Notes on Communication (1):Some fundamentals

Jean-Lou Dupontjl @ jldupont . com

http://www.jldupont.com/

Story board - context setting– Communication between a source and sink(s) requires

one of more interconnected (network) medium to forward the information that constitutes the communication-instance.

– The process by which information is transported from a source to the destination sink(s) requires resources and addressing.

– The resources can either be dedicated to the a communication-instance or shared amongst communication-instances.

– Sharing resources (effectively) require coordination and thus protocols.

In this presentation, we will not be focusing on the transport of information which is encoded in a physical structure (e.g. transport by plane of information contained in a book).

Key Points

• Communication requires Medium air/vacuum, metal strings, optical strings

• Medium is composed of Dimensions

o Time, Frequency, Powero Dimension access is either dedicated or shared

• Information transport is structured

o stream of symbols, stream of packets (group of symbols)

• Sharing implies Coordination implies protocol

Definition

Channel: (sometimes called communications channel) refers to the medium used to convey information from a sender (or source, transmitter) to a receiver (or sink).

Communication-instance: a specific communication between a source and a sink.

Forwarding Process: a component of a system responsible for the transport of information.

Partition: Generally, a partition is a splitting of something into parts. In communications we further assume that partitions can only interact with each other (i.e. exchange information) by established interfaces.

Source: aka transmitter

Sink: aka receiver

Resources

Resource usage can either be dedicated or shared

• Dedicated: the client of the resource controls the said resource for a period of time (either bounded or un-bounded)

• Shared: multiple clients compete to access & use the said resource using a coordination protocol (or apparent lack thereof :-)

Dedicated access is just a special case of Shared access: nothing is really dedicated for an unbounded time period, more likely "leased" for a bounded time period (by a master process).

Medium - Dimensions

A medium consists of 3 domains:

• Frequency Domain• Time Domain• Power Domain

ALL dimensions ALWAYS present

• Often, the power dimension is assumed and abstracted from diagrams Frequency: change in Time Domain• change includes matter level change

o e.g. finger over braille text

Dimensions - Examples

Example 1:

Medium is divided in the frequency domain - total isolation between dimensions

Example 2: ( CDMA )

Medium is divided using "codes" in the Freq+Power plane - no total isolation

Medium - Partitioning

E.g. air/vacuum :• can be spatially partitioned (see figure)• can be temporally partitioned ( time domain multiplexed )• a combination of the above

Medium

• For communication to take place, one or several medium must be used between source(s) and sink(s).

• A medium can either be dedicated or shared

Dedicated• 1 source, m sinks (where m>=1)• Special case of shared

Shared• n sources, m sinks• Requires a coordination protocol (e.g. CSMA/CD)

Medium - Access

Multiple Access (i.e. shared access) requires a coordination protocol active between the sources

• Manage Information Merging => Loss

Information

3 fundamental properties:

• Information can be copied• Information can be modified• Information can be measured

Note:• Erase ~ no copy

Information Property: Copy

Medium: air/vacuum• By nature broadcast• Use of electro-magnetic wave => copy is natural• Partitioning is possible ( spatial, temporal )

Medium: metal / fiber strings• Can be used in "broadcast" configuration too• Easier to dedicate

Information Property: Modify

2 fundamental types:

• Intentional => the usual purpose behind a communication• Un-intentional => environmental cause etc. => yields to loss

Information Property: Measure

Measurement:

• Information Entropy• Algorithmic Entropy ( Kolmogorov )

o Complexity of the algorithm that describes the Information

Information Copy: role of Power

• Distance => power• @ each sink => power

Forwarding

Forwarding can take place in 2 fundamental types of channel:• Destination Un-Aware• Destination Aware

Channel - Destination Un-aware

Representation of a broadcast channel• A) one source accessing the channel @ time t• B) two sources accessing the channel @ time t => loss

Channel - Destination Aware

Representation of a destination aware channel:• A) Information copied to 1 egress• B) Information copied to multiple egresses

Forwarding - Identifier Types

2 fundamental types:• Address• Label

Ultimately, the forwarding process requires an unambiguous way of delivering the information

Note: we are not concerned here with the trivial case "identifier-less broadcast"

Address Types

• Unicast ==> 1 destination• Multicast ==> n destination(s) where n>=0• Broadcast ==> 'all'• Anycast ==> 1 destination from a set

Label Type

A "label" is meant to be interpreted:• can refer to a sequence of hops to a destination• can refer to a communication-instance (aka connection)

o Indirection towards a destination

Connection Types

2 types:• Non-merging (aka Transport Connection)• Merging

Merging: when multiple sources merge their traffic• Result: can't identify individual sources

Merging

Merging is undesirable:• Noise merges into channels ==> loss of information

• Requires additional information (e.g. overhead) to mitigate against merging ==> less efficient

• Root Cause analysis difficult ==> higher OPEX

Merging - Management Aspect

In order to manage communication (e.g. in a network), it is highly desirable to have the capability to find the root cause of faults.• Noise is always present

o If higher than usual, need to locate and mitigateo Noise can (sometimes) have a signature ==> identifiable

• Packet Switching is prone to mergingo multipoint-to-point constructso configuration errors

routing errors