OSI Layers

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 layerís 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††††††††††††††††††††††††† MIDI, HTML

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

senderís 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.