standard- a set of specifications

architecture=model=a modular structure for organizing specifications

Two competing network architecture in MIS today: OSI (multi-vendor) and SNA (IBM)

Layer 1: Physical-> [point-to-point conncections] Concerned with the transmission of a bit stream over a communication channel. Specifications include signal voltages, bit duration, and channel definition. Deals with mechanical and electrical characteristics to activate, maintain, and de-activate the physical conncection. Examples: RS232C, X.25, Ethernet.

Layer 2: Data Link-> [point-to-point conncections] Contains specifications for frames (=blocks=packets), synchronization, and error control. These specifications change the raw bit stream supplied by layer 1 into data. and also addressess reliability and integrity issues. Since Layer 2 handles error control within each packet, the higher layers may assume the each packet is errorFree. [However, a packet could be lost or duplicated. Layer 2 would not know that.] Example: HDLC, SDLC, X.25, Ethernet.

Layer 3: Network-> The network layer makes routing decisions across the communication network that may consist of a number of nodes and possible routes and whose structure is known to the network layer. For example, in packet switched networks, it sets up the circuit for the transmission of each packet and then de-allocates and re-allocates the nodes to other services. At this layer, the computer engages in a dialog with the network and requests network services. This layer contains all the specifications for the transmission and switching technologies needed to build circuits through a network of nodes. In the case of a direct point-to-point link, the network layer may be a null specification. X.25 and ISDN contain network layer specifications.

Layer 4: Transport-> This layer provides error detection and control at the message level across a network of nodes. For instance, in packet switched networks, it handles error-free packet sequencing without losses or duplications as well as re-transmissions for bad or lost packets. Levels 3 and 4 are part of network services. Note: X.25 does NOT contain Transport specifications. These specs are found in TCP/IP, Telenet, and Tymnet.

Layer 5: Session-> The session layer assumes that a reliable virtual point-to-point connection has been made and contains specs for the dialog between the two end systems such as dialog discipline, data grouping, and recovery of an interrupted session. Specs are also included for initiating and concluding a session. Many network specs contain little or no session specs and leave these decisions to the applications. Example: CICS, TOPS, AppleTalk

Layer 6: Presentation-> Provides transformation of data to standardize the application interface. Also provides some network services such as encryption, compression, and text re-formatting.

Layer 7: Application-> This layer plays the same role as the 'application interface' in operating systems. Provides network services to users (applications) of the network in a distributed processing environment: examples transaction server, file transfer protocol, network management, electronic mail, and terminal access to remote applications.

You may think of layers 1 thru 3 as 'the network', layers 4 and 5 as 'network services' and the upper layers as a network operating system. A single enterprise wide network may make use of a mix of different vendor specifications as long as compatibility at the interconnection layers is established. This is the advantage of a modular approach to specifications. For instance, MCDonalds has a X.25 WAN, an Ethernet LAN, and an ISDN at the lower three levels that are linked together at the transport and session layer using TCP/IP. On the other end, a diverse set of applications including POS terminals, Macintosh computers, electronic mail, and CICS may co-exist on the higher levels as long as they are all compatible at the Session layer. Without a modular structure such as OSI, integration of this nature would not be possible. Because of OSI, you may shop for network hardware and software from a variety of vendors and accomodate new services such as document transfer.

Obstacles to OSI: Historically, each computer vendor has developed its own proprietary architecture. Once an MIS installs a vendor-specific system, the migration to an 'open' system such as OSI is expensive and technically challenging.

OSI success story: GM's MAP (Manufacturing Automation Protocol) -> an OSI-based network standard for a manufacturing environment and includes specifications for CAD, numerical control machines, and robots.

Another model: IBM's SNA (System Network Architecture) -> used to be very important in MIS design but now dying out and giving way to OSI. Therefore SNA will not be described in this course but you should know what it is.

Original file name: OSIlecture

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