Notes On Networking 2
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Transcript of Notes On Networking 2
Notes on Networking :Transfer Modes
Jean-Lou Dupontjl @ jldupont . com
http://www.jldupont.com/
Context Setting
The Time Domain is omnipresent
Information is related to change in stateChange implies the Time Domain
Transfer Modes
There are 3 ( and only 3 ) modes:
• Connection-Oriented Circuit-Switching (CO-CS)• Connection-Oriented Packet-Switching (CO-PS)• Connection-Less Packet-Switching (CL-PS)
Why only 3 modes?
To answer this question, some more background information is required.
( hint: it comes down to how communications are identified & delivered )
Information Packaging
2 fundamental ways to package:• single bit• group of bits
Information Packaging
A group of bits can either be interpreted as:
• client information• client information with overhead information
Overhead information serves many purposes:• increases reliability of information• allows for verifying integrity of information etc.
o we will not be focusing on this topic in this presentation
Information Packaging
• Client Information : DATA• Client Information + Overhead Information : SYMBOL
NOTE: the time-domain is present in both cases (CLOCK)
The support of NOTHING
An interface either supports or not the semantic of NOTHING
• Client Information (DATA) is present• Client Information (DATA) is not present
NOTE: Sometimes referred to as IDLE code.
The conveyance of IDLE requires overhead information and thus a symbol.
Information Exchange
2 fundamental ways to exchange:• continuous flow ( aka stream ) => circuit• asynchronous exchange => packet
• Synchronous interface : information is always presento Or at least assumed... the source can get disconnectedo The concept of NOTHING (IDLE) is not supported
• Asynchronous interface: information availability is signaledo Explicit signal is required => the concept of NOTHING
must be supported as it is what defines it in the first place
Circuit
• Continuous stream of DATAo no holes possible (else => loss of information)
• Each DATA from a slot can be extracted from the streamo Need to synchronize in the time-domain to extract
A circuit consistsin timed DATA slots
Packet
A packet is a group of DATA bits delivered over an asynchronous interface.
The DATA bits are packaged in SYMBOLS : • This is to support the IDLE / NOTHING semantic.
Why 3 modes?
( We are half way there answering this question )
• We now acknowledge we have 2 fundamental ways to exchange information through an interfaceo Circuit Interfaceo Packet Interface
• Next, we need to be focusing on delivering the information...
Delivering Information
Communication comes down to copying information from a source to destination(s) (aka sink(s)).
If information can not be identified unambiguously, then valuablecommunication is not possible
To identify communications, labelling is required.
Labelling implies overhead.
Labelling
Labelling consists in information used to deliver the information to the destination(s).
2 fundamental ways:
o Label travels with Client Informationo Label does not travel with Client Information
Transport Container serves as label
Labelling
Label travels with Client Information -- In-band• Overhead information travelling with the Client information• E.g.:
o Address (e.g. IPv4)o Communication Instance Identifier (e.g. ATM VPI:VCI)
Label is related to the Transport Container -- Out-of-band• E.g.
o physical connectiono time-slot
Interfaces & Labelling
Circuit Interface• Only DATA signals => no in-band label• Label information can only be associated with Transport
Container (i.e. the server layer which transport the Client Information)
Packet Interface• Label travels with the Client Information
Note: The Label must travel with the Client Information in case of a packet interface.
Labelling & Identifying Information
Communications traversing an egress Access Point (blue circle /
right-hand side) can either:• be unambiguously identified• be ambiguous
Labelling & Identifying Information
MP2P (MultiPoint-to-Point)• Communications traversing an egress Access Point
supporting MP2P can be unambiguously identifiedo Each communication is labelled uniquely
Connection• A labelled communication which assumes only 1 source• If multiple sources use the same label and converge
towards the egress Access Point, merging occurso Ambiguity => loss of information
Labelling & Identifying Information
Examples
• 1) An communication over an electrical wire (circuit) which is submitted to electro-magnetic interference (noise)o Noise (~information using the same label) merging with
the ongoing communication can result loss
• 2) A communication taking place over label X (label which is interpreted semantically as a communication-instance) receives data from 2 different sources o Merging occurs => loss of information
Labelling & Identifying Information
Thus, we have 2 fundamental cases:
• 1 source is allowed : Connection• >1 sources are allowed : Connection-Less
If >1 sources are allowed (to egress at an Access Point), then additional information is required to identify individual sources
Furthermore, this additional information must be unambiguous.
Recap
Interfaces• Circuit• Packet
Identification• Label travelling with Client Information OR• Label is implicit to the communication channel
Access Point• 1 source only OR• >1 sources
RecapCircuit
-Y
Y-
Packet
Y- (note)
YY
Labelling• Explicit• Implicit
Sources at Egress• Only 1• >1
Note: an implicit label on packet channel defeats the purpose of a packet interface in the first place: a packet's interface value is in it provides the facility to embed additional information (through symbols aka overhead) along side with the client information. The primary purpose of this overhead information is to help direct the communication to its destination(s).
Goal
Before putting it all together, we must remember the ultimate goal:
Communication is only valuableif it is unambiguous
Putting it all together
From the recap page, we can deduce:
• The circuit construct only supports the connection oriented transfer mode
• The packet construct supports both connection and connection-less transfer modes
• The Connection-Less Circuit-Oriented construct does not make sense (see next slide)
Connection-Less Circuit-Switched ?
Why CL-CS does not make sense?
• Connection-Less => no pre-established connection needed before communication takes place between a source and destination(s)
o But we already ruled-out multiple (>1) sources merging their communication in a circuit context