Section 2 : The OSI Reference Model

Question 2

What is the OSI Model Layer 1 (Physical Layer)?

Answer 2

• Layer 1 is called the Physical Layer
• Part of theMedia/Lower Layers (Bit)
• How bits are represented on the medium
  
  • send/receive bits
• Wiring standards for connectors and jacks
• Physical topotology
  
  • Layer 1 devices view a network as physical topology opposed to a logical topology.
• Synchronizing bits
  
  • For two network devices to successfully communicate at the physical layer, they must agree on when one bit stops and another bit starts.
    
    • Asynchronous:
      
      • Wtih this approach, a sender states that it is about to start transmitting by sending a start bit to the receiver. When the receiver sees this, it starts it’s own internal clock to measure the next bits. after the sender transmits its data, it sends a stop bit to say that it is has finished its transmission.
    • Synchronous:
      
      • This approach synchronizes the internal clocks of both the sender and the receiver to ensure that they agree on when bits begin and end. A common approach to make this synchronization happen is to use an external clock (for example, a clock given by a service provider ).The sender and receiver then reference this external clock.
• Bandwidth usage
  
  • ​The two fundamental approaches to bandwidth usage on a network are Broadband and Baseband.
  • Broadband:
    
    • Broadband technologies divide the bandwidth available on a medium(For example, copper or fiber-optic cabling) into different channels. A sender can transmit different communication streams over the various channels. For example, consider frequency-division multiplexing (FDM) used by a cable modem. Specifically, a cable modem uses cetrain ranges of frequencies on the cable coming into your home from the local cable campnay to carry incoming data,another range of frequencies for outgoing data.
  • Baseband:
    
    • Baseband technologies, in contrast, use all the available frequencies on a medium to send data. Ethernet is an example of a networking technology that uses baseband
• Multiplexing strategy
  
  • ​Multiplexing allows mutliple communications sessions to share the same physcial medium. Cable TV,as previous mentioned allows you to receive mutliple channels over a single physical medium(For example, a coaxial cable plugged into the back of your television), Common approaches for multiplexing:
    
    • Time-division multiplexing (TDM):
      
      • TDM supports different communication session (For example, different telephone conversations in a telephony network) on the same physical medium by causing the sessions to take turns. For a brief period, defined as a time slot, data from the first session is sent, followed by the data from the second session. This continues until all sessions have had a turn, and the process repeats itself.
    • Statistical time-division multiplexing(StatTDM):
      
      • A downside to TDM is that each communication session receives it’s own time slot, evn if one of the sessions does not have any data to send at the moment. To make a more efficient use of available bandwidth, StatTDM dynamically assigns time slots to communications sessions on an as-needed basis.
    • Frequency-division multiplexing(FDM):
      
      • FDM divides a medium frequency range into channels, and different communication sessions send their data over different channels. As previously described, this approach to bandwidth usage us called Broadband.

Devices at this layer:

• Hub
• Wireless Access Point (WAP)
• Network Cabling

Question 3

What is the OSI Model Layer 2(Data-Link Layer)?

Answer 3

The Data-Link Layer is unique from other layers in that it has two sublayers of it’s own: MAC and LLC

Media Address Control(MAC):

• Physical Addressing
  
  • ​A common example of Layer 2 address is MAC address, which is a 48-bit address assigned to a device’s network interface card(NIC). MAC address are written in hexadecimal notation (for example 58:55:ca:eb:27:83).
    
    • The first 24-bit of the 48-bit address is the vendor-code. The IEEE registration authority assigns a manufacturer one or more unique vendor codes. You can use the list of vendor codes at http://standards.ieee.org/develop/regauth/oui/oui.txt to identify the manufacturer of a networking device, based on the first half of the devices’s MAC address. The last 24-bits of a MAC address are assigned by the manufacturer, and they act as serial number for the device. NO two MAC addresses in the world should have value.
• Logical topology
  
  • ​Layer 2 devices view a network as logical topology. Examples of logical topology include bus, ring topologies.
• Method of transmitting on the media
  
  • ​With several devices connected to a network, there needs ti be some strategy for deciding when a device sends on the media. Otherwise, mutliple devices send at the same time and thus interfere with another’s transmissions.

Logical Link Control (LLC):

Characteristics of the Logical Link Control (LLC) sublayer include the following:

• Connection services:
  
  • When a device on a network receives a message from another device on the network, that recipient device can give feedback to the sender in the form of acknowledgement message. The two main functions provided by these acknowledgement messages are as follows:
    
    • Flow control:
      
      • Limits the amount of data a sender can send at one time; this prevents the sender from overwhelming the receiever with too much information.
    • Error Control:
      
      • Allows the recipient of data to let the sender know whether the expected data frame was not recieved or whether it was recieved but is corrupted. The recipient figures out whether the data fram is corrupt by mathemetically calculating a checksum of the data recieved. if the calculated checksum does not match the checksum received with the data frame, the recipient of the data draws the conclusion that the data fram is corrupted and can then notify the sender via an acknowledgement message.
• Synchronizing transmissions:
  
  • Sender and receivers of data frames need to coordinate when a data frame is being transmitted and should be recieved.
    
    • Isochronous:
      
      • With Isochronous transmission, network devices look to a common device in the network as a clock source, which creates fixed length time slots. Network devices can determine how much free space if any, is available with a time slot and then insert data into an available time slot, A time slot can accomodate more than one data frame. Isochronous transmission does not need to provide clocking at the begining of a data string (as does synchronous transmission) or for every data frame (as does asynchronius transmission). As a result, Isochronous transmission uses little overhead when compared to asyncronous or synchronous transmission methods.
    • Asynchronous:
      
      • With asynchronous transmission, network devices reference their own internal clocks, and the newtwork devices do not need to synchronize their clocks. Instead the sender places s start bit at the begining of each data frame and a stop but at the end of each data frame. The start and stop bit tell the receiver when to monitor the medium for the presence of bits. An additional bit, called parity bit, might also be added to the end of each byte in a frame to detect an error in the frame. For example, if even parity error detection(as opposed to odd parity error detection) is used, the parity bit (with a value of either 0 or 1) would be added to the end of a byte, causing the total number of 1s in the data frame to be an even number. If the reciever of a byte is configured for even parity error detection and receives a byte where the total number of bits (including the parity bit) is even, the receiver can conclude that the byte was not corrupted during transmission.
    • Synchronous:
      
      • ​Two network devices that want to communicate between themselves must agree on a clocking method to show the beginning and ending of data frames. One aproach to providing this clocking is to use a separate communications channel over which a clock signal is sent. Another approach relies on specific bit combinations of control characters to indicate the beginning of a frame or byte data.
      • Lke asynchronous transmissions, synchronous transmissions can perform error detection. However , rather than using the parity bits, synchronous communication runs a mathematical algorithm on the data to create a cyclic redundancy check (CRC). if both the sender and the reciever calculate the same CRC value fro the same chucnk of data, the receiver can conclude that data was not corrupted during transmission.

NOTE: Using a parity bit to detect errors might not be effective if a byte has more than one error(that is, more than one bit that has been changed from it’s original value.)

Devices examples:

• Switches
• Brdiges
• NICs

NOTE: NICs are not entirely defined at the data link layer because they are partially based on the physical layer standards, such as NIC’s network connector.

Question 4

What is the OSI Model Layer 3 (Network layer)?

Answer 4

The network layer is primarliy concerned with forwarding data based on logical addresses.

• Logical Addressing:
  
  • whereas the data-link layer uses physical addresses to make forwarding decisions, the network layer uses logical addressing to make forwarding decisions. A variety of routed protocols (for example, AppleTalk and IPX) have their own logical addressing schemes. but by far the most widely deployed routed protocol is the Internet Protocol(IP)
• Switching:
  
  • Engineers often associate the term switching with layer 2 technology; however, the concept of switching also exists at Layer 3. A layer 3, three common switching techniques exist:
    
    • Packet Switching:
      
      • With packet switching , a data stream is devided into packets.
      • Each packet has layer 3 header that includes a source and destination Layer 3 address.
    • Circuit Switching:
      
      • Circuit switching dynamically brings up a dedicated communication link between two parties to communicate.
      • An example will be the old land line system(exchange) whereby a connection is created temporalily for the duration of the call.
    • Message Switching:
      
      • Unlike packet dwitching and circuit switching technologies, message switching is usaually not well suited for real-time applications because of the delay involved.
      • Specifically, with message switching, a data stream is divided into messages. Each message is tagged with a destination address, and the messages travel from one network device to another network device on the way to their destination. Because these devices might briefly store messages before forwarding them, a network using message switching is sometimes called a store-and-forward network.
      • to assist with visualizing the concept, message switching is like routing an email message, where the email message might be briefly stored on an email server before being forwared to the recipient.
• Route discovery and selection:
  
  • Becaue Layer 3 devices forwarding decisions based on logical network addresses, a Layer 3 device might need to know how to reach various network addresses, For a example, a common Layer 3 device is a Router. A router can maintain a routing table indicating how to forward a packet based on the packet’s destination network address.
  • A router can have it’s routing table populated via manual configuration (That is, by entering static routes) , via a dynamic routing protocol (for example, RIP,OSPF or EIGRP), or simple by the fact that the router is directly connected to certain networks.
• Connection services:
  
  • Just as the data link layer offers connection services for flow control and error control, connection services also exist at the network layer. Connection services at the network layer can improve the communication reliability, if the data link’s LLC sublayer is not performing connection services.
• The following functions are performed by connection services at the network layer:
  
  • Flow control (also known as congestion control): helps prevent a sender from sending data more rapidly than the receiver is capable of receiving it.
  • Packet reordering: Allows packets to be placed in the prooer sequence as they are sent to the receiver. This might be necessary because some networks support load balancing, where mulitple links are used to send packets between two devices. Because mulitple links exist, packets might arrive out of order.

Devices examples:

• Router
• Multilayer Switches

Question 5

What is the OSI Model Layer 4(Transport Layer)?

Answer 5

Two common transport layer protocols are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

• Transmission Control Protocol(TCP):
  
  • A connection-oriented transport protocol.
  • Connection-oriented transport protocol offer reliable transport, in that if a segment is dropped, the sender can detect that drop and retransmit the dropped segment.
  • Specifically, a receiver acknowledges segments that it receives. Based on those acknowledgements, a sender can decide which segments were successfully received and which segements need to be transmitted.
• User Datagram Protocol(UDP):
  
  • ​A connectionless transport protocol
  • Connectionless transport protocols offer unreliable transport,in fact if a segment is dropped, the sender is unaware of the drop. and no retransmission occurs.
  • • Just as Layer 2 and Layer 3 offer flow control services, Flow control services also exist in Layer 4. The two common flow control apporaches at Layer 4 are Windowing and Buffering:
  • Windowing:
    
    • TCP communications uses windowing, in that one or more segments are sent at one time, and a receive can attest to the receipt of all the segments in a window with a single acknowledgement.
    • If there is a successful acknowledgement of that one segment(That is,the receiver sends an acknowledgement asking for the next segment ), The window size doubles to two segments. Upon successful receipt of those two segments, the next window holds four segments. This exponential increase in window size continues until the receiver does not acknowledge successful receipt of all segments with a certain amount of tim - known as the round-trip-time (RTT) which is sometimes called real transfer time or until a configured maximum window size is reached.
  • Buffering:
    
    • With buffering, a device (for example, a router) uses a chunk of memory (sometimes called a buffer or queue) to store segments if bandwidth is not available to send those segments. A queue has a finite capacity,however, and can overflow (that is, drop segments) in case of sustained network congestion.
• In addtion to TCP and UDP, internet Control Message Protocol (ICMP) is another transport layer protocol you are likely to meet. ICMP is used by utilities such as ping and traceroute

Question 6

What is the OSI Model Layer 5(The Session Layer)?

Answer 6

• Setting up a session:
  
  • Examples of procedures invloved in setting up a session include the following:
    
    • Checking user credentials (for example, username and password)
    • Assigning numbers to a session’s communication flows to uniquely find each other
    • Negotiating services needed during the session
    • Negotiating which device begins sending data
• Maintaining a session:
  
  • Examples of procedures invloved in supporting a session include the following:
    
    • Transferring data
    • re-establishing a disconnected session
    • Acknowledge receipt of data
• Tearing down a session:
  
  • A session can be disconnected based on agreement of the devices in the session. Alternatively, a session might be torn down because one party disconnects (either intentionally or becasue of error condition). If one party disconnects, the other party can detect a loss of communication with that party and tear down it’s side of the session.

H.323 is an example of session layer protocol, which can help set up, support and tear down a voice or video connection. Keep in mind, however, that not every network application neatly maps directly to all seven layers of the OSI Model. The session layer is one of those layers where it might not be possible to name what protocol in each scenario is running in it. Network Basic Input/ Output System (NETBIOS) is one example of a session layer protocol.

NOTE NetBIOS is an application programming interface (API) developed in the early 1980s to allow computer-to-computer communication on a small LAN(Specifically, PC-Network,which was IBM’s LAN technology at the time). Later IBM needed to support computer-to-computer communication over larger Token Ring networks. as a result,IBM enhanced the scalability and features of NetBIOS with NetBIOS emulater named NetBIOS Extended User Interface (NetBEUI)

Question 7

What is the OSI Model Layer 6?

Answer 7

• Layer 6 is called the Presentation Layer
• Part of the Host/Upper Layer (Data)
• Ensures that data transferred from one system’s Application Layer(Layer 7) can be read by the Application Layer on another system.
• Provides character code conversion, data compression and data encryption/decryption
• an example:
  
  • HTM converted to ASCII format
  • JPEG
  • TIFF
  • GIF

Question 8

What is the OSI Model Layer 7?

Answer 8

• Layer 7 is called Application Layer
• Part of the Host/Upper layers (Data)
• Acts as an interface bwteen an appication and end-user protocols
• Privides an interface to communicate with the network interface (API)(Outlook,Chrom etc..)
• Applications do not reside i nthe application layer protocol, but instead interfaces with application-layer protocols.
• Some of the common network services / protocols at this layer:
  
  • Simple Message Transfer Protocol (SMTP)
  • Post Office Protocol (POP)
  • Internet Message Access Protocol (IMAP)
  • Hypertext Transfer Protocol (HTTP)
  • Domain Name System (DNS)
  • Secure Shell (SSH)
  • File Transfer Protocol (FTP)