Chapter 2 Flashcards
– Fundamental network communications model.
Open Systems Interconnection (OSI) reference model
OSI model product of two standards organizations:
– International Organization for Standardization (ISO)
– American National Standards Institute (ANSI)
Is theoretical, not specific hardware or software.
Guidelines analogized to a grammar.
OSI
Accomplishments of the OSI model
– Enabling communications among LANs, MANs, WANs
– Standardizing network equipment
– Enabling backward compatibility to protect investments
– Enabling development of software and hardware with
common interfaces
– Making worldwide networks possible; e.g., the Internet
OSI model consists of seven distinct layers
Physical,
Data Link,
Network,
Transport,
Session,
Presentation,
Application
Set of layers in OSI model is called a
Stack
Called by actual name or placement in stack
Layers
Layers also divided into three groups
– Bottom: handles physical communications
– Middle: coordinates communication between nodes
– Top: involves data presentation
Contact between two network devices
– Communications traverse layered stack in each device
– Each layer handles specific tasks
– Each layer communicates with next layer using protocol
• Layer purpose: transmit and receive signals with data
•Network signals are either analog or digital
•Digital signal generates binary 1s or 0s
Physical Layer
– All data transfer mediums
• wire cable, fiber optics, radio waves, and microwaves
– Network connectors
– The network topology
– Signaling and encoding methods
– Data transmission devices
– Network interfaces
– Detection of signaling errors
Responsibilities of the Physical layer (Layer 1)
– Wave pattern with positive and negative voltages
– Examples: ordinary telephone or radio signal
– Used in WANs that employ analog modems
Analog signal
Physical network problems affect physical layer:
– Example 1: broken cable
– Example 2: electrical or magnetic interference
– Caused by magnetic force fields
– Generated by certain electrical devices
• Fans, electric motors, portable heaters, air-conditioners
Electromagnetic interference (EMI)
– Caused by electrical devices emitting radio waves
• Radio and television stations, radio operators, cable TV
– Problem when frequency matches network signal
Radio frequency interference (RFI)
• Layer purpose: format bits into frames
• Frame: discrete unit of information
– Contains control and address information
– Does not contain routing information
Data Link Layer
– Calculates size of information fields in frame
– Data Link layer at sender inserts value at end of frame
– Receiving Data Link layer checks value in frame
Cyclic redundancy check (CRC): monitor duplication
– Initiates communication link between two nodes
– Guards against interruptions to link
– Link to Network layer may be connection-oriented
Logical link control sublayer (LLC)
– Examines physical (device or _) address in frame
– Frame discarded if address does not match workstation
– Regulates communication sharing
Media access control sublayer (MAC)
MAC address burned into chip on network interface
– Coded as a hexadecimal number; e.g., 0004AC8428DE
• First half refers to _, second half _
•vendor
•unique to device
• Layer purpose: control passage of packets on network
– Physical routes: cable and wireless paths
– Logical routes: software paths
Network Layer
– Optimize physical and logical routes
– Permit routers to move packets between networks
Specific tasks of Network layer
: process of information gathering
– Obtain metrics about location of networks and nodes
Discovery
: logical communication paths
– Send and receive data
– Known only to Network layers between nodes
– Benefit: manage parallel data paths
Virtual circuits
– Checks (and corrects) packet sequence
– Addresses packets
– Resizes packets to match receiving network protocol
– Synchronizes flow of data between Network layers
Extra duties using virtual circuits
• Layer purpose: reliable data transmission
– Ensures data sent and received in same order
– Receiving node sends acknowledgement (“ack”)
Five reliability measures used by protocols
• _ mediates between different protocols
Transport Layer
Transport layer support of virtual circuits
– Tracks unique identification value assigned to circuit
• Value called a port or socket
• Port assigned by Session layer
• Half duplex communications
Session Layer
for dialog control
• Sets up node to separately send and receive
• Analogize to use of walkie-talkies
Two-way alternate mode (TWA)
– Establish and maintain link between two nodes
– Provide for orderly transmission between nodes
– Determine how long node can transmit
– Determine how to recover from transmission errors
– Link unique address to each node (like a zip code)
Multiple goals
– Two-way simultaneous (TWS) for dialog control
• Devices configured to send and receive at same time
– Increases efficiency two-fold
– Made possible by buffering at network interface
Full duplex communications
– Signal can travel in only one direction in a medium
– Not as desirable as either half or full duplex
Simplex alternative
- Primary purpose: manages data formatting
– Acts like a syntax checker
– Ensures data is readable to receiving
Presentation Layer
• 8-bit coding method for 256-character set
• Used mainly by IBM computers
EBCDIC (Extended Binary Coded Decimal Interchange Code)
• 8-bit character coding method for 128 characters
• Used by workstations running Windows XP, Fedora, Linux
ASCII (American Standard Code for Information
Interchange)
: scrambling data to foil unauthorized users
• Example 1: account password encrypted on LAN
• Example 2: credit card encrypted on a LAN
Encryption
• Encryption tool:
Secure Sockets Layer (SSL)
: compact data to conserve space
• Presentation layer at receiving node decompresses data
Data compression
• Connecting workstations to network services
– Link application into electronic mail
– Providing database access over the network
Application Layer
– File transfer, file management, remote access to files
– Remote access to printers
– Message handling for electronic mail
– Terminal emulation
• Services managed by Application layer
– Makes computer visible to another for network access
– Example: access shared folder using redirector
Microsoft Windows redirector
• OSI model enables two computers to communicate
Communicating Between Stacks
Standards provided by OSI models
– Communicating on a LAN
– Communicating between LANs
– Internetworking between WANs and LANs (and WANs)
Constructing a message at the sending node
– Message created at Application layer
– Message travels down stack to Physical layer
– Information at each layer added to message
• Layer information is encapsulated
– Message sent out to network form Physical layer
term for transferred data
Protocol data unit (PDU)
Control data added to PDU as it traverses stack
– Next layer gets transfer instructions from previous layer
– Peer protocols used to communicate with companion layer
• remains after data stripped
Service data unit (SDU)
Key points
– Each layer forms a PDU (from an SDU)
– Each PDU is communicated to counterpart PDU
• Example: workstation accesses shared drive
– Redirector at Application layer locates shared drive
– Presentation layer ensures data format is ASCII
– Session layer establishes and maintains link
– Transport layer monitors transmission/reception errors
– Network layer routes packet along shortest path
– Data Link layer formats frames, verifies address
– Physical layer converts data to electrical signal
• OSI model also applied to network hardware and software communications
Applying the OSI Model
basis for standards and conventions
Request for Comment (RFC)
RFCs managed by
IETF (Internet Engineering Task Force)
RFCs evaluated by
IESG (Internet Engineering Steering Group) within IETF
Two kinds of RFC documents
– Universal Protocol for transferring data on Internet
– Informational RFCs (RFC 2555 provides RFC history)
Two main LAN transmission methods
– Ethernet: defined in IEEE 802.3 specifications
– Token ring: defined in IEEE 802.5 specifications
Ethernet is more widespread than token ring
– Has more high-speed and expansion options
LAN Transmission Methods
High-speed variation of token ring
Fiber Distributed Data Interface (FDDI)
• Leverages bus and star topologies
• Collision occurs if two nodes transmit simultaneously
Ethernet
Control method: of ethernet
Algorithm that transmits and decodes formatted frames
Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
Frames find destination through physical addressing
– Node has unique MAC address associated with NIC
• Functions performed with network drivers
– Network access, data encapsulation, addressing
• Data transmitted in Ethernet encapsulated in frames
Ethernet
Frame composed of six predefined fields
Of ethernet
– Preamble
– Start of frame delimiter (SFD or SOF):
– Destination address (DA) and source address (SA):
– Length (Len)
– Data and pad
– Frame check sequence or frame checksum (FCS)
transport method
– Uses physical star topology and logic of ring topology
– Data transmission up to 100 Mbps
Token ring
hub ensures packet circulated
Multistation access unit (MAU)
Token: specialized packet continuously transmitted
– Size: 24 bits
– Structure: three 8-bit fields
• Starting delimiter (SD)
• Access control (AC)
• Ending delimiter (ED)
– Node must capture token to transmit
– Node builds frame using token fields
– Resulting frame sent around ring to target node
– Target node acknowledges frame received and read
– Target node sends frame back to transmitting node
– Transmitting node reuses token or returns it to ring
Using a token
Node sends frame to indicate problem
– Ring tries to self-correct problem
Beaconing
– Broadcast storms and interference are rare
Token ring networks reliable
– Standard for high-capacity data throughput 100 Mbps
Uses fiber-optic cable communications medium.
Fiber Distributed Data Interface (FDDI)
uses timed token access method
– Send frames during target token rotation time (TTRT)
– Allows for parallel frame transmission
FDDI
uses timed token access method
– Send frames during target token rotation time (TTRT)
– Allows for parallel frame transmission
FDDI
Two types of packets
– Synchronous communications (time-sensitive traffic)
– Asynchronous communications (normal traffic)
• Two classes of nodes connect to FDDI network
– Class A: nodes attached to both rings (hubs)
– Class B: node (workstation) attached via Class A node
•built on topologies and network transmission
– Similar to LAN structure, with greater complexity
– Providers do not provide detailed specifications
WAN Network Communications
WAN network service providers
– Telecommunications companies
• Especially regional telephone companies (telcos or
RBOCs (regional bell operating companies))
– Cable television companies (cablecos)
– Satellite TV companies
Plain old telephone service (POTS)
– Carry most basic WAN communications
– 56-Kbps dial-up access, Integrated Service Digital
Network (ISDN), Digital Subscriber Line (DSL)
Telecommunications WANs
• Topology between RBOCs and long distance carrier
– RBOC provides local access and transport area (LATA)
– IXC lines join RBOC and long distance carrier
• Point of presence (POP) is term for junction
dedicated telephone line for data link
– Example: states use to connect offices to capitol
T-carrier lines
synchronous 56-Kbps service
Alternative to T-carrier
• Architecture consists of star-shaped locations
• Headend is the focal point in the star
• Grouping of antennas, cable connections, satellite dishes, microwave towers
• Distribution centers transfer signals to feeder cables
• Cable modems convert signals for computer use
Cable TV WANs
use of radio, microwaves, satellites
Wireless WANS
– Connect wireless LAN to wireless bridge or switch
– Connect bridge or switch to antenna
– Antenna transmits wave to distant antenna
Topology of radio communications
– Connect microwave dish to LAN
– Dish transmits to microwave dish at remote location
Topology of microwave communication
– Satellite dish transmits to satellite in space
– Satellite relays signal to satellite dish at remote location
Topology of satellite communications
• Switching techniques creating data paths (channels)
WAN Transmission Methods
divides the channels into frequencies instead of time slots
Frequency Division Multiple Access (FDMA)
bandwidth of cable dynamically allocated based on application need
Statistical multiple access
involves creating a dedicated physical
circuit between the sending and receiving nodes
Circuit switching
uses store-and-forward method to
transmit data from sending to receiving node
Message switching
establishes a dedicated logical circuit between the two transmitting nodes
Packet switching
Scenario: new campus needs new network
Ethernet appropriate for all areas of new campus
Designing an Ethernet Network
– Ethernet enjoys widespread vendor/technical support
– Compatible with star-bus topology popular with LANs
– Network upgrades easily to higher bandwidths
– Standards exist for cable and wireless versions
– Ethernet network scales well, adapts well to WANs
– Network devices on old campus may be used
– Many options for Internet connections
Reasons for choosing Ethernet technology
Message travels up stack from
Physical layer
checks address of frame
uses CRC to check frame integrity
Data Link layer
Receives valid frame and sends up stack
Network layer
