PDF of this tutorialDTM is designed for a unidirectional medium with multiple access—for example, a medium with capacity shared by all connected nodes. It can be built on several different local topologies such as ring, double ring, point to point, or dual bus.
Figure 2. DTM Multiplexing Medium with Multiple Access
DTM is based on TDM, whereby the transmission capacity of a fiber link is divided into small time units. The total link capacity is divided into fixed-size frames of 125 microseconds. The frames are further divided into a number of 64-bit time slots. The number of time slots per frame is dependent on the bit rate. Using a bit rate of 2 Gbps, the number of time slots within each frame totals approximately 3900. The selected use of a frame length of 125 microseconds and 64 bits per time slot enables simple adaptation to digital voice and plesiochronous digital hierarchy (PDH) transport.
Figure 3. DTM Multiplexing Format
The time slots within each frame are separated into data slots and control slots. At any point in time, a slot is either a data slot or a control slot. However, if needed, data slots may be converted into control slots and vice versa.
The right to write data slots and control slots is distributed among the nodes attached to the link. Consequently, each node attached to the link will typically have write-access to a set of data and control slots, and these time slots will occupy the same time-slot position within each frame of the link. Each node has write-access to at least one control slot that the node uses for sending control messages to the other nodes. Control messages can be sent upon request from a user served by the node, in response to control messages from other nodes, or spontaneously for network-management purposes. The control slots constitute only a small fraction of the total capacity, while the majority of the time slots are data slots carrying payload. The signaling overhead in DTM varies with the number of control slots but is typically low. For a ring with 20 attached nodes and a bit rate of 2 Gbps, the signaling overhead is typically less than one percent.
As stated, data slots are used for carrying payload data. Each node typically has write-access to a pool of free data slots, which will occupy the same time-slot positions within each frame of the link. When establishing communication channels, a node will allocate a portion of the data slots available in the node's pool of free data slots to the channel, as will be discussed further in the next section.
A summary of DTM fundamentals includes the following:
- The globalization of network traffic and the transmission of integrated data, audio, and video are increasing the demand for network transfer capacity.
- The transmission capacity of optical fibers is today growing significantly faster than processing power, which moves the traffic bottleneck towards processing and buffering in switches and access nodes in the network.
- DTM is a technique designed for full control of network resources. It is built to increase the utilization of optical fibers and minimize the load of the nodes. DTM is also designed to support real-time broadband traffic, multicasting, and the ability to adopt to traffic variations in the network dynamically.
- DTM combines the simple, nonblocking, real-time traffic capabilities of circuit switching with the dynamic resource-handling of packet-switching technology. This covers a gap in available techniques on the market and meets the demands of strict quality of service (QoS) requirements for high-bandwidth communications.
- DTM offers at least three types of reservation schemes: guaranteed bandwidth, on-demand bandwidth, and on-demand bandwidth with best effort.
- The DTM link capacity is divided into frames of 125 microseconds. Each frame is then divided into 64-bit slots. Using a bit rate of 2 Gbps, the number of slots is around 3,900.
- In DTM, data is transported in channels. A channel consists of a number of 64-bit slots.
