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