Content Overview This module presents an overview of WAN technologies. It introduces and explains WAN terminology such as serial transmission, time division multiplexing (TDM), demarcation, data terminal equipment (DTE) and data circuit-terminating equipment (DCE). The development and use of high-level data link control (HDLC) encapsulation as well as methods to configure and troubleshoot a serial interface are presented. Point-to-Point Protocol (PPP) is the protocol of choice to implement over a serial WAN switched connection. It can handle both synchronous and asynchronous communication and includes error detection. Most importantly it incorporates an authentication process using either CHAP or PAP. PPP can be used on various physical media, including twisted pair, fiber optic lines and satellite transmission. Described in this module are configuration procedures for PPP, available options, and troubleshooting concepts. Among the options available is the ability of PPP to use PAP or CHAP authentication. Students completing this module should be able to:
Content 3.1 Serial Point-to-Point Links 3.1.1 Introduction to serial communication WAN technologies are based on serial transmission at the physical layer. This means that the bits of a frame are transmitted one at a time over the physical medium. The bits that make up the Layer 2 frame are signaled one at a time by physical layer processes onto the physical medium. The signaling methods include Nonreturn to Zero Level (NRZ-L), High Density Binary 3, (HDB3), and Alternative Mark Inversion (AMI). These are examples of physical layer encoding standards, similar to Manchester encoding for Ethernet. Among other things, these signaling methods differentiate between one serial communication method and another. Some of the many different serial communications standards are the following:
Content 3.1 Serial Point-to-Point Links 3.1.2 Time-division multiplexing Time-Division Multiplexing (TDM) is the transmission of several sources of information using one common channel, or signal, and then the reconstruction of the original streams at the remote end. In the example shown in the first figure, there are three sources of information carried in turn down the output channel. First, a chunk of information is taken from each input channel. The size of this chunk may vary, but typically it is either a bit or a byte at a time. Depending on whether bits or bytes are used, this type of TDM is called bit-interleaving or byte-interleaving. Each of the three input channels has its own capacity. For the output channel to be able to accommodate all the information from the three inputs, the capacity of the output channel must be no less than the sum of the inputs. In TDM, the output timeslot is always present whether or not the TDM input has any information to transmit. TDM output can be compared to a train with 32 railroad cars. Each is owned by a different freight company and every day the train leaves with the 32 cars attached. If one of the companies has product to send, the car is loaded. If the company has nothing to send, the car remains empty, but it is still part of the train. 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. One TDM example is Integrated Services Digital Network (ISDN). ISDN basic rate (BRI) has three channels consisting of two 64 kbps B-channels (B1 and B2), and a 16 kbps D-channel. The TDM has nine timeslots, which are repeated. This allows the telco to actively manage and troubleshoot the local loop as the demarcation point occurs after the network terminating unit (NTU). Web Links Time-Division Multiplexing http://whatis.techtarget.com/ definition/0,,sid9_gci214174,00.html
Content 3.1 Serial Point-to-Point Links 3.1.3 Demarcation point The demarcation point, or "demarc" as it is commonly known, is the point in the network where the responsibility of the service provider or "telco" ends. In the United States, a telco 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 around the world, the network terminating unit (NTU) is provided and managed by the telco. This allows the telco to actively manage and troubleshoot the local loop with the demarcation point occurring after the NTU. The customer connects a customer premises equipment (CPE) device, such as a router or frame relay access device, into the NTU using a V.35 or RS-232 serial interface.
Content 3.1 Serial Point-to-Point Links 3.1.4 DTE-DCE A serial connection has a data terminal equipment (DTE) device at one end of the connection and a data communications equipment (DCE) device at the other end. The connection between the two DCEs is the WAN service provider transmission network. The CPE, which is generally a router, is the DTE. Other DTE examples could be a terminal, computer, printer, or fax machine. 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. This sequence is then signaled to the remote DTE. Many standards have been developed to allow DTEs to communicate with DCEs. The Electronics Industry Association (EIA) and the International Telecommunication Union Telecommunications Standardization Sector (ITU-T) have been most active in the development of these standards. The DTE-DCE interface for a particular standard defines the following specifications: If two DTEs must be connected together, like two computers or two routers in the lab, a special cable called a null-modem is necessary to eliminate the need for a DCE. For synchronous connections, where a clock signal is needed, either an external device or one of the DTEs must generate the clock signal. The synchronous serial port on a router is configured as DTE or DCE depending on the attached cable, which is ordered as either DTE or DCE to match the router configuration. If the port is