Section 4 : The OSI Network Layer
CSIS 479R Fall 1999
“Network +”
George D. Hickman, CNI, CNE
Objectives
Identify the basic purpose of the OSI Network layerIdentify the uses of the addresses defined at the Network LayerIdentify the characteristics of the three switching methodsIdentify the characteristics of the two route discovery methods
Objectives (con’t)
Identify the characteristics of the two route selection methodsIdentify the characteristics of Network layer connection servicesIdentify the basic purpose of gateway servicesDescribe why connectivity devices are needed on networks
Objectives (con’t)
Describe the function of Ethernet and token ring repeaters
Describe the function of bridges
Explain how a transparent bridge works
Explain how a source-routing bridge works
Explain how switching-hubs work
Objectives (con’t)
Describe how token ring switches work
Describe routing protocols
Identify nonroutable protocols and how to deal with them
Describe combination devices
Decide when to use bridges,switches, and routers
Basic Purpose of theOSI Network Layer
RoutingMoving information between multiple
independent (often dissimilar) networks called internetworks
Routes Data using:SwitchingNetwork Layer Addressing (Logical, not physical or MAC)
Routing Algorithms
Processes and Methods
Logically separate networks must have unique network addressesSwitching determines how connections are madeImplement routing to have “best” data path determinedDifferent levels of connection services can be implemented, based on errors expected
Processes and Methods (con’t)
AddressingLogical NetworkService
SwitchingPacketMessageCircuit
Processes and Methods (con’t)
Route DiscoveryDistance VectorLink-state
Route SelectionStaticDynamic
Processes and Methods (con’t)
Connection servicesNetwork-layer flow controlError controlPacket sequence control
Gateway servicesNetwork layer translation
Network Layer AddressingLogical network address Logically distinguish two networks in an
internetwork Routers connect two networks with different logical
network addresses
Service address Every entity (hardware or software) needs its own
address (port or socket) to send and receive data Ports identity an upper layer software process or
protocol. (Computers can run several concurrently) See figure 4-3 on page 4-10
Switching Method Characteristics
Circuit Switching A Single path is set up for the duration of a single
conversation.
Message Switching Conversations are divided into messages, each of
which MAY take a different path.
Packet Switching Combination of both Messages broken into packets small enough to fit
in RAM. A sequence number is added to each packet Most LANs use Packet Switching
Packet Switching
Virtual Circuit Packet SwitchingLogical connections between sender and
receiverAppears as a point-to-point link, but is actually
a logical switched pathDatagram Packet Switching
Used when no logical connection is required In common use on the internetRelies on the network layer to navigate paths
and correct errors
Route Discovery TermsHop Count Number of routers data must pass through to
reach the destination
Tick count Amount of time required to reach the destination
network
Relative expense An assignable number based on user-defined
costs
Route discovery uses two methods Distance Vector Link-state
Distance VectorRoute Discovery Method
Routers keep tables of routes to various destinations
These tables are communicated to neighboring routers
If router A tells router B that it can reach network 100 in 6 hops, router B would show it can reach network 100 in 7 hops
Updating routing tables is called convergence
Link StateRoute Discovery Method
Considered more intelligent and faster than distance-vectorAfter an initial table is received, routers only broadcast information that has changed (not all routes)Link State Route Discovery requires less network overhead, but can require considerable planning
Route Selection Methods
DynamicRouters can change in response to
changing network conditionsRoute selections are made at each router
StaticData packets always follow a
predetermined pathPath defined by network administrator or
by an assigned network device
Network LayerConnection Services
Data Link-LLC sublayer connection services control the amount of data transmitted and notification of bad frames
Data Link layer connection services manage communication between source and destination devices
Network Layer connection services build on these services, add reliability
Connection Services
Network Layer Flow ControlSimilar to Data Link layer flow control
Can be guaranteed rate or static/dynamic windows
Can involve intelligent path selection (congestion control)
Connection Services (Con’t)
Network Layer Error ControlPrimarily concerned with packet loss,
duplicate packets, altered packetsLost packets handled by acknowledgmentsDuplicate packets are handled by dropping all
but one of the packetsAltered packets are detected (sometimes
corrected) by use of a CRC check CRC is calculated at each router, since the address
field is changed at each hop
Connection Services (Con’t)
Network Layer Packet Sequence ControlUpper layer messages are rebuilt by
putting arrived packets back into the original order
Packet sequence control can take place at Network layer, but at the Transport layer is more common.
Gateway ServicesA Gateway interprets and translates the rules on two separate networksAddress, Route Discovery, Route Selection, and Connection Service rules may vary from one network to anotherBook example: Two networks use data units of different sizes. A Network Layer gateway fragments and reassembles data into acceptable sizes for both networksThe Default Gateway or Default Router is where all packets are sent that are destined for devices not on the local network segment
Connectivity Devices
LAN Media limitations Media may limit number of devices, length of cable
run, or available bandwidth Repeaters and Bridges address these issues
Data destination across multiple networks Routers forward data from network to network
Incompatible Systems Two systems without a common protocol Gateway acts as a translator
Bridges, Repeaters, Switching Hubs, Routing Protocols
Are covered in the book; we will not have class time to cover them
Connection DevicesBroutersBridge and Router in a single deviceCheaper than both devices, more
expensive than either one
Switching Hub and Router CombinationLeading-edge technology, which means
implementation problems
Nonroutable Protocols
Protocols that do not use Network Layer for routingProtocols that use only static routes that cannot be updatedDeal with them by encapsulation (tunneling) or bridging. Routing SNA NetBIOS NetBEUI LAT
Network Layer
This is a major layer in the OSI model, and has many important features we were unable to discuss in class
It is critical that you read the material in the book. Email me if you have any questions about the content.