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    TDM vs. IP

    There are two main techniques for Multiplexing

    • Time Division Multiplexing (TDM)
    • Internet Protocol (IP)
    Time Division Multiplexing (TDM)

    Native TDM is a type of digital multiplexing that transfers two ore more data streams transmitted simultaneously as sub-channels in one communication channel. This is perfect for circuit switched networks such as Public Switched Telephone Network (PSTN) there is a need to transmit multiple subscriber calls along the same transmission medium. TDM allows switches to create channels, also know as tributaries, with a transmission stream. A standard DSO voice signal has a data bit rate of 64Kbits. TDM takes frames of the voice signals and multiplexes them into a TDM frame which runs at a higher bandwidth.

    Internet Protocol (IP)

    The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched network.

    IP is a network layer protocol in the internet protocol suite and is encapsulated in a data link layer protocol (e.g., Ethernet). As a lower layer protocol, IP provides the service of communicable unique global addressing amongst computers.

    Because of the abstraction provided by encapsulation, IP can be used over a heterogeneous network (i.e., a network connecting two computers can be any mix of Ethernet, ATM, FDDI, Wifi, token ring, etc.) and it makes no difference to the upper layer protocols. Each data link layer can (and does) have its own method of addressing (or possibly the complete lack of it), with a corresponding need to resolve IP addresses to data link addresses. This address resolution is handled by the Address Resolution Protocol (ARP).

    FDD vs. TDD

    There are two main techniques for dividing forward and reverse communications channels on the same physical transmission medium:

    • Time Division Duplex (TDD)
    • Frequency Division Duplex (FDD)
    Time Division Duplex (TDD)

    TDD is ideally suited to the transportation of asymmetric traffic by providing adaptive uplink (UL) and downlink (DL) channels that are derived from a singular frequency set. In retrospect, TDD divides the data stream into frames and assigns them different time slots that allow for a shared transmission medium (Same Frequency), while reducing the amount of bandwidth that is required. With this Duplex scheme, the user has the ability to assign values for both UL/DL and reduces overhead by limited the guard band that is required to separate UL and DL channels.

    Product Example:

    • Motorola PTP 600 Series Radio has a TDD Duplex Scheme. This TDD radio can be dynamic or fixed; same or split frequency Tx/Rx.

    Overview: TDD emulates full duplex communication over half duplex communication links. It is ideal for IP, VoIP, and Video.

    Frequency Division Duplex (FDD)

    FDD is a Duplex method that assigns distinct frequency channels for both the transmitter and receiver. Due to the symmetric nature of FDD transmission channels, FDD is the ideal choice for TDM analog voice applications. By proving a 50/50 split between UL & DL, FDD applications greatly reduce latency along with providing simpler radio planning (UL & DL do not hear each other).

    Overview: FDD provides even UP & DL speeds and provides a complete Full Duplex Solution. It is ideal for TDM and Analog voice circuits.

    Example: DragonWave is a FDD (IP) Radio.