1. Network Architecture Layered Architectures Network Protocols
Reference Models
Network Standards
Network Devices
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2. Network Architecture Definition of Network Architecture
A highly structured framework within which networks can be analysed, designed and implemented, incorporating a defined set of layers and protocols
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3. Layered Architectures
Each layer is functionally independent
Each layer has a defined interface to the previous and preceding layer
Each layer builds on the previous layer
Virtual communication takes place between layers at the same level
Layered architectures are often called protocol stacks
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4. Layered Architectures
Advantages
They divide up complex operations into more manageable groups
Changes to one layer can be carried out without having to change all the others
Standard interfaces can be defined for each layer (allows protocols to be mixed and matched)
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5. Interfaces
An interface exists between two adjacent layers. It defines which services and primitives the lower layer offers to the upper layer
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6. Services and Primitives
A layer offers a service to the layer immediately above it
A service is formally specified by a set of operations called primitives which can be thought of as procedure calls with parameters
E.g. The IP network service has two primitives:
send to transmit a packet
deliver to receive a packet
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7. Service Access Points
The layer n SAP is the place where the layer n+1 entity can access the layer n services offered (Network layer SAPs are often referred to as NSAPs)
Each SAP has a unique address called the SAP address (Network layer addressed are often referred to as NSAP addresses)
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8. Protocols
A protocol is a set of mutually agreed rules that allows two peer layer entities to communicate successfully
Protocol Characteristics
Symmetric/Asymmetric
Standard/Proprietary
Connection-oriented / Connectionless
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9. Symmetric and Asymmetric Protocols
Communication between equal peer entities where each entity can invoke the same services
Asymmetric
One entity has access to services that the other does not have. E.g. Client/server
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10. Standard and Proprietary Protocols
A standard protocol is a protocol that is controlled by a recognised standards body
A proprietary protocol is a protocol that is not controlled by a recognised standards body and is usually under the control of a commercial organisation
Customers today are usually wary of proprietary protocols as their use tends to lock them into suppliers
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11. Connection-oriented / Connectionless Protocols
Connection-oriented protocols support three phases:
Connection Establishment
Data Transfer
Connection Termination
In most cases data is received in the same order that it was transmitted
Connectionless Protocols only support one phase (data transfer)
All packets (called datagrams) are routed independently
Packets are often received in an order different to that in which they were transmitted
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12. Service and Protocol Data Units
The SDU is the data unit passed down from layer n+1 to layer n, then passed across the network and up to the peer layer n+1 entity
The PDU is the data passed between peer layer n entities
The layer n PDU consist of the layer n+1 SDU preceded by the layer n header
Some layer n PDUs do not carry any layer n+1 SDUs, but are used to carry control information between layer n entities in the layer n header
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13. Service and Protocol Data Units
Layer n+1 Entity
Layer n Entity
Layer n+1 SDU
Layer n PDU
The layer n PDU consists of the layer n+1 SDU preceded by a layer n protocol header. Protocol headers can include address information, control information and error detecting codes
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14. Generic Protocol Functions
Encapsulation
Segmentation
Connection Control
Ordered Delivery
Flow Control
Error Control
Addressing
Multiplexing
Priority
Grade of Service
Security
Data Compression
Data Encoding
Data Encryption
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15. Encapsulation
When a layer n+1 SDU is transmitted in a layer n PDU, the layer n+1 SDU is said to be encapsulated in the layer n PDU
Encapsulation really means that a layer n protocol header is added to the layer n+1 SDU prior to transmission
Decapsulation is the reverse process where the layer n protocol header is removed before the SDU is passed to layer n+1
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16. Encapsulation Example
Application Protocol
Data Unit (message)
E.g. Addresses
Application
Header
Transport Protocol
Data Unit (segment)
E.g. Port Numbers
Transport
Header
E.g. Network
Addresses
Network Protocol
Data Unit (packet)
Network
Header
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17. Segmentation
Different layers often support different maximum PDU sizes
Sometimes SDUs needs to be segmented (or fragmented) into smaller PDUs by a lower layer protocol and re-assembled back into the original size SDU before being passed back to the upper layer protocol
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18. Segmentation Layer n+1 Entity Layer n Entity Layer n+1 SDU
Layer n PDU 1
Layer n PDU 2
Layer n does not support a large enough SDU size to transmit the
SDU as one PDU. It therefore segments it and reassembles it at the
distant end, before passing the original SDU back to Layer n+1
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19. Connection Control
Connection Control is required for Connection-oriented protocols which must have primitives to establish, terminate and reset connections and use special PDUs to establish and close connections
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20. Ordered Delivery
Protocols need mechanisms to ensure that data can be reordered back into the order that it was originally transmitted
This is usually achieved by including a sequence number in the protocol header
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21. Flow Control and Error Control
Flow control is performed by the receiving entity to limit the rate at which data is sent by the transmitting entity to avoid data loss.
Error Control guards against the the loss of or damage to data or control information
Both these functions are often implemented by using sequence numbers in protocol headers and requiring that the receiver acknowledges receipt of a PDU. The receiver can slow down the transmitter by slowing the rate at which it issues acknowledgements. Sequence numbers can also be used to detect and report the non-delivery or corruption of PDUs
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22. Addressing
All entities that transmit or receive data must have a unique unambiguous address
Most protocols headers have a source and destination address field which identify the peer layer entities involved in the communication
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23. Multiplexing
A protocol can carry SDUs between several different pairs of communicating entities to make more efficient use of lower level services
Protocols support multiplexing by using fields in the protocol header (often addresses) to uniquely identify which PDUs belong to which communication, so that they can be demultiplexed at the receiving end and passed to the correct layer n+1 entity
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24. Priority
Certain messages such as control messages may need to get through with minimum, delay
Protocol headers often contain a priority field so that some messages can be given differential treatment
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25. Grade of Service
Certain classes of data may require a minimum throughput or a maximum delay
Protocol headers often contain a field to specify the grade of service required
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26. Security
Security mechanisms restricting access may be invoked in protocols (E.g. Closed User Groups and authentication fields)
Protocol headers often contain fields that are used to identify and authenticate users and can thus control access
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27. Data Compression
Bandwidth is frequently a scarce resource and some data transmissions are often inefficient and can be compressed before being transmitted (E.g. MPEG 4 for video)
Protocols sometimes support the compression and decompression of data so that the quantity of data transmitted is significantly reduced
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28. Data Encoding
In order to be understood at the receiving end, it is essential that the receiving entity knows how to interpret the data being sent
This is achieved through using a common protocol to encode the data (E.g. ASCII, HTML etc.)
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29. Data Encryption
To keep data (and control information) secure, it is often necessary to use encryption to scramble the data so that other parties cannot understand it
Some protocols support the encryption and decryption of data (and control information)
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