OSI MODEL

Guideline for interoperability used to describe what tasks a protocol suite performs.

Protocol Suite - a set of rules that have been agreed to that allow computers to communicate in some manner with each other. TCP/IP, IPX/SPX are examples.

Provides a model for different hardware and software manufacturers to use in describing and developing their products.

All people seem to need data processing.

Please do not throw sausage pizza away.

Phil Donahue never televises sick people anymore.

People don't need to seek popularity anymore.

My physical datalink to the network transported me to a wonderful session of using presentation applications.

The Application Layer

Refers to the tools an application (such as word processor) has that can be used to accomplish a task. This does not refer to the actual application such as Word, rather the protocols or extensions that allow Word to interface to the network.

The Presentation Layer

Formats data exchange. Character sets are converted (EBCDIC versus ASCII), data encryption, data compression added or removed. Makes the data able to be accessed by the Application layer.

The Session Layer

Defines how computers establish, synchronize, maintain, and end a session. Authentication, data transfer, acks take place here. Any "checkpoints" happen here (Did you get it?). Adjustments to the components that control this layer will greatly influence data transmission speed. The difference between operating at this layer and operating at the Transport layer is that at the session layer the communication considers the entire session, not just a sentence or word... Gateways operate at this level.

The Transport Layer

Checks for errors in the data. Divides long messages into shorter pieces. Appends data that allows the message to be reassembled. Logical address/name resolution. Majority of flow control takes place here.

The Connection Type - There are two connection type, Connection-oriented and connectionless.

Connection oriented - uses acknowledgments and responses to establish a connection between the sending and receiving devices. Acknowledgments are used to acknowledge communication and to maintain the connection during times when data does not flow. A connection service is used to ensure error free communication such as when downloading an application. Much slower than connectionless due to error control activities.

Connectionless services don't have error and flow control. They are very speedy. Used for streaming music, etc. where a few data errors is not likely to be critical.

Name Resolution - Logical Address to Logical Name resolution.

Various ways exist to translate Network Layer addresses to Transport Layer logical names. More later...

Transport Layer Implementation under various protocols.

IPX/SPX Protocol - IPX is connectionless, SPX is a connection type. No name resolution by default. NetWare server must be running and handles the requests from an NDS database.

TCP/IP Protocol - Two transport protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP/IP uses DNS of address to name resolution.

TCP - Connection type. Upper level protocols such as FTP and HTTP require a connection so they use TCP.
UDP - Connectionless. Trivial File Transfer Protocol (TFTP) and Network File System (NFS) require speed.

NetBEUI Implementation - Connectionless only. Uses WINS to support logical names but function only at the higher levels of the OSI model. Non-routable protocol.

The Network Layer

Defines protocols to ensure that data arrives at the correct destination. Logical addressing and translation of logical to physical. Prioritizes data. Packages and tears down packets. Most routing protocols function at this layer.
MAC address is at the Datalink layer (physical address) The network layer assigns a logical address that is protocol dependent, in other words, the physical address is made available to the network protocols regardless of which one it is.

Network Address Formats Example

Netware address (IPX/SPX) - creates a 20 digit address that is composed of two parts. The first is an 8 digit IPX network address. This is created randomly by the Netware installation program or it can be assigned by the network administrator. The second part is the 12 digit MAC address from the NIC. The two sections are separated by a colon so you end up with something that looks like this: 00012345:005A7C11EC59

TCP/IP addresses use a dotted decimal address that looks like this: IP Address 199.217.59.1 Subnet Mask 255.255.255.0 The network and node portion depends on the the numbers in the subnet mask. In this case the three position s of the IP address corresponding to positions containing 255 are the network address. Any other digit in the subnet mask indicates the node number. In this case the Network ID is 199.217.59 and the node is 1.

Routing - Routers are placed in a network to increase efficiency by routing traffic through particular paths to improve throughput. A router contains a table (routing table) that tells which path to take to move data from one network segment to another. Routers will not pass unknown packets or broadcast packets, they have to contain a known destination.

Static Routing - network administrator sets up a routing table. On large networks this is time consuming.
Dynamic Routing - Uses route discovery protocols. Routers send out special packets to other routers in order to discover the best route through the network. Based on what comes back from the other routers, the internal routing table is magically updated with the latest and best information.

There are two protocols used in dynamic routing, distance vector and link state:

Distance vector route discovery - Every 30 seconds a routing table is sent to all routers. Each router examines the table and forwards on to all other routers after appending any changes it wishes to make.
Link state route discovery - Every 5 minutes just an update to a table is sent only to the appropriate routers.

Network Layer Devices - Routers, Brouters, Layer 3 switches:

Router

Makes a choice about how to send data through the network. Bay Networks and Cisco are the two biggest manufacturers. Routers connect small segments to larger networks, connect networks to larger networks. Can connect dissimilar lower layer topologies (ethernet to token ring). Routers also can provide firewall functions. Routers are the most complex device on the network and the price reflects the complexity.

Brouter

Combines the functionality of a bridge and a router. It will route most packets but if it can't it will try to bridge it. Not used much because it does neither job well.

Layer 3 Switches

Performs all of the functions of a layer 2 switch but can also do some limited routing. In a simple LAN the Layer3 switch may be able to replace a router.

The Datalink Layer

Converts raw data into a logical structure. Data transmission synchronization and connection.
Creates, transmits and receives packets.
MAC address (physical address) is found at this layer.
Dictates the logical topology of the network (controls the way packets move through the network).
Describes the method of media access.

Contention - Competing for network resources.
Polling - Controller asks each device in turn if it has anything to do.
Token passing - Token ring (IBM)
Carrier Sense/Multiple Access with Collision Detection (CSMA/CD) - Ethernet
Carrier Sense/Multiple Access with Collision Avoidance (CSMA/CA) - Appletalk

802 Standards

802.1	LAN/MAN Management (and Media Access Control Bridges)
802.2	Logical Link Control
802.3	CSMA/CD - The Ethernet standard!
802.4	Token Bus
802.5	Token Ring - Describes the standard for Token Ring networks.
802.6	Distributed Queue Dual Buss Metropolitan Area Network
802.7	Broadband Local Area Networks - Describes Frequency Division Multiplexing (cable television technology)
802.8	Fiber-Optic LANs and MANs - Describes FDDI and 10BaseFL (fiber Ethernet)
802.9	Integrated Services LAN Interface
802.10	LAN/MAN Security - Virtual Private Network description.
802.11b	Wireless Networks
802.12	Demand Priority Access Method

 

Devices

Bridges - Separates a physical network into two logical networks. Separates traffic into more manageable sections.
Switches - Switching hub (layer 2 switch). Sets up a direct connection between two workstations needing to talk based on their MAC address.

The Physical Layer

Controlling the interface. Includes pin layout, connectors and cable.
Deals with measurable, physical layouts.
Performs signal encoding.
Includes cable media (bounded media).
Conversion of analog electrical signals into 1 and 0.
Responsible for broadbanding or basebanding the signal.

Broadband - Multiple signals on a signal line...channels.
Baseband - Single signal on a line.

Specifies the physical layout (topology).

Bus
Star
Ring
Mesh

Devices

NIC
Transceivers
Repeaters
Hubs

Active
Passive

MAUs

Networking Protocols

TCP/IP - The protocol of choice today. Makes the Internet function. More later...

Addressing - Dotted decimal address 0 to 155. More later... Address resolution protocol (ARP) resolves the IP address to the MAC address on the NIC. Router listens for ARP broadcasts so it knows where to find the MAC address and send the data.
Routing - Router must have three pieces of information. IP of sender, IP of receiver, and IP of the next router to send the data through. Routers need to build a "map" of the network called a routing table in order to know where to send stuff. Routers use RIP (distance vector routing protocol) or OSPF (link state routing protocol, remember???) to build the map.
Interoperability - No protocol in more flexible or interoperable than TCP/IP. Virtually all networked PCs have some form of TCP/IP.
Naming - Named according to the DNS convention. host part (www) and the domain name (rickmudd.com). More later...

IPX/SPX - Novell NetWare proprietary network operating system.

Addressing - IPX handles addressing and routing. Uses 8 digit segment address and 12 digit hexadecimal MAC address to identify an individual machine. Routers need to be given and IPX address also.
Routing - Any TCP/IP router can route IPX/SPX although you may need to load additional software. IPX uses RIP or NLSP (a form of OSPF).
Interoperability - Most operating systems come with support for NetWare so interoperability is great. Some versions of UNIX have no provision to run IPX/SPX.
Naming - Only items that have names are servers. There are strict rules for how they are named. See Novell documentation.

NetBEUI - IBM/Microsoft network operating system.

Addressing/naming - Only the workstation is named. Very simple naming and very quick to configure. No addressing in the same sense that other network operating systems have because there is no routing.
Routing - Not routable.
Interoperability - Only Microsoft and IBM products support NetBEUI

Appletalk - Apple network operating system

Addressing - Each station has an address 24 bits long. 16 bits given to the network and each network support 256 nodes. Addresses are assigned by the computer itself. Self configuring network. Plug it in and it works. No fun at all.
Routing - New version Appletalk 2 includes routing. Routing Table Maintenance Protocol (RTMP) is a distance vector routing protocol. Like RIP for IP and IPX.
Interoperability - Macintosh only although the Windows products can talk with an Appletalk network with additional software installed.
Naming - Each computer associates it's name with its network address. Broadcasts it's existence when it comes on-line. Local router will cache the ID and respond to routing requests based on the broadcast address.

Vocabulary

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