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 Presentation Session Transport Network Data-Link Physical Data Segments Packets Frames Bits Data

Application Presentation Session Transport Network Data-Link Physical THE OSI MODEL LAN Devices & Technologies The 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: 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:  Class B:  Class C:  Class D: (multicasting)  Class E: (experimental)

Network vs. Host NHHH Class A: 2 7 = 126 networks; 2 24 > 16 million hosts NNHH Class B : 2 14 = 16,384 networks; 2 16 > 65,534 hosts NNNH Class C : 2 21 > 2 million networks; 2 8 = 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: 2 4 = = 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: 2 4 = = 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 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

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.

CIDR Notation Classless Interdomain Routing is a method of representing an IP address and its subnet mask with a prefix. For example: /27 What do you think the 27 tells you?  27 is the number of 1 bits in the subnet mask. Therefore,  Also, you know 192 is a Class C, so we borrowed 3 bits!!  Finally, you know the magic number is = 32, so the first useable subnet address is !! Let’s see the power of CIDR notation.

/26 Subnet mask?  Bits borrowed?  Class C so 2 bits borrowed Magic Number?  = 64 First useable subnet address?  Third useable subnet address?  = 192, so

/30 Subnet mask?  Bits borrowed?  Class C so 6 bits borrowed Magic Number?  = 4 Third useable subnet address?  = 12, so Second subnet’s broadcast address?  = 11, so

/28 What kind of address is ?  Class C, so 4 bits borrowed  Last non-zero octet is 240  Magic number is = 16  32 is a multiple of 16 so is a subnet address--the second subnet address!! What’s the broadcast address of ?  = 47, so

/29 What kind of address is ?  Class C, so 5 bits borrowed  Last non-zero octet is 248  Magic number is = 8  Subnets are.8,.16,.24,.32, ect.  So belongs to the third subnet address ( ) 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 ( )