The OSI Model
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Begins at the Application layer where data is generated.
Each layer subsequently adds information to the packet.
Transport layer breaks the data into packets.
Data is passed through the Physical layer to the cable.
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Sending Computer
Receiving Computer
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Application - (layer 7) Provides interfaces (APIs) to the software that enable programs
to use network services. Allows applications to use the network. Handles network
access, flow control and error recovery.
o Protocols and Services - Telnet, DHCP, DNS, FTP, WWW, HTTP, TFTP, SNMP,
SMTP, POP3, IMAP4
o Devices that function at this layer - Gateways
o API – Application Programming Interface: a set of instructions that allows a
program to interact with the operating system.
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Presentation - (layer 6) Translates data into a form usable by the application layer
(ASCII, EBCDIC JPEG, GIF, PICT MIDI, MPEG ). The redirector operates here (software
code that intercepts requests for remote services). Responsible for protocol conversion,
translating and encrypting data, and managing data compression.
o Protocols and Services – Secure Sockets Layer (SSL)
o Devices that function at this layer - Gateways
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Session - (layer 5) Allows applications on connecting systems to establish and maintain
a session. Provides synchronization between communicating computers. Decides which
node can communicate first and for how long. The “traffic cop” on the network.
Monitors the identification of session participants.
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Protocols and Services – Remote Procedure Call (RPC), NetBIOS, Sockets,
Named Pipes
Devices that function at this layer - Gateways
Transport - (layer 4) Responsible for name resolution, packet handling, packet
sequencing and flow control. Ensures error-free delivery. Repackages messages,
breaking arbitrarily long packets into the maximum size that the type of network in
use can handle.
o Protocols - SPX, TCP, UDP, function on this layer
o TCP and SPX - connection-oriented communication for applications to ensure
error free delivery; connection-based - an acknowledgement (ACK) verifies that
the host has received each segment of the message, which results in reliable
delivery service. If the ACK is not received after a given time period, then the
data is resent. If segments are not delivered to the destination device correctly,
then the Transport layer can initiate retransmission or inform the upper layers.
Uses segmentation, flow control, and error checking to insure packet delivery
o UDP - connectionless communications and does not guarantee packet delivery
between transfer points. Connectionless - internal nodes along the message
path do not participate in error correction and flow control. Voice and Video
Transmissions.
o Devices that function at this layer - Gateways function on this layer
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Network - (layer 3) Translates physical addresses (MAC) into logical addresses (IP, IPX).
Responsible for addressing, determining routes for sending, managing network traffic
problems, packet switching, routing, data congestion, segmenting and reassembling
data.
o Protocols - IPX, IP, ICMP, ARP, RARP, RIP, OSPF, EGP, IGMP, IPSec function on
this layer
o IP - Internet Protocol. DOD standard designed for ARPAnet. Based on two
models, the DOD model (4 layers) and the OSI model (7 layers), which is the
Windows NT and Unix default protocol during installation. Requires a lot of
configuration. IP functions on the Internet layer of the DOD model and on the
Network layer of the OSI model. A connectionless protocol responsible for
addressing and routing packets between hosts.
o IPX - Internetwork Packet Exchange. It's the Novell NetWare designed protocol
which is the default protocol during installation. Packet sizes for Ethernet are
approx 1500 bytes, for Token Ring they are approx 4Kb. Performs addressing
and routing functions. Resides in Network Layer. Requires some configuration.
o Services - software & hardware addresses and packet routing between hosts
and networks (IP/IPX); software addresses to hardware addresses are resolved
(ARP/RARP), sends messages and reports errors regarding packet delivery
(ICMP), reports host group membership to local multicast routers (IGMP)
o ARP - used to gather hardware addresses of local hosts and the default gateway,
you can view the ARP cache and check for invalid or duplicate entries
o ICMP - ping - will send ICMP echo packets to verify connections to a remote
host (or local if using the loopback address)
o IGMP - Tracert - used to determine the route a packet took to reach its
destination
o Devices that function at this layer – Routers, Brouters and Layer 3 Switches
function up to this layer
o Routers help organize a large network into terms of logical network segments
using logical network Ids
o A brouter functions as a bridge on the data link MAC sublayer and as a router on
the network layer.
o A routable protocol permits its packets to be sent beyond a single LAN/WAN
segment whereas a non-routable protocol's packets will remain on the
originating LAN segment
o Unique network IDs prevent confusion between devices and help them to
properly direct their packets/datagrams. When a router receives a packet which
is destined for a network ID on the other side, the router will know how to
behave.
o A default gateway is the exit and entry point of a subnet.
o Subnetworks are a division of the entire internetwork which are created to
provide security and/or reduced traffic over a WAN or congested networks
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Data link - (layer 2) Manages physical layer communications between connecting
systems. It’s primary function is to divide data it receives from the Network layer into
distinct frames that can then be transmitted by the Physical layer. The Frame Check
Sequence (FCS) or Cyclic Redundancy Check (CRC) is appended to the frame here. The
Data Link Layer is divided into two distinct sublayers; the Logical Link Control (LLC)
sublayer and the Media Access Control (MAC) sublayer
o Protocols - Ethernet, FDDI, ATM, and Token Ring reside at this layer.
o A frame is a structured package for moving data that includes not only the raw
data, but also the sender’s and receivers network addresses.
o Frame Check Sequence (FCS)– The field in a frame responsible for ensuring that
data carried by the frame arrives intact. It uses an algorithm, such as CRC, to
accomplish this verification.
o Cyclic Redundancy Check (CRC) – An algorithm used to verify the accuracy of
data contained in a data frame.
o LLC - (802.2) Manages link control and defines SAP's (Service Access Points).
802.2 - LLC (Logical Link Control manages link control and defines SAPs); Adds
header fields to identify upper-layer protocols. It provides reliable, intelligent
information to otherwise dumb frames. Also, acts as a switch board to make
sure MAC frames find their way to the right Network layer process.
o 802.3 - (MAC communicates with adapter card) CSMA/CD - Ethernet; Provides
physical layer options including different topologies, media types, data rates
and signaling modes.
o 802.5 - (MAC communicates with adapter card) Token Ring LAN; Uses tokenpassing media access protocol across a physical star, logical ring and differential
Manchester encoding to provide data rates
o MAC - (802.3, 802.4, 802.5, 802.12) Communicates with adapter card.
o MAC addresses, which are a.k.a physical addresses, operate on the data link
layer. Each address is unique 12-digit hexadecimal ID number, which is hard
coded to the network device by the factory, and is used by devices to direct
their packets to other devices.
o Devices that function at this layer – NICs, Switches, WAPs, brouters and bridges
function on this layer using the MAC sublayer
o NIC - Network Interface Card is component added to a computer circuit board
expansion slot and connects directly to network cabling. NOS software is
installed on the workstation to allow communication between the workstation
OS and the server NOS.
o Bridges are used to segment networks. They forward packets based on the
physical address of destination node. Uses RAM to build a routing table based
on physical addresses. Some bridge types are capable of connecting dissimilar
network topologies. Will forward all protocols. Regenerates the signal at the
packet level
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Physical - (layer 1) Encodes the bits onto the physical medium. Defines electrical,
mechanical and procedural formats. Detects and Transmits precise voltages or light
signals over a physical medium. Dictates how the electrical signals are amplified and
transmitted over the wire. Defines cables, cards, and physical aspects. Data bits are
sent.
o Protocols - No protocols function on this layer
o Services - Ethernet (CSMA/CD), Token Ring (token passing), and other
communications occur
o Devices that function at this layer – Repeaters, MAUs, amplifiers, hubs, NICs,
cabling, connectors, gateways and multiplexers function on this layer.
o Hubs - a hub is a wiring concentrator for a LAN or WAN that provides a central
attachment point for network cabling. Coaxial cable doesn't use hubs. There are
three types of hubs: passive (absorbs some signal; no electronics to process data
signal), active (cleans signal; electronics to amplify signal), and intelligent
(managed & switching hubs).
o MAUs - (Multistation Access Unit) an access device used to connect the main
cabling structure to devices in use on a Token Ring network. This device adds
fault tolerance to the network so that a single failure doesn’t stop the whole
network
o Switching hubs - are intelligent hubs which contain circuitry that very quickly
route signals between ports on the hub. This method reduces bandwidth waste
because only the device which needs to receive the packet does, rather than the
entire network segment.
o Repeaters - devices that amplify and regenerate a signal to extend the distance
of a LAN transceivers - connect different Ethernet nodes together in an
organized fashion across an individual Ethernet segment; allows multiple
Ethernet segment nodes to connect to each other to create a segment.
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