Network - Lecture Outline - OSI Model - RLG

7/27/2019 Network - Lecture Outline - OSI Model - RLG

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1-Application File TransferFile Management

Message Handling (e-mail)

2-Presentation Formats data for network

Converts data to a language that the network can

handle.May Encrypts/decrypts

(Codes and decodes graphics and file format

information)3-Session (traffic cop)

Establish and maintain link (the session)

Maintain synchronizationReestablish lost connections

Sets terms of communications

Who talks first

How long each can talk

Often

Terminal-to-

Mainframeconnection

ISP-home

4-Transport (most important layer)

Flow Control selects appropriate rateSequencing reassembles packets in correct order

Sends ACK when data is correct

Sends requests when data is contains errors

Sending will retransmit in ACK not receivedMay break large packets into smaller ones

Transport layer services:

(TCP in TCP/IP)

(SPX (Sequence Packet Exchange - in

IPX/SPX)

5-Network Routs data - Selects Best pathTranslates network to physical address

May segment/reassemble if necessary

Routers

6-Data Link Divides data into frames for transmission

Adds to frame:

Destination address

Source address

Control Information

(data)

Error Checking Information

Bridges,

switches

7-Physical Nic, cables

OSI Model Open System InterconnectGreek for Equal

Pneumonic All

PeopleSeem

To

Need

DataProcessing

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OSI Model Upper LayersGateways Hardware or Software operate in these three layers

The most common is software running on a computer.

Application

Layer

Interfaces with network services ( applications)

Services Include:

File

Print

Message Etc.

PresentationLayer

Formats the data

Character sets are converted

Data is encrypted

Data may be compressed

Usually handles the redirection of data streams

Most standard computer systems use the American Standard Code for Information

Interchange (ASCII). Mainframe computers (and some IBM networking systems) use the

Extended Binary Coded Decimal Interchange Code (EBCDIC). The two are totally

different. Protocols at the Presentation layer can translate between the two.

Session Layer Maintains the session

Establishes, Synchronizes, Maintains and Ends a SessionDoes:

Security Authentication

Connection ID Establishment

Data Transfers (Checkpoints)

Acknowledgment

Connection Release

OSI Middle Layers

Transport Layer Flow Control Handles Errors

Acknowledges (ACK) receipt of data

Resizes messages into smaller or larger segments

(reassembles the messages back to original size)

Handles logical address/name resolution

Establishes one of two types of Connection Services:

Connection-oriented

o Uses acknowledgements and responses to establish and ensure virtual

connection is maintainedo TCP is a connection-oriented protocol (TCP/IP stack)

Used by FTP and HTTP (File Transport Protocol) (HyperText

Transport Protocol)

o SPX is a connection-oriented protocol (IPX/SPX stack)

Connectionless

o No error or flow control less overhead faster

o UDP is a connectionless protocol (User Datagram Protocol - part of TCP/IP



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stack)

Used by TFTP and NFS (Trivial File Transport Protocol and Network

File System)

o IPX is a connectionless protocol

o NetBEUI is a connectionless protocol

Address/Name Resolution

Translates logical address to logical name

DNS (TCP/IP stack protocol) provides cross-platform Address/Name resolution.

(Domain Name Services)

NDS Database (Novel Directory Services) provides address/name resolution at the

Transport Layer in response to higher level NDS Directory Requester

Network Layer

Layer 3

Controls Logical Network Addressing and Routing

Logical Network Addressing

Uses protocol centric addressing (i.e. TCP\IP or IPX)

Duplicate addresses often render both users down

o With Win9X first user is OK second user will be off line

Duplicate TCP/IP addresses are more common than IPX conflictso IPX uses the unique MAC address as part of its address

IPX addresses have two portions

o Network portion

o Node portion

For IPX (8:12 XXXXXXXX:XXXXXXXXXXXX)

o Network portion is an 8 digit hex number The IPX Network Address

Randomly assigned by installation program or manually by SYSOP

o Node portion is the MAC address

For TCP/IP

o A dotted decimal notation is used (XXX.XXX.XXX.XXX)o 4 three digit decimal numbers (or four octets) (decimal #s 1-254)

Routing

Routers create an internetwork out of network segments.

Each routers contain a Routing Table that permits the router to pass data with

specific addresses to a specific port

Routing tables may be either Static or Dynamic

o Static Routing Tables are created and maintained manually by the

administrator. (Time-intensive, expensive) In NT use the ROUTE command

o

Dynamic Routing uses discovery protocols to identify segments in theinternetwork and to update other routing tables most popular routingtechnology

Older Route Discovery Protocols use RIP (Routing InformationProtocol) a Distance Vector Route Discovery method

RIP for TCP/IP

RIP for IPX

Distance Vector Route Discovery method broadcast the routing table

every minute receiving routers add a 1 to the hop count and then



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rebroadcasts it. Creates high network overhead.

Link State Route Discovery method is more efficient.

Sends the table multicast (not broadcast) every five minutes

Other routers only respond with changes to the table (a if there

is a change)

Current Link State Route Discovery protocols:

o OSPF for TCP/IP (Open Shortest Path First )

o

NLSP for IPX (NetWare Link Services Protocol)

Routable Protocols:

Protocol Route Discovery Protocol

IPX RIP or NLSP

TCP/IP RIP or OSPF

XNS RIP

Nonroutable Protocols:

NetBEUI

On a workstation a Default Gateway is the address that packets are sent to if the

address is not located on its segment

Network Layer Devices

Router

Connects segments into an internetwork

Connects LAN to the Internet

Facilitates communications between segments by selecting best route

Can connect dissimilar lower level networks (Ethernet & Token Ring)

Can provide firewall services and packet filtering

Brouter

Designed to bridge dissimilar networks not very effective not used much Layer 3 Switch

Does everything that a Layer 2 Switch does plus basic routing for VPNs

Most routing protocols function at this layer

Translates logical names into physical addresses

Prioritizes data transfer (puts AV data ahead of e-mail)

(Prioritization is know as Quality of Service (QoS)

Routs data from source to destination

Builds and tears down packets

OSI Model Lower LayersData Link LayerLayer 2

Controls:

Controls logical network topology

o Ethernet

o Token Ring

o Etc.

Method of media access

o Contention



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o Polling

o Token Passing

Controls physical addressing

Controls transmission connection and synchronization

Sending Data:

Accepts data frame from Network layer and reformats it to match the network

logical typology (Ethernet, Token-ring, etc). Adds Data Link header and trailer to each new data frame

Manages the flow of data to the Physical layer

(The Physical layer will reject a data frame if the network is busy.)

Listens of an ACK for each data frame it sends and re-sends it if an ACK is not

received.

Receiving Data:

Accept raw data frames from physical layer

Checks destination address on each frame received

Discards all data frames not addressed to it

Performs an error check on accepted data frames

o Send ACK if the data frame is OK

o If it is bad, it does not send ACK and discards the data frame

(The send site will re-sends a frame if the ACK is not received)

Removes Data Link header and trailer from the data frame

Repackages the data frame as required and passes it to Network layer

IEEE 802 divided the Data Link Layer into two Sublayers

(Institute of Electrical and Electronics Engineers)

LLC - Logical Link Control

o Provides error correction and flow control

o 802 Specifications:

802.1 Network management

802.2 Logical Link Control

MAC - Media Access Control (The Data Link layer physical address)

o Has direct communications with NIC

o Provides access control

o 802 Specifications:

802.3 CSMA/CD

802.4 Token Bus

802.5 Token Ring 802.12 Demand Priority

Data Link layer Devices

Bridges

o Connects network segments

o Restricts local traffic to local segment

o Bridges that operate up to the LLC sublayer can connect some dissimilar



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networks (ie Ethernet and Token Ring)

o Bridges that only operate up to the MAC layer can only connect identical

networks

Switches (AKA Layer 2 Switch or Switching Hub)

o Builds a MAC Address table and routs traffic to proper port

Physical Layer

Layer 1

Network Hardware The Physical Topology

Translate data received from the Data Link layer into signals that can be transmitted on the

medium being used. (called signal encoding)

Functional physical interface with the network

Transmission technique

Pin layout and connector type

Physical Layer Devices

Cable media (also called bounded media)

NICs

Transceivers (may convert signal types Example 15-pin DIN AUI to RJ-45

DIN Detusche Industrie Norm (German National Standards body)

Repeaters Used to extend the length of a network passes all data (and noise)

Hubs (also called a concentrator) a multiport repeatero Active (Powered) amplifies signal extends segment length

o Passive (Typically Unpowered) reduces signal strength and segment length

MAUs Multistation Access Unit (also MSAU)

o Used for Token Ring Networks

o Physical Star, Logical Ring

o Max of 33 MAUs can be chained

5-4-3 Rule 5 Network segment Max

4 Repeaters Max3 Populated segments Max

MAC Address Assigned at the Factory (some can be changed in the field)AKA:

Hardware Address

Ethernet Address

Physical Address

NIC Address

Adapter Address

A 12 digit hexadecimal number (six-octet number)

The first 3 octets identify Manufacturer

o The OUI Organizationally Unique Identifer

The last 3 octets are unique numbers for that manfacturero The Universal LAN MAC address

Media Access

CSMA/CD Carrier Sense Multiple Access with Collision Detection

Used by Ethernet

Token Passing Used by Token Ring

The first computer turned on creates a token

The token is then passed from computer to computer until it reaches a computer that

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Only the computer that holds the token can transmit data.

It transmits the data with the token.

When the receiving computer receives the data it sends the token back to the sender.

The sender may then send more data.

When the computer finishes transmitting it passes the token on to the next computer.

(Collisions are avoided)

CSMA/CA Carrier Sense Multiple Access with Collision Avoidance

Used by AppleTalk The sender transmits a request to send (RTS) packet and waits for a clear to send

(CTS) before sending.

IEEE 802

Standard 802.1 LAN/MAN Management (and Media Access Control Bridges)

802.2 Logical Link Control

802.3 CSMA/CD (802.3u includes 100BaseT)

802.4 Token Bus

802.5 Token Ring

802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)

802.7 Broadband Local Area Networks

802.8 Fiber-Optic LANs and MANs

802.9 Integrated Services (IS) LAN Interface

802.10 LAN/MAN Security

802.11 Wireless LAN

802.12 Demand Priority Access Method

802.1 LAN and MAN managementMAC Bridges

Spanning Tree algorithms

802.2 Logical Link Control (LLC)

(Upper half of the Data Link layer link between MAC and Network layer)

802.3 CSMA/CD Carrier Sense Multiple Access with Collision Detection

Ethernet (DIX)

802.3u includes 100BaseT

802.4 Token Bus

Factory Automation Not used for data communicationResembles MAP Manufacturing Automation Protocol (Developed by GM)

802.5 Token RingDeveloped by IBM 1985

Uses MAU

Physical star

Logical ring4Mbps or 16Mbps

Can talk to IMB mainframe computers

Busy networks just get slow do not crash like EthernetUses twisted pair (can uses other cabling) rated by type (not Category).

Token Ring NICs are more expensive than Ethernet NICs

Signals are amplified and regenerated by each NIC or other Token Ring device

802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network

City size networks



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Many-to-many computers

802.7 Broadband LANs

Uses FDM Frequency Division Multiplexing

802.8 Fiber-optic LAN and MAN

Defines use of fiber optics on 802.3-802.6Includes:

FDDI Fiber Distributed Data Interface

10BaseFL

802.9 Integrated Services (IS) LAN Interface

ISDN spec -Integrated Switched Digital Network

ADSL and Cable Modems have eclipsed ISDN

(They are Cheaper, Easier to Setup, and have Better Performance than ISDN)

802.10 LAN/MAN Security

Spec for VPN Virtual Private NetworkUsing public (Internet) network for connections between LAN locations

Encrypts data for transmitting in the open

Cheaper than renting a dedicated line

802.11 Wireless LAN

802.12 Demand Priority AccessDeveloped by HP

Not supported by other manufactures (They use Fast Ethernet with 802.3u)

Assigns higher bandwidth to frames identified with high priorityImportant to real time audio and video transmission

AKA:

100VG (Voice Grade)

100VG-AnyLAN

100Base VG

AnyLAN

Use the OSI reference model to aid in topology decisions

Jun 7, 2001

Mike Mullins CCNA, MCPAuthor's Bio | E-Mail| Archive

2001 TechRepublic, Inc.

Knowing the seven layers of the Open Systems Interconnection (OSI) reference model and understanding where

your network devices fit in that model can prove to be a valuable asset when it comes to adding devices todifferent segments of your network and placing devices between networks. This knowledge enables you to take

control of your topology and keeps you from leaving your network design to your vendors. In this article, wellexplore the most common network devices and discuss where they should reside in a good network topology.

Lets break down the devices by layer and by their function on a network. Figure A provides a good overview.

Figure A

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Where different devices operate on the OSI reference model

Figure A

Layer 1, The Physical Layer: BitsThe Physical Layer provides the mechanical and electrical connections to the network. In other words, it sends

bits down a wire.

Repeaters connect media segments and provide amplification and retransmission of signals with no filtering

and will propagate all errors. Use them only for connecting long segments of wire.

Hubs connect multiple hosts to one segment of wire. All hosts share the same bandwidth. This means that there

is one large collision domain. Use them at points where you would deploy a network sensor, so the device can

see all the traffic on that portion of the network.

Layer 2, The Data Link Layer: Frames

The Data Link Layer splits data into frames for sending on the physical layer and receives acknowledgement

frames. It performs error checking and retransmits frames not received correctly. It provides an error-freevirtual channel to the Network Layer. The Data Link Layer is split into an upper sublayer, Logical Link Control

(LLC), and a lower sublayer, Media Access Control (MAC).

Bridgesconnect different types of networks (token ring, Ethernet, etc.), filter network traffic based on MAC

address, and remove errors from the network. Use them to connect different types of internal networks.

Switches, also known as Multiport Bridges, transfer data between different ports based on the destination

addresses. Each segment or port connection is its own collision domain, but all ports are in the same broadcast

domain. Switches can be used to connect multiple ports to the same destination (i.e., multiple uplink ports), but

only one port can be active at a time. Historically, this is a hardware Layer 2 device and typically operates inone of three modes:

Store and Forward: This mode copies the entire frame into memory, computes the Cyclic Redundancy

Check (CRC) for errors, and then looks up the destination MAC address and forwards the frame. This is



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slow but offers the best solution for error correction without affecting the entire backbone in

retransmission.

Cut-through: This mode reads the destination address of the frame and forwards the frame to the port

connected to that destination MAC address before the entire frame is seen. This is fast but provides very

little error correction and will propagate errors from one collision domain to the next.

Modified Cut-through: This mode reads the first 64 bytes of the frame and then forwards the frame to

a port based on MAC destination address. This is fast and efficient in error correction.

Use Layer 2 switches as your LAN subbackbone device with each host connected to a switched port. Thisdedicates bandwidth and segments the lowest layer of your internal network. Switches are also pretty

inexpensive.

Layer 3, The Network Layer: Datagrams/PacketsThe Network Layer determines the routing of packets of data from sender to receiver. Routes can be static or

dynamic. The Network Layer provides sequencing and flow control of data, selects routes, and provides quality

of service through error detection, recovery, and notification. It also segments collision and broadcast domains.This is where a MAC or hardware address is translated into Internet Protocol (IP) addresses (or other routable

protocol addresses, such as IPX or AppleTalk).

Routers are basically software-based packet-forwarding engines. The Network Layer provides information to

these devices, allowing them to base their forwarding decisions on criteria such as IP, link, or network node

availability and performance. A routers true advantage lies in its flexibility in network protocol.

Use routers for their intended purpose. They were born to bridge networks and separate broadcast domains. This

is your gateway device to the Internet or your linking device between LANs or WANs.

Switches (Layer 3)are nothing more than wire-speed routers. They come in two basic models.

Port switches decide which physical port network traffic needs to go to and direct the traffic

appropriately. Each lane is actually a backplane segment on the switch. Because the switching is

performed locally via logic circuits and at wire speed, port switches are easier and cheaper to implementthan frame switches but give many of the same benefits.

Frame switches examine each Ethernet packet, determine which segment it came from and where it is

going, and send it on its way. These are more expensive than port switches but add a significantperformance boost to your network. They are also known as Learning Switches.

Use these devices at the backbone of your network. Theyre less expensive than routers and much faster. But

read on: Theres another switch that might be a better solution for your network.

Layer 4, The Transport Layer: Segments

The Transport Layer (sometimes referred to as the Host Layer) determines how to use the Network Layer toprovide a virtual point-to-point connection. It creates and dissolves connections between hosts. The most

popular types of transport connection are TCP and UDP. TCP is a point-to-point connection protocol that

delivers messages in the order in which they were sent and guarantees delivery. UDP is a connection-lessprotocol with no guarantee of delivery. The Transport Layer is a legitimate end-to-end layer. In other words, a

program on the source machine carries on a conversation with a similar program on the destination machine.

Layer 4 Switches are sometimes called session switches because they track and maintain individual sessionsfrom start to finish. (This doesnt make them Layer 5 Switches; theyre just aware of sessions.) Layer 4



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Switches make forwarding decisions based on session and application-layer information and provide load

balancing across multiple servers. Layer 4 Switches determine (through different complex and weightedalgorithms) the best server of a cluster to process a service request and bind the session to that servers IP

address until the session is terminated.

They also designate and prioritize traffic by application. Because they are aware at the session level, they

have the ability to prevent unauthorized access to servers. Layer 4 Switches true selling point is handling all

packet processing in hardware. They utilize custom application-specific integrated circuits (ASICs), making

them extremely fast!

Substitute Layer 4 Switches for your Layer 3 Switches only if you intend to operate high-speed intranet

application servers with multiple 100-Mbit or gigabit interfaces.

What you need and what you can afford

Do not let design and implementation of your network become dominated by one specific vendor. While no twonetworks are exactly alike, a variety of vendors exist. So dont settle for a device from one vendor based solely

on company name. Above Layer 2, most devices are very specific in the protocols and configurations they

support. Know the intended purpose of your network and let budget and function dictate design.

What kind of network design tips do you have?Do you use the OSI reference model for topology planning and troubleshooting? We look forward to getting

your input and hearing your experiences regarding this topic. Join the discussion below orsend the editor an e-

mail.

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