1. Chapter 2: Protocols and Architecture
COE 341: Data & Computer Communications (Term 061) Dr. Radwan E. Abdel-Aal
Chapter 2: Protocols and Architecture

2. Agenda Simplified File Transfer Architecture
Need For Protocol Architecture
Key Elements of a Protocol
Simplified File Transfer Architecture
A Three-Layer Model
Protocol Architectures and Networks
Protocols in Simplified Architecture
Standard Protocol Architectures
The OSI Protocol Architecture
The Model
Standardization within the OSI Framework
OSI Layers
The OSI versus the TCP/IP (the Internet) Protocol Architecture

3. What is a Protocol and Why we Need it?
What’s a protocol?
A set of rules or conventions agreed upon and observed by two communicating entities for successful exchange of data between them (think of Human conversation)
Example: File transfer
Source must activate communications path and inform network of the destination address
Source must check if destination is prepared to receive data
Source file transfer application must check if destination file management system will accept and can store file
May need file format translation
Why protocols?
Equipment come from different vendors and run different operating systems… Need a common standard language to communicate properly

4. Need for Architecture (hierarchy) for that Protocol
Benefits of problem decomposition
Large task split into smaller subtasks
(Careful splitting: logical, tradeoffs, effect of changes)
The subtasks are implemented separately in layers forming a stack
Can be developed simultaneously by different teams
Hierarchical nature:
A layer provides/requests services to/from an adjacent layer
Higher layers handle higher-level tasks
Tasks are implemented in both communicating systems
Peer layers in the two systems communicate with each other
Examples from everyday life
High Level, e.g. Applications
Low Level, e.g. Signals, Bits

5. Guidelines for layering
Group functions logically: each layer performs a set of related subtasks
Each layer provides services to the higher layer and requests services from the lower layer
Ideally, changes in one layer should not require changes to other layers, e.g. changing link from wire to optical fibre
Information hiding: Each layer sees only what it needs
Not too few layers:
Not enough splitting of functionality
Not too many layers:
Difficult to manage
Large communication overhead between them

6. Protocol Architecture: Layered Structure
High-level
functions
Communicating Entity
Services Provided
Using formatted data blocks that obey a protocol
Highest Level
Services Requested
Communication
with peer layers
on other entities
Lowest Level
Most primitive
functions
Physical Link

7. Key Aspects of a Protocol
Syntax
Block formats
Semantics
Actual control information for coordination and error handling
Timing
Speed matching, Sequencing

8. Example: Simplified Protocol Architecture
The task of file transfer is broken down into three modules (layers) that handle:
File transfer application  Application layer (Top)
Communication services  Transport layer
Network access  Network access layer

9. Simplified Architecture for File Transfer
A Three-Layer Model
Peer layer communications using the Protocol

10. Network Access Layer
Exchange of data between the computer and the network
Depends on type of network used (LAN, packet switched, etc.)
(should be the only layer that worries about these details)
The sending computer should specify address of destination computer, and possibly level of service, e.g. priority, delay, etc.

11. Transport Layer (communication services)
Reliable exchange of data
Independent of network used (significance)
Independent of application (serves all higher-level applications- sharing of resources)

12. Application Layer
Supports different user applications running on the communicating entity:
e.g. , file transfer, etc.

13. Protocol Architecture and Networks
Applications Layer: contains modules, each supporting an application running on this computer
(SAP)
Each application accesses the services of the transport layer through a SAP or a Port
3 computers communicating over a network
Using a 3-layer protocol architecture

14. Addressing Requirements
Two levels of addressing:
Each computer on the network needs a unique computer address
Each application on a (multi-tasking) computer needs a unique application address within that computer
We identify the application with the SAP it uses:
“Service access point or SAP” in OSI, e.g. SAP 3
“Port” in TCP/IP, e.g. Port 5
Each of the two addressing levels is handled only by the appropriate layer (information hiding):
Computer address handled by the Network Access layer
Application address (SAP #) handled by the Transport layer

15. Protocol Data Units (PDU)
Peer layers communicate (through the lower layers of course!) by exchanging PDU data blocks
At source: Each layer adds its relevant control information (header) to data received from a higher layer to form its PDU
At destination: that PDU is handled by the corresponding peer layer: The relevant control header is removed, used, and the remaining part of the PDU is pushed up to higher layers
Example:
Transport layer may fragment user data into smaller packets
A transport header is appended to each packet, which would include:
Destination SAP
Sequence number of the data fragment
Possibly, error detection code
This makes one transport protocol data unit (PDU)

16. Protocol Data Units (PDUs)
Application
Layer
User Data
Transport
Layer
Address of destination application on the destination computer
Packet sequence #
Error detection code?
What to include in the header?
Network
Access
Layer
Address of destination computer on the network
Network facilities required, e.g. priority
Network

17. Rules for generating PDUs at layers
Layer L generates its PDU by appending control part to data received from the next higher layer (L+1)
Appended control part takes the form of a header
That header will be used by the peer layer L on the destination entity
Both the PDU and the header are labeled (identified) by the layer generating and using them
PDU (L+1) + Header (L)  PDU (L) .
Lower Layers
L+1 L .

18. Operation of a Protocol Architecture
TFER (record, DSAP, Dhost)
PDU Decapsulation
PDU Encapsulation
(Transport PDU)
TFER (transport_PDU, Dhost)
Network
(Network Access PDU)

19. Framework for Standardization
Well-defined interfaces
at layer boundaries
Standards/code for various
layers can be developed
independently
and simultaneously
(Advantage)
Requests
Services
Changes and upgrades in a layer
need not affect other layers

20. Layer-Specific Standards
Service Definitions:
Only as a functional description- not how to be implemented (for flexibility)
For internal use only within this communicating entity (not a protocol) 2
SAP # 1
SAP # 2 …
Here we are dealing with other (foreign) systems!
Need a strict protocol
Addressing
within the same system 1 3

21. So, three standardization elements:
Addressing within the same system
Referencing by SAPs: We name the entity in a layer requesting a service from a lower layer by the SAP used on that lower layer
Service definitions (vertically within a system)
Only Functional description of what is required between layers within the same system
Protocol specification (Horizontally, more strict)
Operates between the same layer on the two communicating entities (peer layers)
May involve different operating systems
So, protocol specifications must be defined precisely
Format of data units
Semantics (meanings) of all fields

22. Service Primitives and Parameters
Services between vertically adjacent layers within the same system are expressed in terms of primitives and parameters
Primitives specify the action to be performed
Parameters pass data and control information required to perform the action
Services are requested by a service user layer
… and performed by a service provider layer
TFER (record, DSAP, Dhost)

23. Primitive Types REQUEST
Source (X)
Destination (Y)
Primitive Types
A: Service User
D: Service User 1 4 2 3
B: Service Provider
C: Service Provider
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
Time
Sequence:
A to B: “Deliver this packet to Y” 1
C to D: “Here is a packet for you!” 2
D to C: “Thanks!” 3
B to A: “Done it!” 4
Which pair of primitives is more important?

24. Timing Sequence for Service Primitives
Source System
Source System
Provider
Provider
Provider
Provider A B C D
Destination System
Destination System
No Response
or Confirm
(Assumed Done!)
Saves time…

25. Standardized Protocol Architectures
Required to allow devices from different manufacturers to communicate (inter-operability)
A win-win environment:
Helps vendors market their products better
Customers can shop around for best deals from different manufacturers as they are sure equipment will work together
Main standards:
OSI Reference model (X.200, 1977)
A theoretical system delivered too late! Never lived up to early promises
TCP/IP protocol suite (Developed for early forms of the internet)
Most widely used (The Internet protocol): A de facto Standard

26. OSI - The Model
Developed by the International Standardization Organization (ISO) in 1977
A 7-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

27. The OSI Seven Layers The Physical Medium End to End Internetworking
(network of networks)
(Internet)
End to End
The Physical Medium

28. The OSI Environment Hardware or software Trailer
Marking end of frame
Note information hiding!
e.g. DL layer does not look inside the N-DPU

29. OSI Layers with examples (1)
Physical Example: 10Base-T for Ethernet LAN
Physical interface between devices
Mechanical: Connectors, etc.
Electrical: Bit representation, V and I levels, Data rates,...
Functional: Functions of individual interface circuits
Procedural: Sequence of events for exchanging bit streams
Data Link (Chapter 7) Example: HDLC
Activating, maintaining and deactivating a reliable data link (link management):
Synchronization, error detection and control, flow control, so that higher layers may assume error free transmission over a direct link

30. OSI Layers with examples (2)
Network Example: X.25 (packet switching)
Data transportation over a network
Higher layers do not need to know about underlying technology (transmission, routing, etc)
Only needs info on destination address and required facilities
Not needed (bypassed) on direct (point-to-point) connections
Transport Example: TCP (Transmission Control)
Exchange of data between end systems
(possibly across multiple networks!)
Without errors
Without loss
Without duplication
Allows different levels of Quality of Service (QoS): Regarding acceptable error rates, maximum delay, priority, and security

31. OSI Layers (3)
Session
Control of dialogues between applications running on end systems
Dialogue disciplines, e.g. full duplex or half duplex
Grouping: Marking of data to indicate different groups
Recovery: Use of data check points
Presentation
Data formats and coding
Data compression
Data Encryption
Application
Means for applications to access the OSI environment
Support for , file transfer, terminal access to remote computers (telnet)

32. Do we always use/need all 7 layers?
Only end points need to have all 7 layers
Point-to-Point on a single link:
Network layer is bypassed
Intermediate nodes in network are not interested in user data!- Will have only the minimum number of required lower layers:
Within the same network: Intermediate node needs only the bottom 3 layers
Between multiple networks: Intermediate node between two networks (router) needs only the bottom … layers

33. Layers for a Relay node in a network
A 3-Layer node
(Network node)
Router
Relay Node, Joins:
Different physical links
Different data links
In the same network
What about a router
that connects
two networks?
(Intermediate Network node is not interested in the data content- so, the lower 3 layers only)

34. TCP/IP Protocol Architecture
Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network ARPANET (mother of the Internet, 1966)
Used today by the Internet
Did not start as a formal model.. but as a working one
Five Layers:
Application layer
Host to host (end to end) Transport layer
Internet layer (multiple interconnected networks)
Network access layer (similar to Data link)
Physical layer
= Same layer exists also in OSI

35. OSI vs. TCP/IP