How To Understand The Layered Architecture Of A Network

C ^{OMPUTER N ETWORKS}

N ^ETWORK A RCHITECTURE ^AND

P ^ROTOCOLS

The Need for Standards

 Computers have different architectures, store data in different formats and communicate at different rates

 Agreeing on a particular standard is difficult

 Defacto Standards

 Standards that exist by virtue of wide spread use

 Products consistent with these standards often have a large market

 Standards adopted by Agencies

 Step 1 – Submit proposal to be considered by an agency

 Step 2 – If proposal has merit and wide spread acceptance, agency make suggestions for modifications to originators

 Step 3 – After rounds of suggestions and modifications the proposal is adopted or refused

Network Architectures



The task of designing a communication network is too complex to be handled as a monolithic unit.



An alternative, a structured approach.

 Divide communication task into manageable parts.

 Need to describe the communication functions in terms of an architecture.



A network architecture defines the relationship and interactions between network services and functions through common interfaces and

protocols.

What is a Protocol?

 A protocol is a set of mutually agreed upon rules that regiment interactions between communicating

entities.

 The key elements of a protocol are:

 Syntax – defines tstructure of information communicated

 Semantics – defines the meaning of the exchanged information, including control and signaling

information

 Timing – defines the time at which data should be exchanged.



Key elements define

is comunicated,

it

is communicated, and

it is communicated.

L ^AYERING

N ^ETWORK A RCHITECTURE

OSI Reference Model – Layered

Architecture of Computer Networks

 International Organization for Standardization (ISO) developed Open Systems Interconnection (OSI)

Reference Model that characterizes and standardizes the internal functions of a communication system

 Model provides a framework for the development protocols for computer communication

 Defines the rules and conventions for various functions within each layer

 Specifies the general relations among these functions.

 Determines the constraints on the types of functions and their relations.

Layered Architecture



Model describes the architecture in terms of abstract layers

 Boundaries between adjacent layers are called interfaces



Each layer performs a related subset of functions required for communication, and adds value to the services provided by lower layers.

 Layer N relies on services of layer N-1 to provide a service to layer N+1

 Service required from lower layer is independent of how that service is implemented

 Information and complexity hiding

Why Layering?



Monolithic non-layered architectures are costly, inflexible, and soon obsolete

 Layering simplifies the design, implementation and testing



Layering provides flexibility for modifying and

evolving protocols and services without having to change layers below

 Protocol in each layer can be designed separately from those in other layers

ISO/OSI Reference Model



The ISO reference model defines seven layers:



Application Layer,



Presentation Layer,



Session Layer,



Transport Layer,



Network Layer,



Data Link Layer, and



Physical Layer.

Physical Data Link

Network

ISO/OSI Reference Model

Physical Data Link

Network Transport

Session Presentation

Application

Physical Data Link

Network Transport

Session Presentation

Application

Physical Data Link

Network

Communication Medium

Router Router

Host Host

Exist in Hosts and Routers

Exist only in

Hosts

Layers, Services & Protocols



Each layer operates according to a protocol to provide a service to the layer above



Logically, each layer communicates directly with its peer

 A peer is the communication entity residing at the same layer in the other communication

system



Physically, each layer communicates with the

layers directly above it and below it

Physical Data Link

Network

ISO/OSI Reference Model

Physical Data Link

Network Transport

Session Presentation

Application

Physical Data Link

Network Transport

Session Presentation

Application

Physical Data Link

Network

Communication Medium

Router Router

Host Host

Physical Layer Data Link Layer

Network Layer Transport Layer

Session Layer Presentation Layer

Application Layer

ISO/OSI Refrence Model

H Data T

H Data H Data

H Data H Data

Data

ISO/OSI L ^AYERS

N ^ETWORK A RCHITECTURE

ISO/OSI Physical Layer



Transmission of raw bits, 0/1, over a

communication medium

Physical Layer Characteristics and Functions



Transmission Medium

 Twisted-pairs, Coaxial cable, Optical fiber, Radio, Satellite, Infrared, …



Aspects of a communications link

 Mechanical – Cable, Wires, Connectors, Pins, ...

 Electrical and Optical – Interfacing, Modulation, Data Encoding, Signal Strength, Voltage levels, Clock Recovery, …

 Functional and Procedural – Activation,

Maintenance and Deactivation of physical links

Data Link Layer Functions

 Transfers frames across direct connections between two adjacent nodes.

 Transform an unreliable physical link into a reliable logical link

 Error free communication over a single link between adjacent nodes

 Speed matching between senders and receivers

 Framing

Data Link Layer

 The DLL is composed of two Sub-Layers

 Logical Link Control (LLC)

 Medium Access Control (MAC)

LLC MAC

Data Link Layer Architecture

Network

Physical

Medium Access Control (MAC)



Contention occurs, when two or more nodes

attempt to access a shared medium at the same time

 Rules must be in place to regiment access



The MAC sublayer defines the rules for accessing the medium



Common MAC Schemes include;:

 Random Access

 Carrier Sense Multiple Access (CSMA)

 CSMA, with Collision Detection (CSMA/CD)

 CSMA, with Collision Avoidance (CSMA/CA)

 Token Passing

 Reservation

LLC Sublayer Main Functions

 Framing – Groups bits into frames, using a header and a trailer

 Flow Control – Prevent sender from overwhelming the receiver’s buffers

 Error Control – Detecting and recovering from bit errors

1011000

LLC

Even Parity

10110001

LLC

10110101

 Single Error Detected

 Retransmission

1011000

LLC

Even Parity

10110001

LLC

10100101

 Double Error Undetected

 Retransmission

Network Layer



Network Layer:

 Data transmission and delivery between hosts

 Controls access to the network

 Provides routing of packets within the network

 Manages contention and bottlenecks within the network



Data Link Layer:

 Error free communication over a single link between adjacent nodes

 Speed matching between senders and receivers

 Framing

Network Layer Main Functions



The network layer provides the functional and procedural means of transferring packet data units from a source to a destination functions



Basic Network Layer functions include:

 Addressing – Network devices must be assigned network addresses for identification

 Routing: Network nodes jointly execute a routing

algorithm to determine “shortest” paths between traffic sources and destinations

 Packet Forwarding – Transfer packets across a routing path

 Congestion Control – To mitigate the impact of traffic surges

Transport Layer



Transport layer supports required functions to enable communications between end systems.



Main functions often performed include:

 Naming: used to differentiate between different communicating processes

 Multiplexing and Demultiplexing:

 TL multiplexes data, received from many local application programs, into as a single data stream before sending it

 TL demultiplexes an incoming stream of data, and directs each data packet to the appropriate recipient application process.

 Connection Management – for Call Setup and Teardown

 Segmentation and Reassembly – To meet the length requirement of the physical network

 Reliability and Flow Control – To deal with packet error and packet loss, delivery out-of-order, and packet duplication

Session Layer



The main function of a session layer is to

establish and maintains a session between two end-users

 Dialog management can be full duplex or half Duplex



The session layer establishes the connection in conjunction with the application

 A video session may have multiple transport connections for Video audio, and textual Data



In case of connection disruption during large file

transfers, SL enables the communication resume

form synchronization points

Presentation Layer



Different computers have different ways of representing the same information



The presentation layer establishes a common ground of communication



Data compression

 Decreasing as much as possible the amount of bits used to represent information



Security

 If an un-authorized person intercepts the message, it is unintelligible

Application Layer



Application Layer: Provides services that are frequently required by applications:



Typical application layers

 E-mail

 File Transfer Protocol (FTP)

 Hypertext Transfer Protocol (HTTP)

 Telnet

P ^EER-

^- P ^EER C OMMUNICATION

N ^ETWORK A RCHITECTURE

Peer-to-Peer Communication Protocols



Layer-N in one system interacts with Layer-N in another system to provide service to Layer-( N+1)



The entities comprising the corresponding layers on different machines are called peer processes.



The communicating entities use a set of rules and conventions called the layer-N Protocol.



Layer-N peer processes communicate by

exchanging Protocol Data Units (PDUs)

Layer N

Layer N-1

Layer N

Layer N-1

SAP SAP

Interface Protocol

Interface Protocol Peer-To-Peer

Protocol

Peer-To-Peer Protocol

End System End System

Service Access Point

Peer-to-Peer Communication

 Communication between peer processes is virtual and actually indirect

 Layer-N+1transfers information by invoking the services provided by Layer-N

 Services are available at Service Access Points (SAP)

 Each layer passes data and control information to the layer below it until the physical layer is reached and transfer occurs

 The data passed to the layer below is called a Service Data Unit (SDU)

 SDU’s are encapsulated in PDU’s

30

Layer N

Protocol Data Unit



A layer accepts a Service Data Unit and adds Protocol Control Information to form a Protocol Data Unit .

Layer N+1 SAP

NSDU NPCI

NPCI

NPDU

Layer N

Layer N+1 SAP

NSDU NPCI

NPCI

NPDU Peer-to-peer

communication

Interface Data Unit



A set of Interface Data and its associated ICI makes an IDU, the data unit passed across

abstract interfaces at the Service Access Point.

SAP

ICI Int. Data ICI Int. Data ICI Int. Data

PCI SDU

PCI SDU PDU

IDU

33

Headers & Trailers

Application Presentation

Session Transport

Network Data Link

Physical

CRC

Application Presentation

Session Transport

Network Data Link

Physical

Data

Data

Data

Data

Data

Data

Data AH

AH

AH

AH

AH

AH PH

PH

PH

PH

PH SH

SH

SH

SH TH

TH

TH NH

DH NH

00101 10101010

ISO/OSI Service Primitives



Interactions between adjacent layers are managed by communication primitives.

 A primitive initiates an action or reports a result.

 These primitives can occur at various instances of the dialogue.

ISO/OSI Service Primitives



Four basic primitives are defined:

 Service.Request(): Invokes the services of the lower layer and passes the needed parameters.

 Service.Indication(): Advises the higher layer of the requested service.

 Service.Response(): Used to reply to an indication issued by the lower layer.

 Service.Confirmation(): Used to acknowledge the completion of a request previously invoked by a higher layer.

ISO/OSI Service Types



Three types of service:

 Confirmed Service

 Involves all four primitives

 Unconfirmed Service

 Involves only Service.Request() and Service.Indication() primitives. Acknowledgment (Service.Response() and Service.Confirm()) are omitted.

 Provider Initiated Service

 Used by the network to signal special conditions, involves only Service.Indication() primitive.

ISO/OSI Confirmed Service

SAP SAP

End System End System

Time Request

Indication

Response Confirm

ISO/OSI Unconfirmed Service

SAP SAP

End System End System

Time Request

Indication

ISO/OSI Provider Initiated Service

SAP SAP

End System End System

Time Indication

Indication

Provider

Segmentation & Reassembly



A layer may impose a limit on the size of a data segment that it can transfer for



Thus, a Layer-N SDU may be too

large to be handled as a single unit by Layer-N-1



The SDU is

segmented into multiple PDUs

Segmentation

Reassembly n-SDU

n-PDU n-PDU n-PDU

n-SDU

n-PDU n-PDU n-PDU

Multiplexing

 Sharing of Layer-N service by multiple Layer-N+1 users

 Multiplexing ID is required in each PDU to determine Layer N+1

Entity

Layer N Entity

Layer N Entity

N-sap N-SAP

N-SDU

N-SDU H

N-PDU

N-PDU N-sap

N-sap

Layer N+1 Entity

H Layer N+1

Entity

Layer N+1 Entity

I NTERNET ARCHITECTURE

N ^ETWORK A RCHITECTURE

Internet Design Philosophy



The approach resulted from extensive research and practical experience of ARPANET, currently the Internet



The fundamental principle of the architecture

 Communication between local and remote processes can be accomplished by first identifying the remote host and then identifying the remote process within the remote host.

Internet Design



The actions of identifying the network and the remote process can be handled independently.

 The task of the network is reduced mainly to routing data between hosts.

 The network need not be concerned with how the data is directed to the remote process within the host.

Internet Layers



Based on the above concept, the architecture was conceived as a hierarchical ordering of

protocols that can be organized into four layers:

 Application layer,

 Transport layer, the

 Internet layer, and

 Network access layer.



Higher layers may bypass an adjacent layer and

directly use the services of a lower layer.

Internet Protocol Architecture

Subnetwork A Subnetwork B Process

Network Interface

Telnet FTP HTTP ToD

User Datagram Protocol (

UDP

Transmission Control Protocol (

TCP

Host-to-Host (Transport)

Internet Protocol (

IP

Internet Control Message Protocol

(

ICMP

Internet (Network)

SNMP

ISO/OSI – Internet Correspondence

ISO/OSI Internet Application

Presentation Session Transport

Network Data Link

Physical

Network Access Internet Host-to-Host

Process

Internet Addressing Concept

Physical

Network Access Protocol

IP TCP

Physical

Network Access Protocol

IP TCP

Host A Host B

IP

NAP 1 NAP 2 Router

Network IP Address

Logical Transport Connection Port

Net 1 Net 2

Physical Address

Internet Architecture

 The IP network layer provides a simple, best effort, datagram service

May not be reliable



TCP supports reliable data delivery

Performance enhancement for a variety of applications:

Telnet, FTP, HTTP, etc

Decision does not impact other applications, if UDP can be used

 Everything else is implemented at

application level

Internet Architecture

Internet Protocol

R O B U S T

FRAGILE

Application

Network

Physical

Summary



Introduced the concept of a protocol



Introduced the main network architectures



Discussed future trends of communication networks



Revisited switching techniques