The OSI Protocol Model
Chapter 2
Protocols & Architecture
• A Protocol Architecture is the layered structure of hardware
& software that supports the exchange of data between systems
• At each layer, one or more common protocols are
implemented in communication systems. Each protocol
provides a set of rules for the exchanges of data between
systems
• Key Functions of a protocol: encapsulation, segmentation &
reassembly, connection control, ordered delivery, flow control,
error control, addressing, and multiplexing
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Functions of Protocols
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Segmentation and reassembly
Encapsulation
Connection control
Ordered delivery
Flow control
Error control
Addressing
Multiplexing
Transmission services
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Segmentation (Fragmentation)
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Data blocks are of bounded size at each layer
Application layer messages may be large
Network packets may be smaller
Splitting larger blocks into smaller ones is segmentation (or
fragmentation in TCP/IP)
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ATM blocks (cells) are 53 octets long
Ethernet blocks (frames) are up to 1526 octets long
Why Fragment?
• Advantages: 1) .More efficient error control; 2) More equitable access
to network facilities; 3) Shorter delays; 4). Smaller buffers needed
• Disadvantages: 1). Overheads; 2). Increased interrupts at receiver;
3). More processing time
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Encapsulation
• Addition of control information to data
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Address information
Error-detecting code
Protocol control: data + control information -> PDU
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Connection Control
• Connection Establishment
• Data transfer
• Connection termination
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Error Control
Ordered Delivery & Flow Control
• Ordered Delivery
PDUs may traverse different paths through network
PDUs may arrive out of order
¾ Sequentially number PDUs to allow for ordering
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• Guard against loss or damage
• Error detection
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• 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
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Sender inserts error detecting bits
Receiver checks these bits
If OK, acknowledge
If error, discard packet
• Retransmission
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If no acknowledge in given time, re-transmit
• Performed at various levels
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Addressing
• Addressing level
• Addressing scope
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Globally unique : Global address identifies unique system
Locally Unique: Multiple simultaneous applications
• Addressing mode
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Unicast address: Sent to one machine or person
Broadcast: Sent to all machines or users
Multicast: Sent to some machines or a group of users
Addressing level
• Level in architecture at which entity is named
• Unique address for each end system (computer)
and router
• Network level address
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• Process within the system
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IP or internet address (TCP/IP)
Network service access point or NSAP (OSI)
Port number (TCP/IP)
Service access point or SAP (OSI)
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Multiplexing
Address Concepts – in TCP/IP Architecture
• Supporting multiple connections on one machine
• Mapping of multiple connections at one level to a single
connection at another: e.g.
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Carrying a number of connections on one fiber optic cable
Aggregating or bonding low speed lines to gain bandwidth
(a)
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(b)
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Trunk
group
MUX
MUX
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Transmission Services
The OSI Protocol Model
Some common service example:
• Different Priorities for Different messages
• Quality of Service Guarantee
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Minimum acceptable throughput
Maximum acceptable delay
• Security: Access Restrictions
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OSI - 7 Layers Model
OSI Layers – Layer 1 - 4
• 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
Provide fundamental network functionalities/services
• Physical: Covers the physical interface between devices & the
rules by which bits are passed from one to another
• Data Link
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• Network: routing/switching; establish/maintain/terminate
connections
• Transport: Exchange of data between end-to-end systems
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Provide a reliable link by Error detection and control
Higher layers may assume error free transmission
Error free; In sequence; No losses; No duplicates; QoS
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Use of a Relay
OSI Layers – Layer 5 - 7
Provide finer network service (more software orientated)
• Session:Control of dialogues between applications in end system
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Dialogue discipline: full-duplex or half-duplex
Recovery: provide checkpoint mechanism.
• Presentation: Defines data formats and coding. e.g.,
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Data compression
Encryption
• Application: provide means for application program to access
OSI environment
¾ management functions; email; FTP
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TCP/IP Protocol Architecture
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PDUs in TCP/IP Architecture
• Application Layer
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Communication between processes or applications
• End to end or transport layer (TCP/UDP/…)
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End to end transfer of data
May include reliability mechanism (TCP)
Hides detail of underlying network
• Internet Layer (IP): Routing of data
• Network Access Layer
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Logical interface between end system and network
• Physical Layer
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Transmission medium
Signal rate and encoding
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Some Protocols in TCP/IP Suite
Further Reading Reference
• Stallings chapter 2
• Comer,D. Internetworking with TCP/IP volume I
• Comer,D. and Stevens,D. Internetworking with
TCP/IP volume II and volume III, Prentice Hall
• Halsall, F> Data Communications, Computer
Networks and Open Systems, Addison Wesley
• RFCs
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