Multiplexing  Consider a typist performs data entry from a terminal to a central computer. The line speed between the terminal and the computer is 9600 b/s.  How fast the typist has to type to keep the line busy?  A channel devoted to a single terminal is typically idle much more than it is active.  A channel can be shared among several terminals to reduce cost. ---> Multiplexing 9600 bits/second x 60 sec/min 10 bits/byte x 10 bytes/word = 5760 words/min

Background  Multiprocessing DTEs (e.g., timesharing computers) are designed to support dozens of remote DTEs (e.g., terminals).  Each remote DTE connection is made through a port, an interface to the central computer. DTE 1 DTE 2 DTE 3 Port E Port L Port S Host DTE Remote DTEs

Background  If individual channels connecting the interactive terminals to the ports on the host are only partly in use, it may be possible to have some or all of the terminals multiplex their communications onto a single channel.  Multiplexer (Mux)  Multiplexing should be Transparent to DTEs being muxed. DTE 1 DTE 2 DTE 3 Port E Port L Port S Host DTE Remote DTEs Mux

Multiplexer Internals  Multiplexing tasks:  Demultiplexing algorithm: - Checking a connection for data - Formatting data according to the protocol used by the channel. - Transmitting data on the channel. Three basic operations: - Reading data from the channel. - Determining the destination of the data. - Forwarding data to the specific connection.

Multiplexing Techniques  Frequency Division Multiplexing (FDM) - dividing the multiplexed channel into a number of unique frequency bands, each one assigned to a pair of communication entities. - FDM can be achieved only if the available bandwidth on the multiplexed channel exceeds the bandwidth needs of all communicating entities. - Not widely used in data communication because of hardware cost. - Examples: cabled TV DTE 1 DTE 2 DTE 3 Port E Port L Port S Host DTE Remote DTEs Mux Hz Hz Hz

Multiplexing Techniques  Time Division Multiplexing (TDM) - Time share the channel between various DTEs. - To prevent information arriving at wrong DTE, both multiplexers must be synchronized. -- a special bit pattern to indicate the start of a new cycle. - When a DTE has no data to send, the corresponding slot is filled with a reserved bit pattern (e.g., NULL). DTE 1 DTE 2 DTE 3 Port E Port L Port S Host DTE Remote DTEs Mux S L E S L

Multiplexing Techniques  Statistical Multiplexing - To overcome the problem of idling DTEs by sending information from a DTE only when it is available. - A DTE does not use a fixed time slot in a cycle. - The multiplexer must be able to determine the intended destination of each byte (data unit) received. - A unique identifier either of the source or the destination of the byte must be included with each byte (data unit). DTE 1 DTE 2 DTE 3 Port E Port L Port S Host DTE Remote DTEs Mux P S I E K 2 G 2 H 1

Multiplexing Techniques  Statistical Multiplexing - Two implications: - The bandwidth is reduced by transmitting identifiers. - Mapping tables are required: A. if the identifier is the source address, the remote multiplexer must map that address into a destination connection. B. if the identifier is the destination address, the local multiplexer must map the local device’s identifier into the destination address. - The bandwidth on the multiplexed channel does not need to exceed the sum of peak bandwidth needs of all communicating entities. - If all devices connected to a multiplexer transmit simultaneously for a sustained period, there may not be sufficient bandwidth to handle all of the traffic. Traffic may be discarded at the multiplexer or buffers may be used to temporarily hold the data. Statistical multiplexing gain

Port Selectors  When there are more remote DTEs than ports on the central host, a device known as a port selector (or front end, switch, or terminal concentrator) is employed to manage connections from remote DTEs to host’s ports.  Connection is sought when a DTE issues some form of signals (a series of characters, carrier signal) to the port selector. DTE 1 DTE 2 DTE 3 Port A Port B Host Remote DTEs Port selector DTE 4