Click to edit Master subtitle style Chapter 2: OSI Specifications Click to edit Master subtitle style

Chapter 2 Objectives The Following CompTIA Network+ Exam Objectives Are Covered in This Chapter: 5.0 Industry standards, practices, and network theory 5.1 Analyze a scenario and determine the corresponding OSI layer • Layer 1 – Physical • Layer 2 – Data link • Layer 3 – Network • Layer 4 – Transport • Layer 5 – Session • Layer 6 – Presentation • Layer 7 – Application 2

Chapter 2 Objectives The Following CompTIA Network+ Exam Objectives Are Covered in This Chapter: 5.2 Explain the basics of network theory and concepts • Encapsulation/de-encapsulation • Modulation techniques o Multiplexing o De-multiplexing o Analog and digital techniques o TDM 3

Internetworking Models In the late 1970s, the Open Systems Interconnection (OSI) reference model was created by the International Organization for Standardization (ISO) to break through this barrier. The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other. The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers. Layered architecture – understand computer processes and see what must be accomplished on any one layer and how 4

Advantages of Reference Models Advantages of using the OSI layered model include, but are not limited to, the following: It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting. It allows multiple-vendor development through standardization of network components. It encourages industry standardization by defining what functions occur at each layer of the model. It allows various types of network hardware and software to communicate. It prevents changes in one layer from affecting other layers, so it doesn’t hamper development and makes application programming easier. 5

The OSI Model The OSI model has seven layers: Application (Layer 7) Presentation (Layer 6) Session (Layer 5) Transport (Layer 4) Network (Layer 3) Data Link (Layer 2) Physical (Layer 1) Its not a physical model, it’s a conceptual and comprehensive fluid set of guidelines 6

OSI Layer Functions Application File, print, message, database, and application services Presentation Data encryption, compression, and translation services Session Dialog control Transport End-to-end connection Network Routing Data Link Framing Physical Physical topology Divided into two groups: top 3 rules of how applications within hosts communicate with each other and end users bottom 4 how data is transmitted from end to end 7

The Upper Layers Application Provides a user interface Presentation Presents data Handles processing such as encryption Session Keeps different applications’ data separate Transport Network Data Link Physical Application: users communicate and interact with the computer application processes, interfaces, APIs -> through OS Presentation: data translation and code formatting Adapts data to standard format before transmission data compression, decompression, encryption, decryption Session setting up , managing, and tearing down session between presentation layer entities coordinates communication between systems organizes their communication through: simplex, half-duplex, full-duplex keeps app data separate from other apps data 8

The Lower Layers Application Presentation Session Transport Provides reliable or unreliable delivery Performs error correction before retransmit Network Provides logical addressing which routers use for path determination Data Link Combines packets into bytes and bytes into frames Provides access to media using MAC address Performs error detection not correction Physical Moves bits between devices Specifies voltage, wire speed, and pin-out of cables Transport, segments and reassembles data into data streams end-to-end data transport services establish logical connections between sending and destination hosts multiplexing establish virtual connections tear down virtual connections TCP/UDP 9

Reliability Reliable data transport employs a connection-oriented communications session between systems, and the protocols involved ensure that the following will be achieved: The segments delivered are acknowledged back to the sender upon their reception. Any segments not acknowledged are retransmitted. Segments are sequenced back into their proper order upon arrival at their destination. A manageable data flow is maintained in order to avoid congestion, overloading, and data loss. 10

A Connection Oriented Session Sender Receiver SYN SYN/ACK ACK How does the Transport layer employ connection oriented sessions: Senders TCP process contacts destination TCP process to establish connection -> this results in a virtual circuit within the handshake, TCP processes agree on amount of information will be sent The virtual circuit setup is called overhead Connection Established Data transfer (Send bytes of Segments) 11

Flow Control 12 Sender Receiver Transmit Buffer full No ready – STOP! Segments processed GO! If a bunch of computers are simultaneously sending datagrams back and forth -> congestion -> bottleneck prevents sending host from overflowing the buffer in receiving host -> loss of data delivered segments are acknowledged back to sender upon receipt if not acknowledged, retransmitted by sender segments are sequenced into proper order on arrival destination manageable data flow maintained to avoid congestion, overload, data loss Transmit 12

Connection Oriented Session A service is considered connection-oriented if: Virtual circuit is setup (three-way handshake). Uses sequencing. Uses acknowledgments. Uses flow control. 13

Windowing Flow Control Sender Receiver Window size of 1 Send 1 Receive 1 Ack 1 Send 2 Receive 2 Ack 2 Slow if: sender waited for ACK after transmission of each segment. Sender allowed to send without receiving ACK is known as Windowing used to control the amount of outstanding, unacknowledged data segments Window size of 3 Send 1 Send 2 Send 3 Ack 4 Send 4 14

Acknowledgements 1 2 3 4 5 6 1 2 3 4 5 6 15 Sender Receiver Send 1 Reliable data deliver guarantees that data wont be duplicated or lost Positive Acknowledgments with Retransmission requires receiving machine to communicate with sender when data received Remember how during the handshake, they share information about size of segment? Send 4 Send 5 Connection lost! Send 6 Ack 5 Send 5 Ack 7 15

The Lower Layers Application Presentation Session Transport Provides reliable or unreliable delivery Performs error correction before retransmit Network Provides logical addressing which routers use for path determination Data Link Combines packets into bytes and bytes into frames Provides access to media using MAC address Performs error detection not correction Physical Moves bits between devices Specifies voltage, wire speed, and pin-out of cables Network logical device addressing transports traffic between devices that aren’t locally attached Routers work at Layer 3 Two types of packets in the network layer: Data packets: transport user data through internetwork, called routed protocols ie: IP and IPv6 Route-Update packets: to update neighboring routers about networks connected to all routers, called routing protocols ie: RIP, RIPv2, EIGRP, OSPF. Used to help build and maintain the routing table 16

Routing at Layer 3 Routing table Routing table 17 3.0 1.0 3.1 1.1 2.1 2.2 1.3 3.3 E0 E0 S0 S0 3.2 1.2 Routing table NET INT Metric 1 E0 2 S0 3 Routing table NET INT Metric 1 S0 2 3 E0 Routing table: network addresses – protocol specific Interface – exit interface metric – distance to the remote network. Different ways of computing this distance: hop count, (bandwidth, delay etc. 17

Routers at Layer 3 18 Internet FastEthernet0/0 Serial0 WAN Services Each router interface is a broadcast domain. Routers break up broadcast domains by default and provide WAN services Each interface represents a separate network – each assigned a unique network identification number, each host on the network will use the same network number Key points: do not forward broadcast or multicast packets use logical address to determine next-hop router to forward packet can use access-lists created by admins to control security can provide layer 2 bridge functions and can route through the same interface layer 3 devices provide connections between VLANS provide QOS for specific types of network traffic 18

The Lower Layers Application Presentation Session Transport Provides reliable or unreliable delivery Performs error correction before retransmit Network Provides logical addressing which routers use for path determination Data Link Combines packets into bytes and bytes into frames Provides access to media using MAC address Performs error detection not correction Physical Moves bits between devices Specifies voltage, wire speed, and pin-out of cables Data link is the physical transmission of data, handles: error notification network topology flow control Uses: hardware addressing (MAC) media access control formats messages into pieces called data frames 19

Data Link Layer (Layer 2) Logical Link Control (LLC) Media Access Control (MAC) 802.11 802.3 802.2 MAC – how packets are placed on the media. physical addressing logical topology – signal path through a physical topology line discipline error notification ordered delivery of frames optional flow control LLC- identifies network layer protocols and encapsulates them flow control sequencing of control bits 802 project 20

The Lower Layers Application Presentation Session Transport Provides reliable or unreliable delivery Performs error correction before retransmit Network Provides logical addressing which routers use for path determination Data Link Combines packets into bytes and bytes into frames Provides access to media using MAC address Performs error detection not correction Physical Moves bits between devices Specifies voltage, wire speed, and pin-out of cables Physical layer: send and receives bits (1, 0) specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link Identify the interface between data terminal equipment DTE -> data communication equipment DCE DTE is the end of the users device DCE is the end of the providers side of communication, usually located at the customer premice (modem or channel service unit/data service unit (CSU/DSU) 21

Data Encapsulation PDU Application Presentation Upper layer data Session TCP Header Upper layer data Segment Transport IP Header Segment Packet Network LLC Header Packet FCS Encapsulation: wrapped with protocol information at each layer Protocol Data Unit (PDU) – hold the control information attached to the data User information -> Data -> segments, reliable connection setup for transmission -> packets, logical address to be routed -> frames, hardware address for unique identify -> bits, digital encoding and cocking scheme Frame Data Link MAC Header Packet FCS 0101110101001000010 Bits Physical 22

Modulation Techniques Modulation is the process of varying one or more properties of a waveform called a carrier signal In current networks, modulation takes digital or analog signals and puts in in another signal that can be physically transmitted Modems – perform both modulation and demodulation operations Analog and Digital modulation use Frequency-Division multiplexing. Several low-pass signals transferred simultaneously over same shared medium Ethernet uses digital baseband modulation or line coding to transfer digital bit stream Time-division multiplexing (TDM) method of transmitting and receiving many independent signals over common signal path through synchronize devices 23

Summary Summary Exam Essentials Section Written Labs Review Questions 24