Digital Interface Standard• Interface ~ the boundary between two pieces of equipment. (a point at which one device connects to another device).• Interface also refers to mechanical, electrical, and functional characteristics of the connection.• There are 2 kinds of interfaces collaborated in the data communications: – Data terminal equipment (DTE) – Data communications equipment (DCE)
Data Terminal Equipment (DTE)• Any device that is a source of or destination for binary digital data.• It can be a terminal, PC, printer, fax, or any other device that generates or consumes digital data.• DTEs do not often communicate directly with one another, they generate and consume information but need an intermediary to be able to communicate.
DTE (cont…)• You have an idea that you want to communicate with your friend. – Brain creates idea but cannot transmit that idea – Brain passes the idea to vocal chord & mouth, which converts to sound waves that can travel through air or to your friend’s ear and from there to his/her brain. – Brains = DTEs ; vocal chords, mouth, ear = DCEs ; air = transmission medium.
DCE• Data communications equipment• Also known as data circuit-terminating equipment.• Any device that transmits or receives data in the form of an analog or digital signal through a network.• DCE takes data generates by a DTE, convert them to an appropriate signal, and then introduces the signal onto the communications link.• I.e. modems
Modem The most familiar type of DCE. Stands for modulator/demodulator. Modulator ~ converts a digital signal into an analog signal Demodulator ~ converts analog signal into a digital signal. Demodulator resembles an analog-to-digital converter (codec), but it does not sample a signal to create a digital.
Bell Modem Standards– 103/113 series– 202 series– 212 series– 201 series– 208 series– 209 series
ITU-T Modem Standards• V.21• V.22bis (2nd generation of V.22 series) ~ two-speed modem (1200 bps or 2400 bps)• V.32 ~ enhanced version of V.29 (9600 bps)• V.32bis ~ (14.4 Kbps / 64-QAM)• V.32terbo ~ (19.2 Kbps / 256 QAM)• V.33 ~ enhanced version of V.32• V.34 ~ (28.8 Kbps / 33.6 Kbps)• V.42• V.42bis
Intelligent/Smart Modems• The purpose of a modem is to modulate and demodulate a signal.• Today’s modem, do more than that.• Intelligent/smart modems contain software to support a number of additional functions such as automatic answering and dialing.
Multispeed Modems• Bell 212 Series or ITU-T V.22 and V.22bis• I.e: Bell 212A modem begins communication with another modem, it tries to communicate using the 1200 bps, and if unsuccessful, it assumes that it has reached a slower modem and tries to communicate using the 300 bps.• This capability allowed users to continue to communicate with older 300 bps.
Error-Correcting Modems• Most modems operating at speed 2400 bps and above have some type of error-correction capability.• 2 approaches to correct errors between modems: – Microcom Network Protocol (MNP) ~ commercially licensed error-checking protocol – Link Access Procedure for Modems (LAPM) ~ V.42 ~ international standard for error correction
Data Compression in Modems• Microcom Network Protocol version 5 (MNP5) ~ can compress data to half its size under ideal conditions.• V.42bis ~ can theoretically compress data to one- fourth its original size under ideal conditions.
Short-Haul Modems• Also called limited distance modems• It can transmit data over twisted pair wire for a few miles comparing to the dial-up modem which can transmit around the world (with the help of public telephone network).• Provides high data rate and no monthly phone line cost.
DTE - DCE• There are usually four basic functional units involved in the communication of data: – DTE and DCE on one end and DCE and DTE on one end. – DTE generates the data and passes them, along with any necessary control characters, to DCE – DCE converts the signal to a format appropriate to the transmission and introduces it onto the network link – When the signal arrives at the receiving end, this process is reversed.
DTEs and DCEs
Remote Access Server (RAS)
Definition• The remote access server (RAS) platform is the foundation on which Internet service providers (ISP) and other telecommunications carriers are bringing to market integrated.• Internet-based versions of traditional services such as voice over IP (VoIP), fax over IP (FoIP), and data over IP.• As the numbers of Internet users grows and as networking technology advances, these services are being successfully marketed as alternatives to those services offered by traditional providers.• In addition to voice and fax for consumers, small offices, and telecommuters, these services include virtual private networks (VPN) for enterprises with many mobile employees or small branch offices.
RAS• Major network-equipment providers today offer a wide range of RAS systems, including some that are touted as VoIP– enabled.• This equipment is in use by service providers of all sizes as well as enterprise IT departments offering remote access to employees.• While technically speaking some of today’s RAS platforms support VoIP, the truth is these first-generation VoIP–enabled RAS platforms are highly inefficient implementations lacking important software features and often require separate hardware modules within an equipment rack to support modem, voice, and fax services.• This requires a priori allocation of hardware resources based on anticipated service needs. As a result, this equipment does not offer an optimum solution with regard to cost and features offered.
Figure 2. Remote Access Server (RAS)Figure 2 illustrates the use of a remote access server (RAS; also called aremote access concentrator) to terminate nearly every type of fax, modem,and voice call, and prepare it for transmission over a packet-switchednetwork.
Limitation of RAS equipment• There are three significant limitations of current RAS equipment: – channel density, – power consumption, and – processing capability.• The combination of these three shortcomings will soon render older RAS equipment useless for many of the newer applications mentioned earlier, including VoIP, fax relay, VPN, and access wholesaling.