Serial Point-to-Point Links

Time Division Multiplexing
Bell Laboratories invented time-division multiplexing (TDM) to maximize the amount of voice traffic carried over a medium. Before multiplexing, each telephone call required its own physical link. This was an expensive and unscalable solution. TDM divides the bandwidth of a single link into separate channels or time slots. TDM transmits two or more channels over the same link by allocating a different time interval (time slot) for the transmission of each channel. In effect, the channels take turns using the link.
TDM is a physical layer concept. It has no regard for the nature of the information that is being multiplexed onto the output channel. TDM is independent of the Layer 2 protocol that has been used by the input channels.
TDM can be explained by an analogy to highway traffic. To transport traffic from four roads to another city, you can send all the traffic on one lane if the feeding roads are equally serviced and the traffic is synchronized. So, if each of the four roads puts a car onto the main highway every four seconds, the highway gets a car at the rate of one each second. As long as the speed of all the cars is synchronized, there is no collision. At the destination, the reverse happens and the cars are taken off the highway and fed to the local roads by the same synchronous mechanism.
This is the principle used in synchronous TDM when sending data over a link. TDM increases the capacity of the transmission link by slicing time into smaller intervals so that the link carries the bits from multiple input sources, effectively increasing the number of bits transmitted per second. With TDM, the transmitter and the receiver both know exactly which signal is being sent.
A MUX at the receiving end reassembles the TDM stream into the three separate data streams based only on the timing of the arrival of each bit. A technique called bit interleaving keeps track of the number and sequence of the bits from each specific transmission so that they can be quickly and efficiently reassembled into their original form upon receipt. Byte interleaving performs the same functions, but because there are eight bits in each byte, the process needs a bigger or longer time slot.
Demarcation Point
Prior to deregulation in North America and other countries, telephone companies owned the local loop, including the wiring and equipment on the premises of the customers. Deregulation forced telephone companies to unbundle their local loop infrastructure to allow other suppliers to provide equipment and services. This led to a need to delineate which part of the network the telephone company owned and which part the customer owned. This point of delineation is the demarcation point, or demarc. The demarcation point marks the point where your network interfaces with the network owned by another organization. In telephone terminology, this is the interface between customer-premises equipment (CPE) and network service provider equipment. The demarcation point is the point in the network where the responsibility of the service provider ends.
The example presents an ISDN scenario. In the United States, a service provider provides the local loop into the customer premises, and the customer provides the active equipment such as the channel service unit/data service unit (CSU/DSU) on which the local loop is terminated. This termination often occurs in a telecommunications closet, and the customer is responsible for maintaining, replacing, or repairing the equipment. In other countries, the network terminating unit (NTU) is provided and managed by the service provider. This allows the service provider to actively manage and troubleshoot the local loop with the demarcation point occurring after the NTU. The customer connects a CPE device, such as a router or frame relay access device, to the NTU using a V.35 or RS-232 serial interface.
From the point of view of connecting to the WAN, a serial connection has a DTE device at one end of the connection and a DCE device at the other end. The connection between the two DCE devices is the WAN service provider transmission network. In this case:
The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network.
The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link. This signal is received at the remote DCE, which decodes the signal back into a sequence of bits. The remote DCE then signals this sequence to the remote DTE.
The Electronics Industry Association (EIA) and the International Telecommunication Union Telecommunications Standardization Sector (ITU-T) have been most active in the development of standards that allow DTEs to communicate with DCEs. The EIA refers to the DCE as data communication equipment, while the ITU-T refers to the DCE as data circuit-terminating equipment.
Cable Standards
Originally, the concept of DCEs and DTEs was based on two types of equipment: terminal equipment that generated or received data, and communication equipment that only relayed data. In the development of the RS-232 standard, there were reasons why 25-pin RS-232 connectors on these two types of equipment needed to be wired differently. These reasons are no longer significant, but we are left with two different types of cables: one for connecting a DTE to a DCE, and another for connecting two DTEs directly to each other.
The DTE/DCE interface for a particular standard defines the following specifications:
Mechanical/physical - Number of pins and connector type
Electrical - Defines voltage levels for 0 and 1
Functional - Specifies the functions that are performed by assigning meanings to each of the signaling lines in the interface
Procedural - Specifies the sequence of events for transmitting data


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