Notes On Networking - Part 1

Notes on Communication (1): Some fundamentals Jean-Lou Dupont jl @ 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 communicationinstance. – 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 communicationinstances. – 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, Power o 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 unbounded) • 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 mitigate o Noise can (sometimes) have a signature ==> identifiable • Packet Switching is prone to merging o multipoint-to-point constructs o configuration errors  routing errors