The OSI model consists of seven layers each of
which can, and typically does, have several sub layers. The CCNA exam should
not cover any sub layers with the exception of the sublayer for Local Area
Network (LAN) Data Links; the names of the OSI model layers and their main
functions are simply good things to memorize. And frankly, if you want to
pursue your Cisco
certifications beyond CCNA, these names and functional areas
will come up continually.
Layer Name Functional
Description Examples
Application
(Layer 7) An
application that communicates with other computers FTP, WWW
browsers,
is
implementing OSI application layer concepts. The Telnet, NFS, SMTP
application
layer refers to communications services to gateways
(Eudora,
applications. For
example, a word processor that lacks CC:
mail), SNMP, X.400
communications
capabilities would not implement code mail,
FTAM
for
communications, and word processor programmers
would not
be concerned about OSI Layer 7. However, if
an
option for transferring a file were added, then the
word
processor would need to implement OSI Layer 7
(or the equivalent layer in
another protocol specification).
Presentation
(Layer 6) This
layer’s main purpose is defining data formats, such TIFF, GIF, JPEG, PICT,
as
ASCII text, EBCDIC text, binary, BCD, and JPEG. ASCII,
EBCDIC,
Encryption is also defined by OSI as a presentation encryption, MPEG,
layer
service. For example, FTP allows you to choose
binary or
ASCII transfer. If binary, the sender and
receiver do
not modify the contents of the file. If ASCII
is
chosen, the sender translates the text from the
sender’s
character set to a standard ASCII and sends the
data. The
receiver translates back from the standard
ASCII to the character set used on the receiving
computer.
Session
(Layer 5) The session layer defines how to start, control, and end RPC, SQL, NFS, NetBios
conversations
(called sessions).
This includes the names,
AppleTalk ASP,
control and
management of multiple bi-directional DECnet
SCP
messages so
that the application can be notified if only
some of a
series of messages are completed. For
example, an
Automated Teller Machine transaction in
which you
get cash out of your checking account should
not
debit your account and fail before handing you the
cash, and
then record the transaction even though you
did not
receive money. The session layer creates ways to
imply
which flows are part of the same session and
which
flows must complete before any is considered complete.
Transport
(Layer 4) Layer
4 includes the choice of protocols that either do TCP, UDP, SPX
or do
not provide error recovery. Reordering of the
incoming data
stream, when packets arrive out of order
is included, as well as reassembly of the data if the
packets fragmented during transmission. For example,
TCP may give a 4200 byte segment of data to IP for
delivery. IP will fragment the data into smaller sizes
if a
4000 byte packet could not be delivered across some
media. So, the receiving TCP might get three different
segments or 1400 bytes apiece. The receiving TCP
might receive these in a different order as well, so
it
reorders the received segments, compiles them into the
original 4200 byte segment, and then is able to move
on
to acknowledging the data.
Layer Name Functional
Description Examples
Network
(Layer 3) This layer
defines end-to-end delivery of packets. To IP,
IPX, AppleTalk, DDP
accomplish this, the network layer defines logical
addressing so that any endpoint can be identified. It
also
defines how routing works and how routes are learned
so the packets can be delivered. The network layer
also
defines how to fragment a packet into smaller packets
to
accommodate media with smaller maximum
transmission unit sizes. The network layer of OSI
defines most of the details that a Cisco router
considers
when routing OSI. For example, IP running in a Cisco
router is responsible for examining the destination IP
address of a packet, comparing that address to the IP
routing table, fragmenting the packet if the outgoing
interface requires smaller packets, and queuing the
packet to be sent out to the interface.
Data
link (Layer 2) The data link (Layer 2)
specifications are concerned Frame
Relay, HDLC,
with getting data across one particular link or medium. PPP, IEEE 802.3/802.2,
The data-link protocols define delivery across an FDDI, ATM, and
IEEE 802.5/
individual link. These protocols are necessarily 802.2
concerned with the type of media in question; for
example, 802.3 and 802.2 are specifications from the
IEEE, which are referenced by OSI as valid data-link
(Layer 2) protocols. These specifications define how
Ethernet works. Other protocols, like High-Level Data
Link Control (HDLC) for a point-to-point WAN link,
deal with the different physical details of a WAN
link.
OSI, like other protocol specifications, often does
not
create any original specification for the data link
layer
but instead relies on other standards bodies such as
IEEE to create new data link and physical layer
standards.
Physical
(Layer 1) These physical
layer (Layer 1) specifications, which are EIA/TIA-232,
EIA/TIA-449,
also typically standards from other organizations that V.35, V.24, RJ45,
are referred to by OSI, deal with the physical Ethernet,
IEEE 802.3, IEEE 802.5,
characteristics of the physical medium. Connectors, FDDI, NRZI, NRZ,
B8ZS
pins, use of pins, electrical currents, encoding, and
light
modulation are all part of different physical layer
specifications. Multiple specifications are sometimes
used to complete all details of the physical layer.
For
example, RJ45 defines the shape of the connector and
number of wires/pins in the cable. Ethernet and 802.3
define the use of wires/pins 1,2,3, and 6. So to use a
category 5 cable, with an RJ-45 connector for an
Ethernet connection, Ethernet and RJ-45 physical layer
specifications are used.