Chapter 2: OSI Reference Model

International Organization for Standardization (ISO)
developed OSI reference model.
Common way to categorize function of a network technology
state what layer (or layers) of the OSI model that technology operates
Bookshelf analogy
not every book (network device) fits into one layer. OSI model is reference model.
OSI Model
1. Physical
2. Data Link
3. Network
4. Transport
5. Session
6. Presentation
7. Application

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OSI Model diagram
binary expressions
made up of bits, where a bit is a single 1 or a single 0.
protocol data unit (PDU)
At upper layers, bits are grouped together. Also known as data service unit
generically used to refer to PDUs. PDUs might have additional name depending on OSI layer.
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Segments, Packets, Frames and Bits.
Transport, Network, Data Link, and Physical layers respectively.
Layer 1: Physical Layer
concerned with transmission of bits on the network along with the physical and electrical characteristics of the network. Wiring standards for connectors and jacks, physical topology, synchronizing bits, bandwidth usage and multiplexing.
Layer 1 Bits:
How represented on medium, in binary expression. Electrical voltage or light can represent 1s and 0s.
current state modulation
presence or absence of light or voltage can represent binary 1 or binary 0 respective
state transition modulation
transition between voltages or presence of light indicates binary value
Other common modulation types
AM (amplitude modulation) uses variation of waveform amplitude to represent original signal. and FM (frequency modulation) uses variation in frequency to represent original signal
Layer 1: Wiring standards for connectors and jacks
Several standards for network connectors. Ex: RJ-45 for 100BASE-TX Ethernet
Layer 1 Physical topology
Layer 1 Devices view a network as a physical topology
Synchronizing bits:
For two networked devices to communicate at physical layer, must agree on when one bit stops and another starts. Two basic approaches: asynchronous and synchronous
sender indicates that about to start transmitting by sending start bit to receiver. When receiver sees, starts own internal clock to measure subsequent bits. After sends data, sends a stop bit to indicate has finished
synchronizes the internal clocks of both sender and receiver to ensure agree on when bits begin and send. Common approach is to use external clock (like ISP clock) which is referenced by both sender and receiver
Bandwidth usage
Broadband: Divide bandwidth available on a medium into different channels. Different communication streams then transmitted over various channels.
Baseband: use all available frequencies on a medium to transmit data. Ethernet is example.
Multiplexing strategy
allows multiple communication sessions to share same physical medium. Cable TV: get multiple channels
Time-division multiplexing (TDM)
supports different communication sessions on same physical medium by causing sessions to take term. Period of time (time slot) data will be sent then data from second session. Each gets a turn (Es: telephone conversations)
Statistical time-division multiplexing (StatTDM)
TDM downside is each communication session receives own time slot, even if not in sure. To make more efficient use, StatTDM dynamically assigns time slots to communications sessions on an as-needed basis
Frequency-division multiplexing (FDM)
divides a medium's frequency range into channels and different communication sessions transmit data over different channels. Called broadband. Cable model: carriers data incoming and outgoing, and ranges for tv stations.
Layer 1 Devices
hubs, wireless access points and network cabling.
Layer 2: Data Link Layer
Has two sub layers, Media Access Control Layer and Logical Link Control
concerned with packaging data into frames and transmitting frames on network, performing error detection/correction, ID network devices and handling flow control.
Media Access Control
Physical addressing, logical topology and transmission methodology
MAC address
48-bit address assigned to device's NIC. Written in hexadecimal notation. First 24 bits are referred to as vendor code.
Logical topology:
layer 2 devices view network as logical topology
Layer two method of transmission
strategy for determining when a device is allowed to transmit on the media
Logical Link Control
Connection services: flow control, error control, and synchronizing transmissions: isochronous and asynchronous
Connection services:
Device on network receives a message from another device, provide ACK message. Two main functions are flow control (limit amount of data sender can send at one time) and error control: allow recipient of data to let send know whether data frame receives or corrupted. Calculated checksum
Synchronizing transmissions
need to coordinate when data frame is transmitted and should be received: three methods are Isochronous, asynchronous and synchronous
network devices look to a common device in the network as a clock source; creates fixed-length time slots. Determine how much free space is available in time slot and insert data into time slot. Can accommodate more than one data frame. No clocking at beginning of data string or for every data frame. Uses little overhead compared to other transmission methods
network devices reference own internal clocks and do not need to synchronize. Start bit at beginning frame and stop bit at end. Parity bit: added to end of each byte in frame to detect an error.
two network devices agree on clocking method to indicate beginning and ending of data frames. Use separate communications channel over which clock signal is sent. Another approach relies on specific bit combinations or control characters.
Cyclic redundancy check (CRC)
mathematical algorithm on the data, if sender and receiver calculate same CRC value for same data chunk, data not corrupted during transmission
Layer 2 devices
switches, bridges and NICs
Layer 3: the Network Layer
Primarily concerned with forwarding data based on logical addresses. Also does switching, route discovery and selection, connection services, bandwidth usage and multiplexing strategy
Logical addressing
network layer uses logical addressing to make forwarding decisions. A variety of routed protocols have their own logical addressing schemes but most widely deployed is Internet Protocol (IP)
often associated with Layer 2 technologies but also exists at layer 3. Switching is making decisions about how data should be forwarded. Layer 3 common switching: packet switching, circuit switching and message switching
Packet Switching
a data stream is divided into packets. each packet has a Layer 3 header, which includes source and destination address. Another term for packet switching is routing
Circuit Switching
dynamically brings up a dedicated communication link between two parties for those parties to communicate. Ex: phone call, telephone company switching equip connects home phone with phone system you calling. the circuit only exists for duration of call
Message Switching:
usually not well suited for real-time applications because of delay involved. Data stream is divided into messages. each message tagged with destination address and messages travel from one network device to another on way to destination. Devices might briefly store messages before forwarding: called store-and-forward network. Ex: routing an email message: store in server before forward to recipient.
Route discovery and selection:
Layer three device makes forwarding decisions: needs to know how to reach various network addresses. Router can maintain a routing table indicate how to forward a packet based on destination network address. Routing table manually or via dynamic routing protocol (RIP, OSPF or EIGRIP)
Connection services
connection services at network layer can improve communication reliability in event data link's LLC sublayer is not performing connection services: Flow control (congestion control) and packet reordering: place packets in sequence as sent
Layer 3 devices:
routers and multilayer switches. Most common Layer 3 protocol is IPv4.
Layer 4: Transport Layer
acts as dividing line between the upper and lower layers of the OSI. Messages take from upper layers (5-7) and are encapsulated into segments for transmission to lower layers (1-3). Data streams from lower layers are decapsulated and sent to layer 5 depending on protocol. TCP/UDP, Windowing and Buffering.
Transmission Control Protocol (TCP)
Connection-oriented transport protocol. Provide reliable transport, if segment is dropped, sender can detect that drop and retransmit that dropped segment. Receiver acknowledges segments that it receives.
User Datagram Protocol (UDP)
connectionless transport protocol; provides unreliable transport, in that if a segment is dropped, sender is unaware of the drop and no retransmission occurs.
flow control. TCP uses windowing, one or more segments are sent at one time, and a receive can acknowledge the receipt of all the segments in a window with a single ack. TCP can use sliding window, window size beings with one segment. Size increases until receiver does no ACK within certain time period: round-trip time (RTT) or real transfer time.
With buffering, a device allocates a chunk of memory (called a buffer or queue) to store segments if bandwidth is not currently available to transmit those segments. Queue has a finite capacity and can overflow (drop segments) in event of congestion.
ICMP (Internet Control Message Protocol)
used by utilities such as ping and traceroute.
Layer 5: Session Layer
responsible for setting up, maintaining and tearing down sessions. A session can be thought of as a conversation that needs to be treated separately from other sessions to avoid intermingling of data from different conversations.
Setting up a session
ex: checking user credentials, assigning numbers to a session's communications flows to uniquely ID each flow, negotiating services required during the session, negotiating which device begins sending data
Maintaining a session
Ex: transferring data, reestablishing a disconnected session, acknowledging receipt of data
Tearing down a session
disconnected based on mutual agreement, or torn down because one part disconnects (intentionally or because of error condition)
Session layer protocol example:
H.323: set up, maintain, and tear down a voice or video connection. NetBIOS (Network Basic Input/Output System) computer to computer communication on small PAN.
Layer 6: The Presentation Layer
responsible for formatting of data being exchanged and securing data with encryption
Data formatting
some applications might format text using ASCII (American Standard Code for Information Interchange) while other applications might format text using EBCDIC (Extended Binary Coded Decimal Interchange Code). Presnetaion layer is responsible for formatting the text (or other types of data) in a format that allows compatibility between the communicating devices
encryption is used to scramble (encrypt) data in a way that if data were intercepted, third party is unable to read (decrypt)
Layer 7: Application Layer
provides application service to a network. end user applications do not reside in application layer. Application layer supports services used by end-user application. Email is application layer service that does reside at application layer, Outlook does not live at application layer.Application layer advertises available services
Application services
examples of the application services residing at the application layer include file sharing and email
Service advertisement
some application's services (ex: networked printers) periodically send out advertisements, making availablity of service known to other devices on network. Other services register themselves and their services with a centralized directory (ex Microsoft Active Directory)
TCP/IP Stack
Has four defined layers. Application, Transport, Internet and Network Interface.
Network Interface
Technologies addressed by Layer 1 and Layer 2 of OSI. Also called network access layer
maps to layer 3 of the OSI Model. TCP/IP stack focuses on IP as the protocol to be routed through a network. Fields in IP packet for source and destination IP address. Time to Live (TTL) decremnted each time packet is routed from one netwrok to another. If reaches 0, discarded
maps to layer 4 of OSI model. Two primary protocols are TCP and UDP.
Layer 5,6, and 7 of OSI model.
Common Application Protocols in the TCP/IP Stack
identifiable by unique port numbers, TCP port 80. When send traffic to remote website, packet send out needs destination IP and port humber for HTTP, Source IP.
Well known ports
Ports numbered 1023 and below. Ports 1023 and above are called ephemeral ports. Maximum port value is 65,535.