OSI Model - SRM Institute of Science and Technology4).pdf · Open Systems Interconnection (OSI) reference model is the result of this effort. In 1984, the Open Systems Interconnection

OSI Model

UNIT‐I

TCP/IP Protocol Suite 2

OSI MODEL

Communication ArchitectureStrategy for connecting host computers and othercommunicating equipment.

Defines necessary elements for data communicationbetween devices.

A communication architecture, therefore, defines astandard for the communicating hosts.

A programmer formats data in a manner defined by thecommunication architecture and passes it on to thecommunication software.

Separating communication functions adds flexibility, forexample, we do not need to modify the entire host softwareto include more communication devices.

OSI Model

Layer ArchitectureLayer architecture simplifies the network design.

It is easy to debug network applications in a layeredarchitecture network.

The network management is easier due to the layeredarchitecture.

Network layers follow a set of rules, called protocol.

The protocol defines the format of the data beingexchanged, and the control and timing for the handshakebetween layers.

OSI Model

Open Systems Interconnection (OSI) Model

International standard organization (ISO) established acommittee in 1977 to develop an architecture for computercommunication.

Open Systems Interconnection (OSI) reference model is theresult of this effort.

In 1984, the Open Systems Interconnection (OSI) referencemodel was approved as an international standard forcommunications architecture.

Term “open” denotes the ability to connect any twosystems which conform to the reference model andassociated standards.

OSI Model

OSI Reference Model

The OSI model is now considered the primary Architecturalmodel for inter‐computer communications.

The OSI model describes how information or data makes itsway from application programmes (such as spreadsheets)through a network medium (such as wire) to anotherapplication programme located on another network.

The OSI reference model divides the problem of movinginformation between computers over a network mediuminto SEVEN smaller and more manageable problems .

This separation into smaller more manageable functions isknown as layering.

OSI Model

OSI Reference Model: 7 Layers

OSI Model

Chapter Outline

3.1 Wired Local Area Network

3.2 Wireless LANs

3.3 Point-to-Point WANs

3.4 Switched WANs

3.5 Connecting Devices

3-1 WIRED LOCAL AREA NETWORKS

A local area network (LAN) is a computer network that isdesigned for a limited geographic area such as abuilding or a campus. Although a LAN can be used asan isolated network to connect computers in anorganization for the sole purpose of sharing resources,most LANs today are also linked to a wide area network(WAN) or the Internet.

The LAN market has seen several technologiessuch as Ethernet, token ring, token bus, FDDI, and ATMLAN, but Ethernet is by far the dominant technology.

Topics Discussed in the Section

IEEE Standards

Frame Format

Addressing

Ethernet Evolution

Standard Ethernet

Fast Ethernet

Gigabit Ethernet

Ten‐Gigabit Ethernet

Figure 3.1 IEEE standard for LANs

Figure 3.2 Ethernet Frame

Figure 3.3 Maximum and minimum lengths

Minimum length: 64 bytes (512 bits)

Maximum length: 1518 bytes (12,144 bits)

Figure 3.4 Ethernet address in hexadecimal notation

Figure 3.5 Unicast and multicast addresses

The broadcast destination address is a special case of the multicast address

in which all bits are 1s.

The least significant bit of the first byte defines the type of address.

If the bit is 0, the address is unicast; otherwise, it is multicast.

Define the type of the following destination addresses:a. 4A:30:10:21:10:1Ab. 47:20:1B:2E:08:EEc. FF:FF:FF:FF:FF:FF

SolutionTo find the type of the address, we need to look at the secondhexadecimal digit from the left. If it is even, the address is unicast. If itis odd, the address is multicast. If all digits are F’s, the address isbroadcast. Therefore, we have the following:a. This is a unicast address because A in binary is 1010 (even).b. This is a multicast address because 7 in binary is 0111 (odd).c. This is a broadcast address because all digits are F’s.

Example 3.1

Show how the address 47:20:1B:2E:08:EE is sent out on line.

SolutionThe address is sent left-to-right, byte by byte; for each byte, it issent right-to-left, bit by bit, as shown below:

Example 3.2

← 11100010 00000100 11011000 01110100 00010000 01110111

Figure 3.6 Ethernet evolution through four generations

• Carrier Sense• Listen to the wire is anybody talking

• No – transmit

• Yes – say nothing– Try again later

• Multiple Access• Possible for two to hear nothing and transmit at the same time

• Possible collision

• What to do when a collision occurs• Shutup

• Wait a while

• Try again

CSMA/CD ‐ Carrier Sense Multiple Access with Collision Detection

Figure 3.7 Space/time model of a collision in CSMA

T ime T ime

BA C D

Figure 3.8 Collision of the first bit in CSMA/CD

In the standard Ethernet, if the maximum propagation time is25.6 μs, what is the minimum size of the frame?

SolutionThe frame transmission time is Tfr = 2 × Tp = 51.2 μs. Thismeans, in the worst case, a station needs to transmit for aperiod of 51.2 μs to detect the collision. The minimum size ofthe frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This isactually the minimum size of the frame for Standard Ethernet,as we discussed before.

Example 3.3

Figure 3.9 CSMA/CD flow diagram

Figure 3.10 Standard Ethernet implementation

STP – Shielded twisted pair (copper)Fiber – Fiber optic cable (glass)UTP – Unshielded twisted pair (copper)

Figure 3.11 Fast Ethernet implementation

In the full-duplex mode of Gigabit Ethernet, there is no collision;

the maximum length of the cable is determined by the signal attenuation

in the cable.

Figure 3.12 Gigabit Ethernet implementation

Pin No. conductor color Name

1 white and orange TX_D1+

2 orange TX_D1‐

3 white and green RX_D2+

4 blue BI_D3+ **

5 white and blue BI_D3‐ **

6 green RX_D2‐

7 white and brown BI_D4+ **

8 brown BI_D4‐ **

Ethernet Pin OutPC to Hub

Ethernet Crossover CablePC to PC

Current Technology

• Used to be:• Wrong cable – no communications

• Now:• Ethernet adapters sense the cabling and auto configure themselves

• Auto‐MDIX

3-2 WIRELESS LANS

Wireless communication is one of the fastestgrowing technologies. The demand for connectingdevices without the use of cables is increasingeverywhere. Wireless LANs can be found on collegecampuses, in office buildings, and in many publicareas. In this section, we concentrate on twowireless technologies for LANs: IEEE 802.11wireless LANs, sometimes called wireless Ethernet,and Bluetooth, a technology for small wireless LANs.

IEEE 802.1

MAC Sublayer

Addressing Mechanism

Bluetooth

Figure 3.13 Basic service sets (BSSs)

Figure 3.14 Extended service sets (ESSs)

• Carrier Sense• Listen to the wire is anybody talking

• No – transmit

• Yes – say nothing– Try again later

• Multiple Access• Possible for two to hear nothing and transmit at the same time

• Possible collision

• How to avoid a collision (politeness)• I want to talk

• Ok talk

• Everyone else shut up

CSMA/CA ‐ Carrier Sense Multiple Access with Collision Avoidance

Figure 3.15 CSMA/CA flow diagram

Figure 3.16 CSMA/CA and NAV

Figure 3.19 Hidden station problem

The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

Figure 3.20 Use of handshaking to prevent hidden station problem

3-3 POINT-TO-POINT WANS

A second type of network we encounter in theInternet is the point-to-point wide area network. Apoint-to-point WAN connects two remote devicesusing a line available from a public network such asa telephone network. We discuss traditional modemtechnology, DSL line, cable modem, T-lines, andSONET.

65K Modems

DSL Technology

Cable Modem

T Lines

Figure 3.26 56K modem

ADSL is an asymmetric communication technology designed for residential users; it

is not suitable for businesses.

Figure 3.27 Bandwidth division

Figure 3.28 ADSL and DSLAM

DSLAM – Digital subscriber line access multiplexer ADSL ‐ Asymmetric digital subscriber line

Figure 3.29 Cable bandwidth

Figure 3.30 Cable modem configuration

Figure 3.31 PPP frame

3-4 SWITCHED WANS

The backbone networks in the Internet can beswitched WANs. A switched WAN is a wide areanetwork that covers a large area (a state or acountry) and provides access at several points to theusers. Inside the network, there is a mesh of point-to-point networks that connects switches. Theswitches, multiple port connectors, allow theconnection of several inputs and outputs.

Switched WAN technology differs from LANtechnology in many ways.

3-5 CONNECTING DEVICES

LANs or WANs do not normally operate in isolation.They are connected to one another or to theInternet. To connect LANs and WANs together weuse connecting devices. Connecting devices canoperate in different layers of the Internet model. Wediscuss three kinds of connecting devices: repeaters(or hubs), bridges (or two-layer switches), androuters (or three-layer switches).

Repeaters

Bridges

Routers

Figure 3.40 Connecting devices

Figure 3.41 Repeater or hub

A repeater forwards every bit; it has no filtering capability.

A bridge has a table used in filtering decisions.

A bridge does not change the physical (MAC) addresses in a frame.

Figure 3.42 Bridge

Figure 3.43 Learning bridge

M MM M

A router is a three-layer (physical, data link, and network) device.

A repeater or a bridge connects segments of a LAN.

A router connects independent LANs or WANs to create an internetwork (internet).

Figure 3.44 Routing example

A router changes the physical addresses in a packet.

OSI Model - SRM Institute of Science and Technology4).pdf · Open Systems Interconnection (OSI) reference model is the result of this effort. In 1984, the Open Systems Interconnection

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