You have been introduced to internetworking.
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Internetworks have evolved in response to changing demands and new technologies.

Internetworks tie LANs and WANs, computer systems, software, and related devices together to form the corporate communications infrastructure.

Today's network users demand high bandwidth, low delays, and multimedia capabilities from global internetworks.

The global internetwork must be reliable, easily managed, and be able to connect many disparate networks.

LANs connect computers within a limited geographical area. WANs use the services of telecommunications carriers to operate beyond the LAN's geographic scope.

Devices you would find on both LANs and WANs include

   • routers
   • ATM switches

Devices you would find specifically on a LAN include

    • repeaters
    • bridges
    • hubs
    • Ethernet switches
    • gateways

Devices you would find specifically on a WAN include

    • X.25 or Frame Relay switches
    • modems, CSU/DSU devices, TA/NT1 devices
    • communications servers
    • multiplexors
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You have been introduced to the OSI reference model.
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The OSI reference model is made up of seven layers:
 
    • the application layer
    • the presentation layer
    • the session layer
    • the transport layer
    • the network layer
    • the data-link layer
    • the physical layer

Layers in one system communicate with their peer layers in other systems.

The information to be sent descends through the layers of the sending system and is encapsulated with headers and trailers from each layer as appropriate. At the physical layer it is transformed into electircal signals and sent over the physical medium. At the other end it ascends through the other system's layers and the headers and trailers are stripped off until the message reaches the second application.

The physical layer of the OSI model defines the physical and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

The data-link layer of the OSI model is responsible for providing reliable transit of data across a physical link.

The data-link layer is divided into two sublayers:

    • the logical link control (LLC) sublayer
    • the media access control (MAC) sublayer

Layer three of the OSI model, the network layer, is responsible for providing consistent end-to-end packet delivery services to the transport layer.

Routers determine an optimal path to a destination using routing algorithms and routing tables.

The transport layer of the OSI model provides mechanisms for
 
    • multiplexing upper layer applications
    • the establishment, maintenance, and
      orderly termination of virtual circuits
    • information flow control
    • transport fault detection and recovery

The main function of the session layer is to control data conversations between devices. These are known as "sessions".

The presentation layer is concerned with
 
    • the format and representation of user data
    • the data structure (or syntax) used
      by programs
    • data encryption
    • standards for image and sound formats

The application layer
 
    • identifies and establishes the availability
      of the intended communication partner
    • synchronizes the sending and receiving
      applications
    • establishes agreement on procedures
      for error recovery and control of
      data integrity
    • determines whether sufficient resources
      for the intended communications exist
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You have been introduced to some common topologies.
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Ethernet is a very common LAN technology. It provides data transport across the physical link joining devices on a LAN. The Ethernet specification describes a carrier sense multiple access with collision detection (CSMA/CD) LAN.

After Ethernet, the next most popular LAN specification is Token Ring. Token Ring networks operate by passing a small frame, called a ^Rtoken^r, around the network. Although logically a ring, the Token Ring network is physically laid out as a star.

Whereas Ethernet broadcast media provides opportunistic access, Token Ring provides fair deterministic access.

FDDI provides a very reliable network environment which operates over dual counter-rotating rings. It was developed for high speed, high bandwidth requirements. FDDI operates at 100 Mbps over optical fiber medium.

ATM is a high speed, high bandwidth cell relay technology. It can support demanding applications such as:
 
    • interactive multimedia
    • real-time video services
    • client/server databases
    • interconnection of existing networks
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You have been introduced to some common LAN and WAN protocols.
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The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE).

WAN data-link protocols such as Frame Relay and X.25 describe how frames are carried between systems on a single data link.

The common data-link encapsulations associated with synchronous serial lines are
 
    • Synchronous Data Link Control (SDLC)
    • Point-to-Point (PPP)
    • High-Level Data Link Control (HDLC)
    • Link Access Protocol, Balanced (LAPB)

X.25 provides a synchronous, bit-serial full-duplex point-to-point circuit for data transmission between the DTE and the DCE.

X.25 defines three basic types of packet service:
 
    • a switched virtual circuit
    • a permanent virtual circuit
    • a connectionless, datagram service

Frame Relay was developed to meet the needs of bursty data traffic. It is a service designed to exploit the high quality and high bandwidths offered by optical fiber links.

TCP/IP is a suite of protocols incorporating protocols at the network, transport, and application layers of the OSI model. IP and TCP are the most important of these.

IP provides connectionless, best-effort delivery routing of datagrams and TCP guarantees packet delivery and data integrity.

Two main protocols in Novell's NetWare are IPX and SPX.

IPX is a network layer protocol that provides a very fast connectionless service. It provides network layer services such as addressing and routing.

SPX is a transport layer protocol. It is a connection-oriented protocol, providing error checking and windowed flow control. It ensures reliable delivery by retransmitting information that has not been correctly received.

AppleTalk is a suite of networking protocols for the Apple Macintosh. It works with Apple's own lower-layer protocol, LocalTalk, but can also run over the standard lower-layer protocols such as Ethernet and Token Ring.

Banyan Virtual Integrated Network Service (VINES) implements a distributed network operating system.

It provides three transport layer services:
 
    • unreliable datagram service
    • reliable message service
    • data stream service

DECnet is described by the DNA architecture. DNA is fully OSI-compliant and supports a wide variety of protocols.
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You have been introduced to the basics of routing.
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A router is a device that routes packets across an internetwork. It makes decisions about the route to a packet's destination and it relays it from one data link to another.

Path determination enables the router to evaluate available paths to a destination and determine which to use. Routers determine the path to a destination using ^Rrouting algorithms^r, which initialize and maintain ^Rrouting tables^r containing route information.

The switching function of a router allows it to accept packets on one interface and forward them to a second interface.

Routing algorithms use one or more of the following metrics:

    • path length
    • reliability
    • delay
    • bandwidth
    • load
    • communication cost

Node addresses and network addresses should be unique in order to avoid conflicts on an internetwork.

Many protocols use a two-part addressing scheme. The first part of the address identifies the network and the second part the node. How the address parts are interpreted and what authority allocates them differs from one protocol to another.

Routing algorithms often have one or more of the following goals:

    • optimality
    • simplicity
    • robustness
    • rapid convergence
    • flexibility

In ^Rclassless addressing^r the prefix can be of any length. The length of the prefix is indicated either by a network mask or by a number following the address.

In ^Rclassful addressing^r only a few specified prefix lengths are allowed, possibly extended by subnet masks. In standard IP classful addressing

    • class A has a prefix length of 8 bits and starts
         with 0
    • class B has a prefix length of 16 bits and starts
        with 10
    • class C has a prefix length of 24 bits and starts
         with 110

The particular design goals of the algorithm affect the operation of the resulting routing protocol.

^RRouted protocols^r are protocols that are routed over an internetwork.  They carry trafic accross the network.

^RRouting protocols^r are protocols that implement routing algorithms in order to allow routers to direct routed protocol packets.
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You have been introduced to the different types of routing algorithm.
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^RStatic routing^r is administered manually. A network administrator enters the route information into the router and must manually update it whenever there is a change in the topology of the network.

^RDynamic routing^r algorithms automatically adjust in real time. Whenever there is a change in the topology of the network they send out update messages to the other routers. Each router then re-runs its routing algorithm and adjusts its routing table.

^RDistance vector^r routing algorithms determine the direction and distance to any link in the internetwork. Each router periodically sends all, or a portion of, its routing table to its nearest neighbors only.

^RLink state^r routing algorithms maintain a complex database of topology information. Each router sends only that portion of the routing table that describes the state of its own links.

^RBalanced hybrid^r routing algorithms are a combination of the best of both. They use distance vectors to determine the best paths to destination networks, but allow network topology changes to trigger the routing updates. Like link state routing algorithms they converge rapidly. However they have a lower CPU, bandwidth, and memory overhead.
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