Transcript William Stallings Data and Computer Communications

Chapter 1
Introduction
Supplementery Slides
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A Communications Model
Source
generates data to be transmitted
Transmitter
Converts data into transmittable signals
Transmission System
Carries data
Receiver
Converts received signal into data
Destination
Takes incoming data
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Communications Tasks
Transmission system utilization
Addressing
Interfacing
Routing
Signal generation
Recovery
Synchronization
Message formatting
Exchange management
Security
Error detection and correction
Network management
Flow control
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Simplified Communications
Model - Diagram
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Key Communications Tasks
 Transmission System Utilization
 Interfacing
 Signal Generation
 Synchronization
 Exchange Management
 Error detection and correction
 Addressing and routing
 Recovery
 Message formatting
 Security
 Network Management
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Simplified Data
Communications Model
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Networking
Point to point communication not usually
practical
Devices are too far apart
Large set of devices would need impractical number
of connections
Solution is a communications network
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Simplified Network Model
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Wide Area Networks
Large geographical area
Crossing public rights of way
Rely in part on common carrier circuits
Alternative technologies
Circuit switching
Packet switching
Frame relay
Asynchronous Transfer Mode (ATM)
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Circuit Switching
Dedicated communications path established for
the duration of the conversation
e.g. telephone network
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Packet Switching
Data sent out of sequence
Small chunks (packets) of data at a time
Packets passed from node to node between
source and destination
Used for terminal to computer and computer to
computer communications
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Frame Relay
Packet switching systems have large overheads
to compensate for errors
Modern systems are more reliable
Errors can be caught in end system
Most overhead for error control is stripped out
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Asynchronous Transfer Mode
ATM
Evolution of frame relay
Little overhead for error control
Fixed packet (called cell) length
Anything from 10Mbps to Gbps
Constant data rate using packet switching
technique
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Integrated Services Digital
Network
ISDN
Designed to replace public telecom system
Wide variety of services
Entirely digital domain
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Local Area Networks
Smaller scope
Building or small campus
Usually owned by same organization as
attached devices
Data rates much higher
Usually broadcast systems
Now some switched systems and ATM are being
introduced
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Protocols
Used for communications between entities in a
system
Must speak the same language
Entities
User applications
e-mail facilities
terminals
Systems
Computer
Terminal
Remote sensor
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Key Elements of a Protocol
Syntax
Data formats
Signal levels
Semantics
Control information
Error handling
Timing
Speed matching
Sequencing
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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
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Simplified File Transfer
Architecture
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A Three Layer Model
Network Access Layer
Transport Layer
Application Layer
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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.)
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Transport Layer
Reliable data exchange
Independent of network being used
Independent of application
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Application Layer
Support for different user applications
e.g. e-mail, file transfer
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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
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Protocol Architectures and
Networks
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Protocols in Simplified
Architecture
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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
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Network PDU
Adds network header
network address for destination computer
Facilities requests
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Operation of a Protocol
Architecture
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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.
Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer
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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.
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Network Access Layer
Exchange of data between end system and
network
Destination address provision
Invoking services like priority
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Internet Layer (IP)
Systems may be attached to different networks
Routing functions across multiple networks
Implemented in end systems and routers
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Transport Layer (TCP)
Reliable delivery of data
Ordering of delivery
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Application Layer
Support for user applications
e.g. http, SMPT
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TCP/IP Protocol Architecture
Model
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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
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OSI Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
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OSI v TCP/IP
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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
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Standards Organizations
Internet Society
ISO
ITU-T (formally CCITT)
ATM forum
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Further Reading
Stallings, W. Data and Computer
Communications (6th edition), Prentice Hall
1999 chapter 1
Web site for Stallings book
www.shore.net/~ws/DCC6e.html
Web sites for IETF, IEEE, ITU-T, ISO
Internet Requests for Comment (RFCs)
Usenet News groups
comp.dcom.*
comp.protocols.tcp-ip
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Networking
Configuration
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Service Primitives and
Parameters
Services between adjacent layers expressed in
terms of primitives and parameters
Primitives specify function to be performed
Parameters pass data and control info
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Primitive Types
REQUEST
A primitive issued by a service user to invoke some service and to
pass the parameters needed to specify fully the requested service
INDICATION
A primitive issued by a service provider either to:
indicate that a procedure has been invoked by the peer service
user on the connection and to provide the associated parameters,
or
notify the service user of a provider-initiated action
RESPONSE
A primitive issued by a service user to acknowledge or complete
some procedure previously invoked by an indication to that user
CONFIRM
A primitive issued by a service provider to acknowledge or
complete some procedure previously invoked by a request by the
service user
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Timing Sequence for Service
Primitives
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Networking
Point to point communication not usually
practical
Devices are too far apart
Large set of devices would need impractical number
of connections
Solution is a communications network
Wide Area Network (WAN)
Local Area Network (LAN)
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Wide Area Networks
Large geographical area
Crossing public rights of way
Rely in part on common carrier circuits
Alternative technologies
Circuit switching
Packet switching
Frame relay
Asynchronous Transfer Mode (ATM)
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Circuit Switching
Dedicated communications path established for
the duration of the conversation
e.g. telephone network
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Packet Switching
Data sent out of sequence
Small chunks (packets) of data at a time
Packets passed from node to node between
source and destination
Used for terminal to computer and computer to
computer communications
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Frame Relay
Packet switching systems have large overheads
to compensate for errors
Modern systems are more reliable
Errors can be caught in end system
Most overhead for error control is stripped out
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Asynchronous Transfer Mode
ATM
Evolution of frame relay
Little overhead for error control
Fixed packet (called cell) length
Anything from 10Mbps to Gbps
Constant data rate using packet switching
technique
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Local Area Networks
Smaller scope
Building or small campus
Usually owned by same organization as
attached devices
Data rates much higher
Usually broadcast systems
Now some switched systems and ATM are being
introduced
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LAN Configurations
Switched
Switched Ethernet
May be single or multiple switches
ATM LAN
Fibre Channel
Wireless
Mobility
Ease of installation
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Metropolitan Area Networks
MAN
Middle ground between LAN and WAN
Private or public network
High speed
Large area
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Protocols and Architecture
Characteristics
Direct or indirect
Monolithic or structured
Symmetric or asymmetric
Standard or nonstandard
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Direct or Indirect
Direct
Systems share a point to point link or
Systems share a multi-point link
Data can pass without intervening active agent
Indirect
Switched networks or
Internetworks or internets
Data transfer depend on other entities
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Monolithic or Structured
Communications is a complex task
To complex for single unit
Structured design breaks down problem into
smaller units
Layered structure
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Symmetric or Asymmetric
Symmetric
Communication between peer entities
Asymmetric
Client/server
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Standard or Nonstandard
Nonstandard protocols built for specific
computers and tasks
K sources and L receivers leads to K*L protocols
and 2*K*L implementations
If common protocol used, K + L
implementations needed
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Use of Standard Protocols
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Functions
Encapsulation
Segmentation and reassmebly
Connection control
Ordered delivery
Flow control
Error control
Addressing
Multiplexing
Transmission services
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Encapsulation
Addition of control information to data
Address information
Error-detecting code
Protocol control
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Segmentation (Fragmentation)
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
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Why Fragment?
Advantages
More efficient error control
More equitable access to network facilities
Shorter delays
Smaller buffers needed
Disadvantages
Overheads
Increased interrupts at receiver
More processing time
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Connection Control
Connection Establishment
Data transfer
Connection termination
May be connection interruption and recovery
Sequence numbers used for
Ordered delivery
Flow control
Error control
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Connection Oriented Data
Transfer
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Ordered Delivery
PDUs may traverse different paths through
network
PDUs may arrive out of order
Sequentially number PDUs to allow for ordering
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Flow Control
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
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Error Control
Guard against loss or damage
Error detection
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
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Addressing
Addressing level
Addressing scope
Connection identifiers
Addressing mode
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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)
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Address Concepts
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Addressing Scope
Global nonambiguity
Global address identifies unique system
There is only one system with address X
Global applicability
It is possible at any system (any address) to identify
any other system (address) by the global address of
the other system
Address X identifies that system from anywhere on
the network
e.g. MAC address on IEEE 802 networks
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Connection Identifiers
Connection oriented data transfer (virtual
circuits)
Allocate a connection name during the transfer
phase
Reduced overhead as connection identifiers are
shorter than global addresses
Routing may be fixed and identified by connection
name
Entities may want multiple connections - multiplexing
State information
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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
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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
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Transmission Services
Priority
e.g. control messages
Quality of service
Minimum acceptable throughput
Maximum acceptable delay
Security
Access restrictions
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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
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The OSI Environment
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OSI as Framework for
Standardization
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Layer Specific Standards
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Elements of Standardization
Protocol specification
Operates between the same layer on two systems
May involve different operating system
Protocol specification must be precise
Format of data units
Semantics of all fields
allowable sequence of PCUs
Service definition
Functional description of what is provided
Addressing
Referenced by SAPs
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OSI Layers (1)
Physical
Physical interface between devices
Mechanical
Electrical
Functional
Procedural
Data Link
Means of activating, maintaining and deactivating a
reliable link
Error detection and control
Higher layers may assume error free transmission
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OSI Layers (2)
Network
Transport of information
Higher layers do not need to know about underlying
technology
Not needed on direct links
Transport
Exchange of data between end systems
Error free
In sequence
No losses
No duplicates
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OSI Layers (3)
Session
Control of dialogues between applications
Dialogue discipline
Grouping
Recovery
Presentation
Data formats and coding
Data compression
Encryption
Application
Means for applications
to access
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Use of a Relay
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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
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TCP/IP Protocol Architecture(1)
Application Layer
Communication between processes or applications
End to end or transport layer (TCP/UDP/…)
End to end transfer of data
May include reliability mechanism (TCP)
Hides detail of underlying network
Internet Layer (IP)
Routing of data
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TCP/IP Protocol Architecture(2)
Network Layer
Logical interface between end system and network
Physical Layer
Transmission medium
Signal rate and encoding
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PDUs in TCP/IP
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Some Protocols in TCP/IP Suite
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