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CMPT 371
Data Communications and Networking
Summary Switching ,throughput, Multiplexing
Protocol layers
© Janice Regan, CMPT 128, Jan 2007
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What is a Protocol
 A set of definitions and rules defining the
method by which data is transferred between
two or more entities or systems.
 The key elements of a protocol are:

Syntax: definitions of data format, size and content
of messages or packets (headers and data)
 Semantics: control, responses to messages or
packets, coordination, error handling
 Timing/Synchronization: maintaining
synchronization between the communicating entities
(speed matching, sequencing, recovery from timing
related errors )
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Characteristics of Protocols: 1
 Direct: data passes from source to destination without
active intervention (point to point or multipoint link)
Source
Set Switch
Fixed connection
Destination
 Indirect: data from source must pass through
intermediate active entities to reach the destination
(switched networks)
Source
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Network
Active
Intermediate
nodes
Destination
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Characteristics of Protocols: 2
 Monolithic: Single protocol for all aspects of data
transfer
 Structured: uses a set of protocols interacting based
on a chosen architecture. (Layered, client server …)
 Consider computer networks, A lot of complexity

Hosts , routers, switches, applications, varied
hardware and software, varied communication
needs
 Any chance a single monolithic protocol is the best
way to organize and manage the Internet?
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Characteristics of Protocols: 3
 Symmetric: communications between peers
that is the same regardless of which entity is
source and which is destination (peer to peer)
Source/Destination
Destination/Source
 Asymmetric: unequal source and receiver
(client – server)
Source: Controls Transfer
Destination:
 Internet needs to support both
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Characteristics of Protocols: 4
 Standard: independent of hardware and application

Used by all members of network, no need for
translation
 Operates in a given layer regardless of operating
system or system hardware (may require multiple
implementations)
 Nonstandard: designed for specific hardware or
application
 Must translate between nonstandard protocols
increasing load on system rapidly as number of
nonstandard systems increases
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Internet Protocol Choices
 Structured (layered): uses a set of protocols,
several different protocols per layer
 Standardized: uses same protocols, may use
different implementation on different hardware
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Figure 2.3 Stallings (2003)
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Layered Protocol Architecture
 Flexible protocols can become very complicated
very quickly.





Reduce complexity, separate communication
functions into modules.
Layer these modules using clean interfaces
Each module becomes a separate protocol layer
Each layer provides services to the layer above
through an interface that hides the complexities of
how the service is implemented.
With one or more protocols per layer the complete
network architecture is known as a protocol stack
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Standardization
 Clearly defined functions for each layer
allow for independent and simultaneous
development of multiple layers (Lower
layers = more detail)
 Protocol specification for each layer and
for interfaces between layers must be
exact
 Format of data units
 allowed sequence of PDUs
 Semantics (meanings) of all fields
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Standardization
 Particular operations are localized in a
given layer regardless of operating
system or system hardware (may require
multiple implementations)
 Service Definition: standards for services
provided by a layer, using a given set of
protocols, to the layer above it
 Addressing: Provide the defined services
to entities with particular addresses
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Layered models
 Each layer performs a different set of communications
functions.
 Each layer encapsulates and hides complexity from the
layer above it
 Each layer uses the layer below it the perform more
primitive and detailed functions
 Clear interfaces between layers make independent
changes in one layer independent of the other layers
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Encapsulation
 The addition of control information to data
before passing the resulting frame to the
next protocol level
 A PDU (protocol data unit) may or may not
contain data but will contain control
information such as




Addresses
Error detection checksums or redundancy checks
Sequence numbers
Other control information
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The OSI Environment
Janice Regan
© Sept. (2003)
2007-2013
Figure
2.7 Stallings
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TCP/IP Protocol Suite
 TCP/IP is the most commonly used
protocol stack on the internet
 TCP/IP uses a layered structure, with
fewer layers than the OSI model
 TCP/IP protocols for a given layer use the
services available from lower levels to
complete the tasks they are responsible
for
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Janice Regan © Sept. 2007-2013
Figure 1.11 Stallings (2000)
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Application Layer
 Application Layer: Working
environment / support for applications
 Provides services used directly by
applications
 Hides the complexities of services provided
by the other layers from the user
 Provides an interface to communications
between processes or applications on
separate hosts
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Transport Layer
 Manages exchange of application data
between end systems
 Connection oriented reliable delivery (TCP)
 Manages connections and error free, In
sequence, loss and duplication free data transfer
 Best effort delivery (UDP)
 Packet oriented, connectionless
 Process level addressing, Segmentation,
Multiplexing / De-multiplexing
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Network Layer
 manages how data moves from one host
to another in a network
 Hides specifics of network technology from
higher levels
 Addressing, forwarding and routing in the
network. How to get your packet from A to B
 Fragmentation / Reassembly
 Error detection and handling
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Link Layer
 Data Link Layer: making the physical link
reliable and usable
 Operates on frames containing messages
sent between directly connected devices.
 Bit level error detection and handling (finds
corruption that happens during transmission)
 Link control and management, Link level
addressing
 Media access control, sharing the physical
media between multiple devices
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Physical Layer
 Physical Layer: raw bit stream service.
Operates on bits
 Mechanical: properties of connection to
network (connector)
 Electrical: representation of bits as voltages
(encoding and signaling)
 Functional: Defines functions performed by
particular circuits at the interface between
the system and the transmission media
 Procedural: Defines how bit streams are
exchanged through the physical media
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Some Application layer Protocols
 Users most commonly directly use application
layer protocols like Hypertext Transfer Protocol
(HTTP), File Transfer Protocol (FTP), Simple
Mail Transfer Protocol (SMTP); Telnet,
 Other common application layer protocols help
facilitate the use and management of TCP/IP
networks: These include Domain Name System
(DNS), the Routing Information Protocol (RIP),
the Simple Network Management Protocol
(SNMP).
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TCP-IP Protocol Architecture
application
transport
Application protocol
Transport protocol
application
transport
network
Network
network
Data link
Data link
Data link
Physical
Physical
Physical
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Encapsulation PDUs for TCP/IP
application
Application data
transport
TCP/UDP
Application data
TCP
segment
network
IP TCP/UDP
Application data
IP
datagram
Data link
Ethernet IP TCP/UDP
Application data
Ethernet
frame
Physical
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Encapsulation: PDU Headers
 TCP segment: User data +Transport Header
 Destination and source process address of
each occurrence of an application, called a
port
 Sequence Number used to manage packet
flow, deliver packets in the correct order
 Checksum preserves reliability of data transfer
and assure delivery to correct destination
 Lengths, flags …
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Encapsulation: PDU Headers
 IP Datagram: TCP segment + IP Header
 Destination and/or source network address
(IP address)
 Protocol Identification

Indicate transport protocol used
 Flow and error control information (including
fragmentation)
 Error detection (header only) information
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Encapsulation: PDU Headers
 Ethernet Frame: IP Datagram + Ethernet
header
 Next hop and source Ethernet address
 May be hardware specific (works for
hardwares other than Ethernet)
 Flow and sharing for the physical media
 Error detection information
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Using a relay (router)
application
application
transport
transport
network
network
Network
Data link
Physical
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Data link
Data link
Physical
Physical
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Encapsulation at each level
application
Application data
transport
TCP/UDP
Application data
TCP
segment
network
IP TCP/UDP
Application data
IP
datagram
Data link
Ethernet IP TCP/UDP
Application data
Ethernet
frame
Physical
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Using a relay (switch)
application
application
transport
transport
network
network
Data link
Data link
Physical
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Data link
Physical
Physical
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History
 Please be sure to read the excellent
summary in your text book
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