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| Open Systems Interconnection (OSI) Model |
| Layer |
Descriptions and Responsibilities |
| Application |
Application-oriented layer
Where users communicate with the computer
Services
Application – Process locally or distributed
Database Access – Network Dbs storage and retrieval
File – Store, Move, Control Access, Retrieve
Message – Transfer of text, graphics, audio, video
Print – Send data to local or network printer
Also advertises any services, and determines if the services should be handled locally or remotely
Examples:
Electronic Data Interchange (EDI) – Specialized standards/processes that flow tasks between businesses: accounting, shipping/receiving,
order, inventory
eMail gateways – smtp, X.400
Financial Transaction Services – Gears to the financial industry, where information is gathered and sold to subscribers: investments,
market trading, commodities, currency exchange rates, and credit card rates.
Internet Navigation Utilities – Gopher, WAIS
Special Interest Bulletin Boards – Internet Chat Rooms
WWW – Browsers (Netscape Navigator, Internet Explorer, Mosaic)
|
| Presentation |
Application-oriented layer
Defines how standard data should be formated
Network redirector makes files and file server visible to clients
Network redirector makes printers appear 'local'
Presents data to the Application Layer
Translator for coding and conversion
Processes
Character-code conversion – Converts binary data (EBCDIC) to and from ASCII
Data compression – Compresses data
Data encryption – Encrypts data
Data expansion – Decrypts data
Interpretation of Graphical commands
Examples:
JPEG (Joint Photographers Expert Group)
MIDI (Musical Instrument Digital Interface) – digital music
MPEG (Moving Picture Experts Group) – standard for compression and coding movies
PICT (Mac Quickdraw graphics)
QuickTime – a standard for audio and video applications
TIFF (Tag Image File Format) – high-resolution bit-mapped images
|
| Session |
Application-oriented layer
Communications management
Controlling dialogues
– checkpoint in case of network failure — only data AFTER failure is resent
– data synchronization
– determines which computers are talking to one devices or nodes
– name lookup and security
– who can send vs. who can receive
Coordinates communications
– simplex, half-duplex, full-duplex
Establish, Maintain, Synchronize and Terminate connections
– determines what protocols will be used
Keeps data from different applications separate
Examples
AppleTalk Session Protocol (ASP) – Used to establish and maintain communications from an AppleTalk client and a server
Digital Network Architecture Session Control Protocol (DNA SCP) – DECnet session-layer protocol
Network File System (NFS) – Developed by Sun Microsystems; transparent access to remote resources for TCP/IP and UNIX workstations
Remote Procedure Call (RPC) – A broad client/server redirection tool; clients create procedures, and servers perform them
Structures Query Language (SQL) – Developed by IBM; defined a simpler way to access information on both local and remote systems
X Window – For communicating with remote UNIX workstations from an intelligent terminal as if it were located locally
|
| Transport |
Reliable data transfer – TCP is reliable; UDP is not reliable
– End-to-end data transport services
– Ensures packets are delivered error free, in sequence and no loss or duplications
Establishes a logical connection between source and destination computers on an internetwork
Services in this layer, segment and reassemble data (from upper layers) and combine them into a single data stream
Flow Control
– can issue a "Not Ready" indicator to prevent buffer over flow/lost data
– prevents the sending host from overflowing the buffer on the receiving host
– segments are re-sequenced into proper order once received
– segments not received are retransmitted
– segments received are acknowledged back to the sender
Connection-Oriented service
– "telephone like" — established and acknowledged
1) 'Device A' establishes a connection with 'Device B'
2) The application on 'A' and 'B' notify the respective OSes they are ready to transfer, and the OSes confirm the transfer approval
3) The connection is fully established and transfer begins, once the synchronization is complete
During the transfer, the two devices periodically check with each other
AKA — '3-way handshake' by Cisco
Connectionless service
– "snail-mail like" — does not establish a connection, it is assumed (not as reliable)
Windowing
– From the sending machine, there is a period of time during the sending of a data segment and it's acknowledgement known as a "window"
– A 'window' size of one, will transmit one segment and then wait for acknowledgement
– A 'window' size of three, will transmit three segments and then wait for acknowledgement
|
| Network |
Translates Logical network addresses into Physical addresses
Responsible for routing and network addressing on an internetwork
Transporting traffic to devices that are not locally attached
Breaks down larger packet into smaller ones for the Data Link layer
Two Types of packets
Data Packets
– Used to transport User data via Routed Protocols (ip and ipx)
Route Update Packets
– Used to update neighbor routers via Routing Protocols (RIP, EIGRP, OSPF)
– Helps build and maintain routing tables
Broadcast Domains
– Breakup collision domains (via layer-2 switches)
– Each interface must be assigned a unique network address
– Each machine on a segment must use the same network number
Routers and Gateways
– Addresses logical and service addresses
– Broadcasts/Multicast packets are NOT forwarded by the router
– Connections services: network layer flow control, error correction, packet sequence
– Circuit, message and packet switching
– Gateway services
– Quality of Service (QoS - priority) for network traffic can be provided by routers
– Route discovery and selection
– Routers can use an access list for security (Administrator created)
– Routers provide layer-2 bridging functions, can route through the same interface (if needed)
– Routers use a logical address to determine the next hop
– Virtual LANs (VLANs) are connected via layer-3 devices
Routing Tables
Network address
– The routing table must maintain a table for each routing protocol for each addressing scheme
Interface
– Exit interface when the data packet reaches it's destination
Metric
– Distance to the remote network.
– Each protocol uses it's own way of computing the distance
Examples: hop count, bandwidth, delay of the line, tick count (1/18 of a second)
|
| Data Link (802.1) |
Adds header containing source and destination address
– Uniquely identifies devices by physical address on the local network
– Makes sure that data is delivered to the proper destination
— disregards other information
Establishes and Maintains communications channel (LLC)
Translates Network layer bits for the Physical layer
Organize data into frames* (logical data)
– error-free transmission of frames
– a Cyclic-Redundancy Check (CRC) can be added to the frame
— damaged frames in this layer can be re-requested from sender
— can also detect lost frames
Control Information
– frame type
– routing
– segment information
Functions/Responsibilities
– data flow control
– error detection
– link management
Two Sublayers (submitted by the IEEE)
Logical Link Control (LLC - 802.2) – provides Service Access Points (SAPs); identifies the Network layer protocol and
encapsulates them; a LLC header tells the Data Link layer what to do with the packet once a frame has been received; can define flow control (low-level –
'does the channel exist or not') and sequence of control bits
Media Access Control (MAC - 802.3) – defines how packets are placed on the media; defines the physical addressing (embedded on
chip in NIC; a unique 12-digit number); defines logical topologies; Other features on this sublayer: line discipline, error notification (not correction),
ordered delivery of frame, optional flow control
– CSMA.CD (802.3)
– Token Bus (802.4)
– Token Ring (802.5)
– Demand Priority (802.12)
Switches and Bridges
– Both work on the Data Link layer
– Filter the MAC address
– Layer-2 devices propagate layer-2 storm broadcasts; to stop this, use layer-3 device (router)
– Layer-2 switching is hardware based because of Application-Specific Integrated Circuits (ASICs);
– Low latency up to gigabit speed
– Routers can provide translation services
– Switches can NOT translate between different media, must have the same ethernet frame type
*Frames – contain the physical address of source and destination NICs, plus all other data used in upper layers
|
| Physical |
Physical Network Structure/Transmission service
Communicates directly with the physical media
Two responsibilities - to send and receive bits (1's and 0's)
Addresses
Bit synchronization: between sender and receiver
Media bandwidth: baseband or broadband
Multiplexing: several data channels into one
Network Connection type: multipoint or point-to-point
Physical Topologies: bus, star, ring
Signaling: analog or digital
Termination: prevents signal from reflecting back; indicates last device on segment
Specifications for
Electrical - voltage levels
Functional - ping assignments
Mechanical - connector type: electrical, optical, radio
Procedural - handshake
– requirements for activating, maintaining and deactivating the physical links between end systems
Interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) is identified
– service available to DTE are usually accessed via a modem or Channel Service Unit/Data Service Unit (CSU/DSU)
Examples
RS-232C/D, hubs*, simple active hubs, terminators, couplers, cables and cabling, connectors, repeaters,
multiplexers, transmitters, receivers, transceivers
*Hubs– Really only multiple port repeaters; Do NOT look at any traffic, and broadcast it to all ports
|
| Mnemonics for OSI layers |
| Top-to-Bottom > All People Seem To Need Data Processing |
| Bottom-to-Top > People Don't Need To See Prince Albert |
| Bottom-to-Top > Please Do Not Take Sales Peoples Advice |
| IEEE 802 Categories |
| 802.1 – Internetworking |
| 802.2 – Logical Link Control (LLC) |
| 802.3 – Carrier Sense with Multiple Access and Collision Detection (CMSA/CD, or EtherNet) |
| 802.4 – Token Bus LAN |
| 802.5 – Token Ring LAN |
| 802.6 – Metropolitan Area Network (MAN) |
| 802.7 – Broadband Technical Advisory Group |
| 802.8 – Fiber-Optic Technical Advisory Group |
| 802.9 – Integrated Voice/Data Networks |
| 802.10 – Network Security |
| 802.11 – Wireless Networks |
| 802.12 – Demand Priority Access LAN, 100baseVG-AnyLAN |
|
