1. Identify and describe the functions of each of the seven layers of the OSI reference model.

OSI Model

Application

• Identifies and establishes the resources availability of the intended communication devices.
• Synchronizes sending and receiving applications.
• Agrees on error control and data integrity of communicating applications.
• Provides system-independent processes or program services to end users.
• User interface (Telnet, SMTP, FTP, etc)

Presentation

• Negotiates data transfer syntax.
• Encoding / decoding (ASCII, EBCDIC, PICT, JPEG, TIFF, MPEG, MIDI).

Session

• Establishing, managing, and terminating sessions between applications.
• Coordinates communication between systems.
• Protocols: NFS (Network File System), SQL, RPC, X Window.

Transport

• Mechanism for multiplexing upper-layer application, establishing session, and building / tearing down virtual circuits.
• Flow control:
  • Upon receiving the segments, the recipient sends an acknowledgment back to the sender.
  • Any segments not acknowledged are retransmitted.
  • Segments are sequenced back into their proper order upon arrival at their destination.
  • A manageable data flow is maintained in order to avoid congestion, overloading, and the loss of any data.
• Acknowledgement and windowing.
• Hides detail of any network-dependent information from the higher layers by providing transparent data transfer.
• Reliable data transport, error checking and recovery, flow control, multiplexing.

Network

• Routing, logical addressing.

Data Link

• Putting 1s and 0s into a logical group.
• Two IEEE sublayers:
  • LLC (logical link control): Manage communications.
  • MAC (media access control): Manages device addressing and access to the physical layer.
• Reliable transit of data across the physical network.

Physical

• Bit synchronization.
• Defines the physical topology.
• Build, maintain, and break physical connections.

3. Describe data link addresses and network addresses, and identify the key differences between them.

4. Identify at least 3 reasons why the industry uses a layered model.

1. It clarifies general function rather than how to do it.
2. It divides the complexity of networking into more manageable sub-layers.
3. It uses standard interfaces to enable ease of interoperability.
4. It allows changes in one layer to occur without changing others.
5. It allows specialization to help the industry progress.
6. It facilitates systematic troubleshooting.

5. Define and explain the 5 conversion steps of data encapsulation.

1. User information is converted into data.
2. Data is converted into segments.
3. Segments are converted into packets or datagrams.
4. Packets or datagrams are converted into frames.
5. Frames are put into bits.

6. Define flow control and describe the three basic methods used in networking.

1. Buffering
2. Source-Quench Messages
   1. A network device receives too many segments and starts discarding them.
   2. Each time the receiving device drops a segment, it sends a source-quench message to the transmitting device.
   3. The transmitting device receives a source-quench message for each segment that was discarded. It does not stop transmitting, however; it just slows down the process until source-quench messages are no longer being sent from the receiving device.
   4. Once the transmitting device stops receiving source-quench messages, it slowly increments the number of segments being sent until it starts receiving the source-quench messages again.
3. Windowing
   1. The transmitting device sends three segments to the destination device.
   2. After receiving the three segments, the destination device sends an acknowledgment to the transmitting device.
   3. Once the transmitting device receives the acknowledgment, it then sends three more segments.
   4. If the transmitting station does not receive an acknowledgment from the receiving station, it will resend the segments after a time-out period. Since the receiving device may have overflowed its buffers, the transmitting device resends the segments at a slower rate.

7. List the key internetworking functions of the OSI Network layer and how they are performed in a router.

When packets are moving

• Logical address do not change.
• Physical address changes at each hop.

Physical address movement between hosts

• Host interface on source network.
• Router interface on source network.
• Router interface on destination network.
• Host interface on destination network.