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Cisco Hierarchical Model:
For more information about this, please read our separate tutorial titled "The Cisco Hierarchical Model".

OSI Model:
The OSI model is a layered model and a conceptual standard used for defining standards to promote multi-vendor integration as well as maintain constant interfaces and isolate changes of implementation to a single layer. It is NOT application or protocol specific. In order to pass any Cisco exam, you need to know the OSI model inside and out.

The OSI Model consists of 7 layers as follows:

Layer	Description	Device	Protocol
Application	Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes.	Gateway	NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk
Presentation	Performs protocol conversion, encryption and data compression	Gateway and redirectors	NCP, AFP, TDI
Session	Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release	Gateway	NetBios
Transport	Repackages messages into smaller formats, provides error free delivery and error handling functions	Gateway	NetBEUI, TCP, SPX, and NWLink
Network	Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management.	Router and brouter	IP, IPX, NWLink, NetBEUI
**Data Link	Packages raw bits into frames making it transmitable across a network link and includes a cyclical redundancy check(CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers.	Switch, bridge and brouter	None
Physical	Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. It also includes specifications for certain mechanical connection features, such as the adaptor connector.	Multiplexer and repeater	None

Here is an easy way to memorize the order of the layers:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.

Class	Range	Explanation
A	1-126	IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. A class C is for smaller networks with the first 3 octets making up the netid and the last octet comprising the hostid. The later two classes aren’t used for networks.
B	128-191
C	192-223
D	224-239 (Multicasting)
E	240-255 (Experimental)

A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and netid. The default subnet masks are as follows:

Class	Default Subnet	# of Subnets	# of Hosts Per Subnet
Class A	255.0.0.0	126	16,777,214
Class B	255.255.0.0	16,384	65,534
Class C	255.255.255.0	2,097,152	254

In these cases, the part of the IP address blocked out by 255 is the Net ID.

3COM’s IP addressing tutorial is just superior. It covers basic IP addressing options as well as subnetting and VLSM/CIDR.

IPX/SPX:
IPX will also be an important issue to consider in network management given the fact there many companies still use Netware servers. There are two parts to every IPX Network address - the Network ID and the Host ID. The first 8 hex digits represent the network ID, while the remaining hex digits represent the host ID, which is most likely the same as the MAC address, meaning we do not need to manually assign node addresses. Note that valid hexadecimal digits range from 0 through 9, and hexadecimal letters range from A through F. FFFFFFFF in hexadecimal notation = 4292967295 in decimal.

Sequenced Packet Exchange(SPX) belongs to the Transport layer, and is connection-oriented. It creates virtual circuits between hosts, and that each host is given a connection ID in the SPX header for identifying the connection. Service Advertisement Protocol(SAP) is used by NetWare servers to advertise network services via broadcast at an interval of every 60 minutes by default.

OSI Layer 2 Protocols

Ethernet / IEEE 802.3
Token Ring / IEEE 802.5
FDDI Fiber Distributed Data Interface
PPP Point-to-Point Protocol
X.25
Frame Relay
ISDN - Integrated Services Digital Network

Ethernet

CSMA /CD - Carrier Sense Multiple Access / Collision Detection local area network
A broadcast network - all frames that cross a segment can be heard by all machines on the segment
All machines have equal access, they wait for a quiet time to transmit, collisions occur when two machines transmit at the same time. When a collision occurs both the machines apply a back off time that prevents them from colliding again
Services both the Physical and Data Link layers

Ethernet frame (read right to left)
FSC	Data	Type	Source	Destination	SOF	Preamble
Frame Check Sequence	Data to be handed to the upper layer protocol	Defines which upper layer protocol will receive the packet	MAC Address	MAC Address	Start of Frame	Beginning of the frame
4 bytes	46-1500 bytes	2 bytes	6 bytes	6 bytes	1 byte	7 bytes

IEEE 802.3

Also a broadcast network
More concerned with the Physical Layer and how it talks to the Data Link Layer

802.3 frame (read right to left)
FSC	Data	Length	Source	Destination	SOF	Preamble
Frame Check Sequence	Contains 802.2 header and the encapsulated data to be passed to an upper layer protocol that is defined in data field	Defines how many bytes the 802.2 header and data field is	MAC Address	MAC Address	Used to synchronize the frame reception portions of all the machines on the segment	Beginning of the frame
4 bytes	46-1500 bytes	2 bytes	6 bytes	6 bytes	1 byte	7 bytes

Token Ring / IEEE 802.5

Specifies baseband signaling, token passing, and data rates
Equal time is allotted to each station. achieved by passing a token around the ring

Tokens

A small frame, a station cannot transmit data unless in possession of the token
The Token is passed around the ring, each station waits for the token, if the station doesn't have anything to transmit, it passes the token along. If it does have something to transmit, it change the frame to a start of frame identifier and appends data to the frame. When the token is changed, no other station on the ring can use the token to transmit data. This eliminates collisions. The altered frame is passed along the ring until it reaches the destination address. The destination station copies the data and the original frame continues around the token until it reaches the source station. At the source, the token is removed and a new token is sent out onto the network. This way each station has an equal opportunity to transmit data.
Fault Recovery - an Active Monitor is designated by the workstations on the ring
Active Monitor - responsible for token monitoring, token generation when token disappears, purging recycled frames, and other ring maintenance

Token Frame (read right to left)
End Delimiter	Access control	Start Delimiter
Indicates end of frame	Defines frame type	Indicate start of frame
1 byte	1 byte	1 byte

Data / Control Frame (read right to left)
Frame Status	End Delimiter	FCS	Data	Source	Destination	Frame Control	Access Control	Start Delimiter
Can relay 2 statuses Frame copied or address recognized	Indicates end of frame	Error checking, used in CRC calculations	All data that is to be encapsulated. length is determined by how long the station can hold a token	Packet's address	Packet's address	Info Regarding Frame Control	Assigns Priority to the frame, distinguishes frame type	Alerts each station that the frame is Data/Control Frame
1 byte	1 byte	1 byte	varies	6 bytes	6 bytes	1 byte	1 byte	1 byte

FDDI

Physical media is Fiber Optic, dual ringed, operates at 100Mbps Bandwidth
Uses tokens to control media access, uses a ring topology
One ring is Primary ring flows clockwise
Other ring is secondary ring flows counterclockwise

Specifications

Physical Layer Medium Independent (PMD) - defines physical transmission characteristics - light levels, allowable bit errors, decibel loss, actual physical equipment
Physical Layer Protocol (PHY) - encodes and decodes data, before and after transmission over the physical medium
MAC - bridges FDDI with layer 2 of OSI model, responsible for how FDDI runs as layer 2 technology, the physical device, and the way data is presented to the physical layer
Station Management (SM) - responsible for station and ring configuration, fault recovery and other management duties

PPP

Used to transfer data over serial point-to-point links, accomplishes this by using a layer 2 serial encapsulation HDLC ( High Level Data Link Compression)
Uses LCP (Link Control Protocol) to manage the serial connection
Network Control Protocols are used to allow PPP to use other protocols from layer 3, thus enables PPP to assign IP addresses dynamically

PPP frame (read right to left)
FCS	Data	Protocol	Control	Address	Flag
Used for CRC calculations	Information handled by the upper layer protocols	Indicates the encapsulated data's protocol	Request a transmission of user data	Holds a broadcast address	Specifies beginning of frame
1 byte	varies	2 bytes	1 byte	1 byte	1 byte

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OSI Layer 2 Protocols