- Remote access technologies are normally lower cost home-based Internet access if compared to corporate Internet access (eg: leased lines, Frame Relay, ATM). Most remote access technologies use the PSTN for basic physical access, eg: modems, ISDN, and DSL.
- Public Switched Telephone Network (PSTN) was built to support telephones voice traffic.
Microphones were being used to convert sound waves into analog electrical signals, and being transported over the PSTN network to the speaker of the telephone at the other end. A circuit must be established prior to communication.
- The core of the PSTN has evolved to use digital signals instead of analog signals, which results in higher bandwidth and lower calling cost. Analog-to-digital (A/D) conversion is normally performed by voice switches located at the Telco’s Central Offices (COs).
- Codec (Encoder + Decoder) is an A/D converter (ADC) that is being used for processing voices.
Figure 21-3: Analog Voice Calls over a Digital PSTN
- The original standard for the conversion of analog voice signals to digital signals is Pulse-Code Modulation (PCM), which defines that an analog voice signal should be sampled 8000 times per second by an A/D converter. Sampling rate is the rate at which the samples of a specific waveform amplitude are collected within a specific period of time (eg: one second).
- The PCM standard defines the use of 256 different binary values per sample (to represent different combination of amplitude, frequency, and phase). 256 values = 8 bits. 1 second = 8 x 8000 = 64kbps. This is the basic transmission speed that represents the necessary bandwidth for a single PCM-encoded voice call. Due of this specification, the first digital PSTN was built on a speed of 64kbps, and a single 64kbps channel was denoted as the Digital Signal Level 0 (DS0).
Analog Modems
- They allow 2 computers to communicate with digital signals, but without any physical changes on the local loops and Telco’s Cos (transmit analog signals). A modem converts or encodes the digital signals from a computer to analog signals, and sends the analog signals to the PSTN.
Another modem at the other end decodes the analog signals received from the PSTN back to digital signals, and sends the digital signals to the computer at the other end.
- Modems are used in analog lines while CSUs/DSUs are used in digital lines.
Phone 1 Telco Phone 2
Voice Switch
@ CO
Local Loop (Analog)
Local Loop (Analog) T1 Line
(Digital)
PSTN
PCM Codec (A/D conversion)
PCM Codec (A/D conversion)
Telco Voice Switch
@ CO
- Modem Modulation + Demodulation.
Modulation is the process of encoding a set of binary digits into an analog electrical signal.
Demodulation is the process of decoding an analog electrical signal into a set of binary digits.
- During the establishment of a modem dial-up circuit, a modem signals the tones associated with the telephone keypad. These tones, which are generated by phones as well, are interpreted as dual-tone multi-frequency (DTMF) tones by a CO switch. Once the circuit is established by the Telco, the modems at both ends must agree the modem standard to use for communication.
There are many modem standards and most modems support several standards. Modems are able to probe, negotiate, and use the best standard that is supported by the modems at both ends.
Figure 21-4: Modulation Schemes - Modulation scheme with combination of AM and FM:
Amplitude Frequency Binary Code
Low Low 00
Low High 01
High Low 10
High High 11
- Amplitude modulation (AM) represents info by varying the amplitude of the carrier signal.
Frequency modulation (FM) represents info by varying the frequency of the carrier signal.
- Baud is a measure of the number of symbol changes or signaling events per second. A baud is a single encoded analog energy that can represent 1 or more bits. To archive higher bit rates, modems use certain modulation techniques to encode more than 1 bit in the analog signal.
With the AM + FM combined modulation scheme in Figure 21-4, a modem which is running at 14000 baud per second can transmit data at 28000bps.
- Most computers use PPP as the data link (L2) protocol over the L1service provided by modems.
PPP is the good choice as modems traffic is transmitted asynchronously, while PPP support both synchronous and asynchronous communications.
- Other reasons why PPP is being used in modem communications are:
i) PPP provides the capability to dynamically assign an IP address for a device connected to the other end of a PPP link.
ii) PPP supports authentication (via PAP and CHAP), which is commonly being used to authenticate dial-in users for the access to the ISP network.
- V.92 is the current modem specification defined by International Telecommunications Union (ITU). It transmits at 56kbps (downstream) and 33kbps (upstream), which is same as V.90 but offers a reduced handshake time (faster connection setup), increased compression throughput, and the on-hold feature.
Amplitude Modulation Frequency Modulation Phase Modulation AM + FM Combined Volume
LowHighLow High Low High Low Phase Shifts
Time
0 1 0 1 0 1 1 0 0 1 0 1 00 01 10 11
Digital Subscriber Lines (DSL)
- DSL just defines how to transmit data between a customer site and the local CO. Data do not flow through DS0 channels inside the PSTN, but through some ISP IP networks. It does not need to be compatible with the core of PSTN and hence is able to provide higher transmission rates.
The speed does not degrade when more users are added to the network.
Figure 21-5: DSL Connection
- The local loop connects to the DSL Access Multiplexer (DSLAM) at the local CO, which is responsible for splitting the data and voice signals from the local loop.
- DSL allows concurrent data transmission and voice call, as it doesn’t use the voice frequencies.
Phones generate analog signals at 0–4000Hz, while DSL modems generate analog signals higher than 4000Hz – the interference is minimal. Filters are often being used to prevent interference. - DSL is considered an always-on Internet connection service. It does not require a circuit to be
setup before data transmission as with modems and ISDN.
- Its downstream speeds range from 1.5–8Mbps, while upstream speeds range from 64–800kbps.
- The ADSL maximum supported distance to home is 18000 feet (or ~5500 meters).
- Other DSL standards or variants include HDSL (High bit rate DSL), IDSL (ISDN DSL), MSDSL (Multi-rate Symmetric DSL), PDSL (Powerline DSL), RADSL (Rate-Adaptive DSL), SDSL (Symmetric DSL), SHDSL (Single-Pair High-Speed DSL), UDSL (Uni-DSL), and VDSL (Very High Speed DSL). Annex A is referred to as ADSL over PSTN (UK) while Annex B is referred to as ADSL over ISDN (Germany).
- DSL routers usually include the DSL modem features, and other networking features, eg: IP
Analog Phone
DSL Modem or
Router PC
Local Loop L2 – ATM & PPP L1 – DSL Ethernet
DSLAM ISP IP
Router
Voice Switch
IP Network
PSTN
House / Office
Local CO DSL Low-Pass Filter
(LPF)
- DSL is a L1 service. It uses ATM as the L2 protocol for the communication between the DSL modem (or router) and the ISP router. DSL uses PPP over ATM (PPPoA) protocol to encapsulate PPP frames in the ATM AAL5 (ATM Adaptation Layer 5) cells. DSL transmit ATM cells over DSL, instead of SONET. ATM cells are received and processed by the ISP router.
PPP and ATM are both L2 protocols, but they provide different functions in DSL connections.
PPP provides features such as address assignment, authentication, encryption, and compression.
- A DSL modem (or router) uses PPP over Ethernet (PPPoE) when performing bridging function for a PC and a DSLAM when the PC connects to the DSL network using a PPPoE client.
The WAN IP address is dynamically assigned to the PC instead of the DSL router.
Cable Modems
- This remote access technology does not use a phone line for physical connectivity. It transmits
computer data in digital signals over cable TV coaxial cabling (analog). A splitter called f-connector, is being used to split data received from drop cable to multiple cable TVs and the
cable modem in a subscriber house.
- Cable Modem Termination System (CMTS) is deployed at various concentration points or hubs in the cable network to provide high-speed Internet access, voice, and other networking services to home and business subscribers. The Cisco uBR (Universal Broadband Router) CMTS Series is designed for deployments at MTUs (multi-tenant units), eg: apartments and hotels.
- The speed degrades as more subscribers are added to the network, as the bandwidth is shared among subscribers.
- Downstream data referred to as the data going toward the home.
Upstream data referred to as the data going outward the home.
- Cable networks broadcast downstream traffic (logical bus networks). Security is a main concern.
- CATV (Community Antenna Television) standards use Multimedia Cable Network System (MCNS) as the data link protocol for arbitration, addressing, and error detection.
- As the cables could be very long, the CSMA/CD algorithm used in Ethernet is not effective to detect collisions in cable networks. MCNS defines some multiplexing methods – Time-Division Multi Access (TDMA), where home users are allocated time slots per second to send upstream data, and Frequency-Division Multiplexing (FDM) to receive downstream data.
FDM is a form of signal multiplexing where multiple baseband signals are modulated at different frequency and bundled as a composite signal that could be transmit over a single physical circuit.
FDM was normally used in traditional analog telephone networks. Modern telephone networks that employ digital transmission use Time-Division Multiplexing (TDM) instead of FDM.
- Multiplexing is the process of converting multiple logical signals into a single physical signal for transmission across a physical channel.
- MCNS defines the following encoding schemes (QAM = Quadrature Amplitude Modulation):
i) QAM-64 6 bits per baud.
ii) QAM-256 8 bits per baud.
- MISC Notes: Analog modems and DSL support both symmetric and asymmetric speeds; while ISDN, Frame Relay, and cable modems can only run across synchronous links or circuits.