Application Presentation Session Transport Network Data-Link Physical THE OSI MODEL Where We’ve Been Chapter 1—Review By: Allan Johnson.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

THE OSI MODEL

Where We’ve Been

Chapter 1—Review

By: Allan Johnson

Table of Contents

• Review the OSI Model

• Encapsulation

• LAN Devices & Technologies

• Transport Layer

• IP Addressing

Why A Layered Model?• Reduces complexity• Standardizes

interfaces• Facilitates modular

engineering• Ensures interoperable

technology• Accelerates evolution• Simplifies teaching &

learning

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Application Layer Provides network services

(processes) to applications.

For example, a computer on a LAN can save files to a server using a network redirector supplied by NOSs like Novell.

Network redirectors allow applications like Word and Excel to “see” the network.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Presentation Layer Provides data

representation and code formatting.

Code formatting includes compression and encryption

Basically, the presentation layer is responsible for representing data so that the source and destination can communicate at the application layer.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Session Layer Provides inter-host

communication by establishing, maintaining, and terminating sessions.

Session uses dialog control and dialog separation to manage the session

Some Session protocols: NFS (Network File System) SQL (Structured Query

Language) RCP (Remote Call Procedure) ASP (AppleTalk Session

Protocol) SCP (Session Control Protocol) X-window

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Transport Layer Provides reliability, flow control,

and error correction through the use of TCP.

TCP segments the data, adding a header with control information for sequencing and acknowledging packets received.

The segment header also includes source and destination ports for upper-layer applications

TCP is connection-oriented and uses windowing.

UDP is connectionless. UDP does not acknowledge the receipt of packets.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Network Layer Responsible for logically

addressing the packet and path determination.

Addressing is done through routed protocols such as IP, IPX, AppleTalk, and DECnet.

Path Selection is done by using routing protocols such as RIP, IGRP, EIGRP, OSPF, and BGP.

Routers operate at the Network Layer

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Data-Link Layer Provides access to the media Handles error notification,

network topology issues, and physically addressing the frame.

Media Access Control through either... Deterministic—token passing Non-deterministic—broadcast

topology (collision domains)

Important concept: CSMA/CD

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Physical Layer Provides electrical,

mechanical, procedural and functional means for activating and maintaining links between systems.

Includes the medium through which bits flow. Media can be... CAT 5 cable Coaxial cable Fiber Optics cable The atmosphere

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Application

Presentation

Session

Transport

Network

Data-Link

Physical

THE OSI MODEL

Encapsulation

Peer-to-Peer Communications

Table of Contents

Peer-to-Peer Communications• Peers communicate using the PDU of their

layer. For example, the network layers of the source and destination are peers and use packets to communicate with each other.

Application Application

Presentation Presentation

Session Session

Transport Transport

Network Network

Data-Link Data-Link

Physical Physical

Data

SegmentsPacketsFramesBits

DataData

Application

Presentation

Session

Transport

Network

Data-Link

Physical

THE OSI MODEL

LAN Devices & TechnologiesThe Data-Link & Physical Layers

Table of Contents

Devices• What does it do?

Connects LAN segments;

Filters traffic based on MAC addresses; and

Separates collision domains based upon MAC addresses.

What layer device?

Devices• What does it do?

Since it is a multi-port bridge, it can also Connect LAN

segments; Filter traffic based

on MAC addresses; and

Separate collision domains

However, switches also offer full-duplex, dedicated bandwidth to segments or desktops.

What layer device?

Devices• What does it do?

Concentrates LAN connections from multiple devices into one location

Repeats the signal (a hub is a multi-port repeater)

What layer device?

Devices• What does it do?

Interconnects networks and provides broadcast control

Determines the path using a routing protocol or static route

Re-encapsulates the packet in the appropriate frame format and switches it out the interface

Uses logical addressing (i.e. IP addresses) to determine the path

What layer device?

Media Types

LAN Technologies

Three Most Common

Used Today in

Networking

Ethernet/802.3• Cable Specifications:

10Base2 Called Thinnet; uses coax Max. distance = 185 meters (almost 200)

10Base5 Called Thicknet; uses coax Max. distance = 500 meters

10BaseT Uses Twisted-pair Max. distance = 100 meters

10 means 10 Mbps

Ethernet/802.3• Ethernet is broadcast topology.

What does that mean? Every devices on the Ethernet segment sees

every frame. Frames are addressed with source and

destination ______ addresses. When a source does not know the destination

or wants to communicate with every device, it encapsulates the frame with a broadcast MAC address: FFFF.FFFF.FFFF

What is the main network traffic problem caused by Ethernet broadcast topologies?

Ethernet/802.3• Ethernet topologies are also shared

media.• That means media access is

controlled on a “first come, first serve” basis.

• This results in collisions between the data of two simultaneously transmitting devices.

• Collisions are resolved using what method?

Ethernet/802.3• CSMA/CD (Carrier Sense Multiple Access

with Collision Detection)• Describe how CSMA/CD works:

A node needing to transmit listens for activity on the media. If there is none, it transmits.

The node continue to listen. A collision is detected by a spike in voltage (a bit can only be a 0 or a 1--it cannot be a 2)

The node generates a jam signal to tell all devices to stop transmitting for a random amount of time (back-off algorithm).

When media is clear of any transmissions, the node can attempt to retransmit.

Address Resolution Protocol• In broadcast topologies, we need a way to

resolve unknown destination MAC addresses.

• ARP is protocol where the sending device sends out a broadcast ARP request which says, “What’s you MAC address?”

• If the destination exists on the same LAN segment as the source, then the destination replies with its MAC address.

• However, if the destination and source are separated by a router, the router will not forward the broadcast (an important function of routers). Instead the router replies with its own MAC address.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

THE OSI MODEL

Transport Layer

A Quick Review

Table of Contents

Transport Layer Functions• Synchronization of the connection

Three-way handshake

• Flow Control “Slow down, you’re overloading my

memory buffer!!”

• Reliability & Error Recovery Windowing: “How much data can I

send before getting an acknowledgement?”

Retransmission of lost or unacknowledged segments

Transport’s Two Protocols• TCP

Transmission Control Protocol

Connection-oriented

Acknowledgment & Retransmission of segments

Windowing Applications:

Email File Transfer E-Commerce

• UDP User Datagram

Protocol Connectionless No

Acknowledgements

Applications: Routing Protocols Streaming Audio Gaming Video

Conferencing

Application

Presentation

Session

Transport

Network

Data-Link

Physical

THE OSI MODEL

IP Addressing

Subnetting Review

Table of Contents

Logical Addressing• At the network layer, we use logical,

hierarchical addressing.• With Internet Protocol (IP), this address

is a 32-bit addressing scheme divided into four octets.

• Do you remember the classes 1st octet’s value? Class A: 1 - 126 Class B: 128 - 191 Class C: 192 - 223 Class D: 224 - 239 (multicasting) Class E: 240 - 255 (experimental)

Network vs. Host

N H H H

Class A: 27 = 126 networks; 224 > 16 million hosts

N N H H

Class B : 214 = 16,384 networks; 216 > 65,534 hosts

N N N H

Class C : 221 > 2 million networks; 28 = 254 hosts

Why Subnet?• Remember: we are usually dealing

with a broadcast topology.• Can you imagine what the network

traffic overhead would be like on a network with 254 hosts trying to discover each others MAC addresses?

• Subnetting allows us to segment LANs into logical broadcast domains called subnets, thereby improving network performance.

Stealing Bits• In order to subnet, we must steal or

“borrow” bits from the host portion on the IP address.

• First, we must to determine how many subnets we need and how many hosts per subnet.

• We do this through the power of 2 For example, I need 8 subnets from a Class C:

24 = 16 - 2 = 14 subnets Remember: we subtract 2 because these subnets are

not used How many host do we have?

It’s a Class C, so 4 bits are left: 24 = 16 - 2 = 14 hosts Remember: we subtract 2 because one address is the

subnet address and one is the broadcast address

Subnet Mask• We determine the subnet mask by adding up

the decimal value of the bits we borrowed.• In the previous Class C example, we borrowed

4 bits. Below is the host octet showing the bits we borrowed and their decimal values.

128 64 32 16 8 4 2 1

1 1 1 1

We add up the decimal value of these bits and get 240. That’s the last non-zero octet of our subnet mask.So our subnet mask is 255.255.255.240

Last Non-Zero Octet• Memorize this table. You should be able to:

Quickly calculate the last non-zero octet when given the number of bits borrowed.

Determine the number of bits borrowed given the last non-zero octet.

Determine the amount of bits left over for hosts and the number of host addresses available.

Bits Borrowed

Non-Zero Octet Hosts

2 192 623 224 304 240 145 248 66 252 2

CIDR Notation• Classless Interdomain Routing is a

method of representing an IP address and its subnet mask with a prefix.

• For example: 192.168.50.0/27• What do you think the 27 tells you?

27 is the number of 1 bits in the subnet mask. Therefore, 255.255.255.224

Also, you know 192 is a Class C, so we borrowed 3 bits!!

Finally, you know the magic number is 256 - 224 = 32, so the first useable subnet address is 197.168.50.32!!

• Let’s see the power of CIDR notation.

202.151.37.0/26• Subnet mask?

255.255.255.192• Bits borrowed?

Class C so 2 bits borrowed• Magic Number?

256 - 192 = 64• First useable subnet address?

202.151.37.64• Third useable subnet address?

64 + 64 + 64 = 192, so 202.151.37.192

198.53.67.0/30• Subnet mask?

255.255.255.252• Bits borrowed?

Class C so 6 bits borrowed• Magic Number?

256 - 252 = 4• Third useable subnet address?

4 + 4 + 4 = 12, so 198.53.67.12• Second subnet’s broadcast address?

4 + 4 + 4 - 1 = 11, so 198.53.67.11

200.39.89.0/28• What kind of address is

200.39.89.32? Class C, so 4 bits borrowed Last non-zero octet is 240 Magic number is 256 - 240 = 16 32 is a multiple of 16 so 200.39.89.32

is a subnet address--the second subnet address!!

• What’s the broadcast address of 200.39.89.32? 32 + 16 -1 = 47, so 200.39.89.47

194.53.45.0/29• What kind of address is 194.53.45.26?

Class C, so 5 bits borrowed Last non-zero octet is 248 Magic number is 256 - 248 = 8 Subnets are .8, .16, .24, .32, ect. So 194.53.45.26 belongs to the third subnet

address (194.53.45.24) and is a host address.

• What broadcast address would this host use to communicate with other devices on the same subnet? It belongs to .24 and the next is .32, so 1

less is .31 (194.53.45.31)