven ordered layers as shown in the figure.

Within a single machine, each layer calls upon the services of the layer below it. For example, the Network layer uses the services of the Data Link layer and provides services for the Transport layer.

The 7-layer OSI model has not succeeded in practice. The 5-layer TCP/IP Protocol Suite is the

communication model widely used in practice.

TCP/IP PROTOCOL SUITE

The layers in the TCP/IP protocol suite do not exactlymatch those in the OSI model. Two layers in the OSImodel (Presentation layer and Session Layer) do notexist in the TCP/IP Protocol Suite.

The TCP/IP protocol suite is made of five layers:physical, data link, network, transport, and application.The next slide shows these five layers with some of theprotocols (or applications) that are used in each layer.

TCP/IP Protocol Suite

An exchange using the OSI model

Each layer in the sending device adds its own information to the message it receives from the layer just above it, then passes the whole package to the layer just below it. The added information in each layer takes the form of a header and possibly a trailer.

Starting from layer 7, the data unit moves down from layer to layer. When the data unit reaches layer 1 (physical layer),it is converted to an electromagnetic or a radio signal and transported along the wired or wireless transmission medium.

Seven layers of the OSI model

• The passing of data down through the layers of the sending device and back up through the layers of the receiving device is accomplished by an interface between each pair of adjacent layers.

• As long as a layer provides the expected services to the layer above it, implementation of the functions within this layer can be modified without requiring changes to the surrounding layers. This is called modular design.

• Between machines, layer X on the receiving machine is the only layer that can understand and process the header added by layer X on the sending machine. This is called peer-to-peer communication (see figure on next slide).

• At the receiving machine, the message is unwrapped layer by layer, with each layer removing the header and possibly the trailer created by its peer layer.

• As the message travels from the sender to the receiver, it may pass through many intermediate nodes. These nodes are usually routers and involve only the first three layers of the OSI model.

The interaction between layers in the OSI model

2.14

Logical Connections between Layers in the TCP/IP Protocol Suite

Logical connections

� The logical connection between layer X on the receiving machine and layer X on the sending machine is similar to the OSI peer-to-peer communication between these two layers.

2.15

Identical Objects in the TCP/IP Protocol Suite

Identical objects (messages)

Identical objects (segment or user datagram)

Identical objects (datagram)

Identical objects (frame)

Identical objects (bits)

Identical objects (datagram)

Identical objects (frame)

Identical objects (bits)

packet packet

The layer in the source machine and the corresponding layer in thedestination machine use identical objects. For example the two datalinklayers use data frames with identical format.

Description of Each Layer

In this section we describe the functions of each layer inthe communication stack of the TCP/IP Protocol Suite.

Physical LayerData Link LayerNetwork LayerTransport LayerApplication Layer

Topics discussed in this section:

Physical Layer

The figure below shows the position of the physical layer with respect to the transmission medium and the data link layer.

The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

The physical Layer is concerned with the following:

• Representation of bits – the type of encoding (how 1s and 0s are changed to signals).

• Data Rate – the transmission rate – the number of bits sent each second.

• Link configuration – i.e. point-to-point dedicated link or multipoint shared link.

• Physical topology- how devices are connected to make a network (mesh, ring, bus, star, hybrid).

• Transmission mode – simplex, half-duplex, full-duplex.

Physical Layer

Data Link LayerThe Data Link Layer adds a header containing the physical (MAC) addresses of the source node and destination node. It also adds a trailer for bit error detection. The MAC address of a node is of length 48 bits (12 hexadecimal digits) and is the address assigned to the Network Interface Card of the node.

The data link layer is responsible for moving frames from one hop (node) to the next.

Responsibilities of the data link layer:

• Framing: divides the stream of bits received from the network layer into manageable units called frames.

• Flow control: if the data sending rate is more than the rate at which the data are absorbed by the receiver, the data link layer imposes a flow control to avoid overwhelming the receiver.

• Error Control : The data link layer uses a mechanism to detect and retransmit damaged or lost frames. Error control is usually achieved by adding a trailer to the end of the frame.

• Access Control:when multiple devices are connected to the same link, the data link medium access control (MAC) protocol is used to determine which device has control over the link (i.e., can transmit).

Data Link Layer

Hop-to-hop Delivery

To send data from node A to node F, three frame deliveries are made. First, A sends a frame to B (router). Second, B sends a new frame to E. Finally E sends a new frame to F.

The frames sent by the three nodes A, B and E have different source and destination address values in the header.

Specifically the destination addresses in these frames are B, E and F, respectively.

Network Layer

The network layer (IP Protocol) is responsible for the delivery of individual packets from

the source host to the destination host.

Source-to-destination delivery

The network layer is particularly useful when the source node and destination node are attached to different networks. In this case, the network layer ensures that each packet is routed correctly from its point of origin to its final destination.

Logical Addressing: the network layer of the sender adds a header containing the logical IP addresses(of length 32 bits) for both the sender and receiver nodes.Routing: the network layer of the connecting devices (called routers or switches) route or switch the packets to their final destination.

Transport Layer

The transport layer is responsible for the delivery of a message from one process to another.

From application layer To application layer

Process-to-process delivery of a message

Two popular transport protocols are: UDP and TCP

Responsibilities of the transport layer:

• Process-to-process delivery:the transport layer header includes a service-point address, called the port number, that ensures the correct delivery to a specific process running on the destination computer. The network layer gets each packet to the correct computer and the transport layer delivers whole messages (collection of ordered data segments) to the correct application process.

• Connection Control: in the connection-oriented (TCP) mode, the transport layer handles connection establishment and termination.

• Error Control : like the data link layer, the transport layer uses a mechanism to detect and retransmit damaged or lost segments. Error control is done on an end-to-end basis and not across each single link.

• Flow Control: like the data link layer, the transport layer is responsible for flow control, which is done on an end-to-end basis and not across each single link.

Transport Layer

Application Layer

The application layer is responsible for providing a variety of services to the user.

HTTP HTTPFTP FTPSMTP SMTP

From transport layerTo transport layer

ADDRESSING

Four levels of addresses are used in an internet employingthe TCP/IP protocols:─ physical or MAC address (48 bits)─ logical or IP address (32 bits)─ port address (16 bits)─ specific address (application dependent)

Addresses in TCP/IP

Relationship of layers and addresses in TCP/IP

In the figure on the next slide, a node with physicaladdress 10 sends a frame to a node with physical address87.

The two nodes are connected by a link (bus topologyLAN).

Note: the values 10 and 87 are used as simplified MAC addresses inthis example. Real MAC addresses consisting of 48 bits do not havesmall values like 10 or 87.

Example 1

Physical addresses

As the figure shows, the computer with physicaladdress 10 is the sender, and the computer withphysical address 87 is the receiver.

Most local-area networks use a 48-bit (6-byte) physicaladdress written as 12 hexadecimal digits; every byte (2hexadecimal digits) is separated by a colon, as shownbelow:

Example 2

04:01:02:01:2C:4BA 6-byte (12 hexadecimal digits) physical address.

The physical address of a computer is the address of its network interface card, which is a unique address assigned bye the manufacturer.

Exercise: represent the least significant byte (hexadecimal value 4B) in binary and in ASCII ( American Standard Code for Information Interchange).

Preamble SDDestination

Address

Source

AddressLength Information Pad FCS

6 bytes 6 bytes 4 bytes

Example: Ethernet uses 48-bit physical addresses

• Preamble: a sequences of seven octets that repeats the bit pattern 10101010 .

• SD: start delimiter equal to the bit pattern 10101011.

• Destination Address: physical address of 48 bitsidentifying the station or stations that are to receive the frame. It can be a unicast, multicast, or broadcast address.

• Source Address: physical address of 48 bits identifying the station that originated the frame. It must be a unicast address.

• Length: indicates the number of bytes in the information field.

• Information field: carries the data payload.

• Pad: ensures that the frame size is at least 64 bytes.

• FCS: the frame check sum is used for error checking .

Ethernet Frame Format

2.34

The hardware component that connects a computer to a network is called the network interface card (NIC), also known as the network interface controller, network adapter, or LAN adapter. Because of Ethernet popularity, most computers use Ethernet NICs for network connectivity.

In the past, Ethernet NICs were commonly implemented on expansion cards that plug into a computer bus, but most newer computers have a network interface built into the motherboard.

Network Interface Card (NIC)

Network Interface Card

Ethernet hardware addresses are 48 bits, expressed as 12 hexadecimal digits. For example

00-C0-4F-78-9A-BC

The most significant 6 digitscorrespond to the vendor and the least significant 6 digitsspecify the interface serial number for that vendor.

The most significant 24 bits (6 hexadecimal digits) of an EthernetMAC (physical) address correspond to the vendor code.

Use your browser to view the Ethernet vendor codes posted on thefollowing web page

http://standards.ieee.org/develop/regauth/oui/oui.txt

Notice that a large vendor needs multiple 24-bit codes. Forexample some of the codes assigned to Cisco are

009086 , 009092 , 0090A6

Exercise

The figure on the next slide shows a part of an internet with two routersconnecting three LANs.

Each device (computer or router) has a pair of addresses (logical andphysical) for each connection. The logical address is represented by aletter (e.g., A) and the physical address is represented by a number (e.g.,10). For example, the notation A/10 represents the two addresses of thesending machine.

In this case, each computer is connected to only one link and thereforehas only one pair of addresses.

Each router, however, is connected to three networks (only two areshown in the figure).

So each router has three pairs of addresses, one for each connection.

Example 3

An IPv4 address is a logical address of length 32 bits

The physical addresses change from hop to hop, but the logical addresses remain the same from the source to destination.

IPv4 Packet Format

Source Address (32 bits): gives the IPv4 address of the source machine

Destination Address (32 bits): gives the IPv4 address of the destination

Example: IPv4 uses 32-bit logical addresses

IPv4 uses 32-bit logical addresses.

IPv6 uses 128-bit logical addresses.

In CNT 3004, we will assume IPv4 is used.

IPv4 Addresses� IP addresses are represented by a 32-bitunsigned binary value,

which is usually expressed in a dotted decimalformat, e.g., 193.205.80.5.

� The binary format of the 32-bit IP address 193.205.80.5 is:

193 . 205 . 80 . 511000001 .11001101. 01010000.00000101

� An easier way to remember IP addresses is by assigning a domain name to each address, e.g.,

www.ucf.eduwhich is resolved through the Domain Name System (DNS) to the IP address

132.170.0.0

TCP Header Format

Example: TCP and UDP use 16-bit Port addresses

The figure on the next slide shows two computers communicatingvia the Internet.

The sending computer (with IP address A) is running threeprocesses at this time with port addresses a, b, and c. Thereceiving computer (with IP address P) is running two processesat this time with port addresses j and k.

Process a in the sending computer needs to communicate withprocess j in the receiving computer.

Note that although the physical addresses change from hop tohop, logical and port addresses remain the same from the sourceto destination.

Example 4

Port addresses

Example 5

A port address is a 16-bit address represented by onedecimal number as shown below.

753

The above 16-bit port address is represented as one single number.