Multiplexing

Multiplexing
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Getting Started

The Concept:
- Multiple messages/signals on a single (shared) channel
Roles:
- Multiplexer (MUX) - combines the messages/signals
- Demultiplexer (DEMUX) - splits the messages/signals

Multiplexing on Circuits

Frequency-Division Multiplexing (FDM):
- A frequency band is devoted to each connection (e.g., analog cable television)
Time-Division Multiplexing (TDM):
- Each connection is allocated a time slot

Multiplexing on the Internet

Network/Internetwork Layer (IP):
- Responsible for routing and delivering frames/datagrams
Transport Layer:
- Responsible for (among other things) delivering the segments to the appropriate process/application (which is identified by fields in the segment's header)
- The receiver identifies the process/application and delivers the segment (i.e., demultiplexes)
- The sender includes the destination process/application fields in the header and hands the segment off to the network layer (i.e., multiplexes)

An Analogy

The Setting:
- Several people live at the same address
Multiplexing:
- The local post office bundles all mail to the same address
Delivery:
- The letter carrier delivers the bundle to the address
Demultiplexing:
- Someone at the address divides the bundle and puts it into different stacks

Ports

Types:
- Source - used to identify the process/application that sent the segment
- Destination - used to identify the receiving process/application
Details (UDP and TCP):
- Two octets (i.e., 0-65535)
- "Well-known" port numbers are in the range 0-1023 (e.g., FTP uses 20 and 21, HTTP uses 80)

Sockets

The Concept:
- What you "plug into" to get segments where they need to go
UDP:
- A socket is specified by a two-tuple consisting of the receiving IP address and port
TCP:
- A socket is specified by a four-tuple consisting of the sending IP address and port and the receiving IP address and port