Download Week One

Data Communications
Architecture Models
What is a Protocol?
• For two entities to communicate successfully, they
must “speak the same language”.
• What is communicated, how it is communicated,
and when it is communicated must conform to
some mutually acceptable conventions.
• These conventions are referred to as a protocol.
Key Elements of a Protocol
• Syntax
– Data formats
– Signal levels
• Semantics
– Control information for coordination
– Error handling
• Timing
– Speed matching
– Sequencing
Protocol Architecture
• Task of communication broken up into
modules
• For example file transfer could use three
modules
– File transfer application
– Communication service module
– Network access module
Simplified File Transfer
Architecture
A Three Layer Model
• Application Layer
• Transport Layer
• Network Access Layer
Application Layer
• Support for different user applications
• e.g. e-mail, file transfer
Transport Layer
• Reliable data exchange
• Independent of network being used
• Independent of application
Network Access Layer
• Exchange of data between the computer and
the network
• Sending computer provides address of
destination
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
Addressing Requirements
• Two levels of addressing required
• Each computer needs unique network
address
• Each application on a (multi-tasking)
computer needs a unique address within the
computer
– The service access point or SAP
Protocols in Simplified
Architecture
Protocol Data Units (PDU)
• At each layer, protocols are used to communicate
• Control information is added to user data at each
layer
• Transport layer may fragment user data
• Each fragment has a transport header added
– Destination SAP
– Sequence number
– Error detection code
• This gives a transport protocol data unit
Network PDU
• Adds network header
– network address for destination computer
– Facilities requests
Operation of a Protocol
Architecture
TCP/IP Protocol Architecture
• Developed by the US Defense Advanced Research
Project Agency (DARPA) for its packet switched
network (ARPANET)
• Used by the global Internet
• No official model but a working one.
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Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer
TCP/IP Protocol Suite
• Dominant commercial protocol architecture
• Specified and extensively used before OSI
• Developed by research funded US
Department of Defense
• Used by the Internet
Physical Layer
• Physical interface between data
transmission device (e.g. computer) and
transmission medium or network
• Characteristics of transmission medium
• Signal levels
• Data rates
• etc.
Network Access Layer
• Exchange of data between end system and
network
• Destination address provision
• Invoking services like priority
Internet Layer (IP)
• Systems may be attached to different
networks
• Routing functions across multiple networks
• Implemented in end systems and routers
Transport Layer (TCP)
• Reliable delivery of data
• Ordering of delivery
Application Layer
• Support for user applications
• e.g. http, SMTP
TCP/IP Protocol Architecture
Model
Some Protocols in TCP/IP Suite
OSI Model
• Open Systems Interconnection
• Developed by the International
Organization for Standardization (ISO)
• Seven layers
• A theoretical system delivered too late!
• TCP/IP is the de facto standard
OSI - The Model
• A layer model
• Each layer performs a subset of the required
communication functions
• Each layer relies on the next lower layer to
perform more primitive functions
• Each layer provides services to the next higher
layer
• Changes in one layer should not require changes
in other layers
OSI as Framework for
Standardization
OSI Layers
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Application
Presentation
Session
Transport
Network
Data Link
Physical
The OSI Environment
OSI v TCP/IP
Standards
• Required to allow for interoperability between
equipment
• Advantages
– Ensures a large market for equipment and software
– Allows products from different vendors to
communicate
• Disadvantages
– Freeze technology
– May be multiple standards for the same thing
Standards Organizations
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Internet Society
ISO
ITU-T (formally CCITT)
IEEE
ANSI
Functions of Standards
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Encapsulation
Segmentation and reassembly
Connection control
Ordered delivery
Flow control
Error control
Addressing
Multiplexing
Transmission services
Encapsulation
• Addition of control information to data
– Address information
– Error-detecting code
– Protocol control
Segmentation (Fragmentation)
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Data blocks are of bounded size
Application layer messages may be large
Network packets may be smaller
Splitting larger blocks into smaller ones is
segmentation (or fragmentation in TCP/IP)
– ATM blocks (cells) are 53 octets long
– Ethernet blocks (frames) are up to 1526 octets long
• Checkpoints and restart/recovery
Why Fragment?
• Advantages
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More efficient error control
More equitable access to network facilities
Shorter delays
Smaller buffers needed
• Disadvantages
– Overheads
– Increased interrupts at receiver
– More processing time
Connection Control
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Connection Establishment
Data transfer
Connection termination
May be connection interruption and recovery
Sequence numbers used for
– Ordered delivery
– Flow control
– Error control
Connection Oriented Data
Transfer
Ordered Delivery
• PDUs may traverse different paths through
network
• PDUs may arrive out of order
• Sequentially number PDUs to allow for
ordering
Flow Control
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Done by receiving entity
Limit amount or rate of data
Stop and wait
Credit systems
– Sliding window
• Needed at application as well as network
layers
Error Control
• Guard against loss or damage
• Error detection
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Sender inserts error detecting bits
Receiver checks these bits
If OK, acknowledge
If error, discard packet
• Retransmission
– If no acknowledge in given time, re-transmit
• Performed at various levels
Addressing
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Addressing level
Addressing scope
Connection identifiers
Addressing mode
• Let’s look at two of these in more detail
Addressing level
• Level in architecture at which entity is named
• Unique address for each end system (computer)
and router
• Network level address
– IP or internet address (TCP/IP)
– Network service access point or NSAP (OSI)
• Process within the system
– Port number (TCP/IP)
– Service access point or SAP (OSI)
Address Concepts
Addressing Mode
• Usually an address refers to a single system
– Unicast address
– Sent to one machine or person
• May address all entities within a domain
– Broadcast
– Sent to all machines or users
• May address a subset of the entities in a domain
– Multicast
– Sent to some machines or a group of users
Multiplexing
• Supporting multiple connections on one
machine
• Mapping of multiple connections at one
level to a single connection at another
– Carrying a number of connections on one fiber
optic cable
– Aggregating or bonding ISDN lines to gain
bandwidth
Transmission Services
• Priority
– e.g. control messages
• Quality of service
– Minimum acceptable throughput
– Maximum acceptable delay
• Security
– Access restrictions
Review Questions
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What are the layers of the TCP/IP model?
What are the layers of the OSI model?
What is meant by encapsulation?
Trace an FTP command as it moves down
through the layers, across the medium, and
up the layers on the receiving side.