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MCITP MCSE and CCNA Certification Notes Series :

MCSE 2016 Boot Camp : Azure Boot camp : CCNA / CCNAX Boot Camp:

CCNA Boot Camp

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.

Transport Layer

The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery. Services located in Transport layer both segment and reassemble data from upper layer applications and unite it onto the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an Internetwork. It also hides details of any network dependent information from the higher layer by providing transparent data transfer.

Flow Control

-Data integrity is ensured by maintaining flow control and allowing users the option to request reliable data transport between systems. Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Reliable data transport employs a connection-oriented communication session between systems. The protocols ensure that the following are achieved:

-segments delivered are acknowledged to sender upon delivery.

-non acknowledged segments are re-sent.

-segments are put back in sequence upon arrival at their destination.

-a manageable data flow is maintained to avoid congestion, overloading, and data loss.

Multiplexing

The Transport layer is responsible for providing mechanisms for multiplexing upper layer applications. Multiplexing enables data from several applications to be transmitted onto a single physical link.

Virtual Circuits

Virtual circuits are established, maintained, and terminated by the transport layer.

Error Checking and Recovery

Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur.

Acknowledgments

-Positive acknowledgement with retransmission ensures that reliable data delivery by requiring a receiving machine to send an acknowledgment message to the sender when it receives data. The sending machine documents each segment sent and waits for an acknowledgment before sending the next segment. Using windowing, the machine will transfer an agreed upon number of segments. If the receiving machine receives all the segments intact, it will request the next segment of the next window. If it misses a segment, it will request the missing segment and will transmit a request for the next segment of the next window, when the first window's segments are all received.

-During a transfer, congestion can occur because high speed computers can generate data faster that the network can transfer it or because many computers are using the network and sending datagrams through a single gateway. When a machine receives a flood of datagrams, it stores them in a buffer. If the buffer fills, all additional datagrams are discarded. Transport can issue a "not ready" signal to stop a device from transmitting additional segments. Once the buffer is emptied, it sends a "ready" transport indicator. When the waiting machine receives this "go" signal, it continues where it left off. To avoid failures in data transfers, the receiving host acknowledges every segment it receives.

Connection-Oriented Communications

-In reliable transport operations

One device first establishes a connection oriented session with is peer (Initiation)

Both host's application programs begin by notifying their individual Operating Systems that a connection is about to be initiated. (Synchronization)

The two Operating Systems communicate by sending messages over the network confirming that the transfer is approved and both sides are ready for it to take place. (Negotiating)

Once the synchronization is complete, a connection is fully established and data transfer begins. (Established)

The data transfers. While the information is being transferred between hosts, the two machines periodically check in with each other, communicating through their protocol software to ensure that all is going well and that data is being received properly.

Windowing

-A window is the number of segments that can be sent without receiving an acknowledgement. Windowing can increase the throughput for data exchanges by limiting the number of acknowledgments needed for total segments transferred. Example: if the window size is three then an acknowledgment is required after the third segment is transferred.

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Transport Layer