24-07-2012, 04:22 PM
Broadband
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Abstract
Broadband in telecommunications is a term which refers to a signaling method which includes or handles a relatively wide range of frequencies which may be divided into channels or frequency bins. The wider the bandwidth, greater is the information carrying capacity. In radio, for example, a very narrow-band signal will carry Morse code; a broader band will carry speech; a still broader band is required to carry music without losing the high audio frequencies required for realistic sound reproduction. A television antenna described as "normal" may be capable of receiving a certain range of channels; one described as "broadband" will receive more channels. In data communications a modem will transmit a bandwidth of 64 kilobits per seconds (kbit/s) over a telephone line; over the same telephone line a bandwidth of several megabits per second can be handled by ADSL, which is described as broadband
Keywords—
DSL - Digital Subscriber Line
DSLAM - Digital Subscriber Line Access Multiplexer
IP - Internet Protocol
SDSL - Symmetric Digital Subscriber Line
ADSL - Asymmetric Digital Subscriber Line
I. INTRODUCTION
Broadband in data communications may have the same meaning as above, so that data transmission over a fiber optic cable would be referred to as broadband as compared to a telephone modem operating at 600 bits per second.
However, broadband in data communications is frequently used in a more technical sense to refer to data transmission where multiple pieces of data are sent simultaneously to increase the effective rate of transmission, regardless of actual data rate. In network engineering this term is used for methods where two or more signals share a medium.
Various forms of Digital Subscriber Line (DSL) services are broadband in the sense that digital information is sent over one channel and voice over another channel sharing a single pair of wires. Analog modems operating at speeds greater than 600 bit/s are technically broadband. They obtain higher effective transmission rates by using multiple channels with the rate on each channel limited to 600 baud. For example, a 2400 bit/s modem uses four 600 baud channels (see baud). This is in contrast to a baseband transmission where one type of signal uses a medium's full bandwidth such as 100BASE-T Ethernet.
Ethernet, however, is the common user interface even to DSL data links. Ethernet provisioned over cable modem often is a competitive alternative to DSL, especially in the small office/home office market.
Users who need more than DSL or cable modem speeds will often use metro Ethernet, when available, rather than older and often more expensive (per megabit) than T-carrier (or E-carrier in appropriate parts of the world, or Asynchronous Transfer Mode. Metro Ethernet is usually implemented over a metropolitan all-optical network.
II. Broadband working
Broadband utilizes various techniques and components to work properly. However, broadband in data communications is frequently used in a more technical sense to refer to data transmission where multiple pieces of data are sent simultaneously to increase the effective rate of transmission, regardless of data signaling rate. In network engineering this term is used for methods where two or more signals share a medium.
A. Multiplexing
Communications may utilize a number of distinct physical channels simultaneously; this is multiplexing for multiple access. Such channels may be distinguished by being separated from each other in time (time division multiplexing or TDMA), in carrier frequency (frequency division multiplexing (FDMA) or wavelength division multiplexing (WDM)), or in access method (code division multiple access or CDMA). Each channel that takes part in such a multiplexing exercise is by definition narrowband (because it is not utilizing the whole bandwidth of the medium), whereas the whole set of channels taken together and utilized for the same communication could be described as broadband.
B.Digital Subscriber Line Access Multiplexer
A Digital Subscriber Line Access Multiplexer (DSLAM) allows telephone lines to make faster connections to the Internet. It is a network device, located near the customer's location, that connects multiple customer Digital Subscriber Line (DSL) s to a high-speed Internet backbone line using multiplexing techniques. By locating DSLAMs at locations remote to the telephone company central office (CO), telcos are now providing DSL service to consumers who previously did not live close enough for the technology to work.
• Residential/commercial source: DSL modem plugged into the customer's computer.
• Local loop: the telephone company wires from a customer to the telephone company's central office, often called the "last mile".
• Main Distribution Frame (MDF): a wiring rack that connects outside subscriber lines with internal lines. It is used to connect public or private lines coming into the building to internal networks. In a telco CO, the MDF is generally in proximity to the cable vault and not far from the telephone switch.
• DSLAM: a device for DSL service. Sending on the customer or downstream side, it intermixes voice traffic and VDSL traffic onto the customer's DSL line. Receiving on that side, it accepts and separates outgoing phone and data signals from the customer. It directs the data signals upstream towards the appropriate carrier's network, and the phone signals towards the voice switch.
• From the DSLAM the telephone wires, now cleansed of DSL signals, go through the MDF again to the voice switch so the customer will have dial tone phone service. Old-fashioned voice signals pass between voice switch and subscriber line through DSLAM, which does not disturb them but adds a higher frequency signal to carry data for Internet service.
Role of the DSLAM
The DSLAM at the CO collects the digital signals from its many modem ports and combines them into one signal, via multiplexing.
Depending on the product, DSLAMs connect DSL lines with some combination of Asynchronous Transfer Mode (ATM), frame relay or Internet Protocol networks.
In terms of the OSI 7 Layer Model, the DSLAM acts like a massive network switch, since its functionality is purely Layer 2.
The aggregated signal then loads onto backbone switching equipment, traveling through an access network (AN) — also known as a Network Service Provider (NSP) — at speeds of up to 10 Gbit/s and connecting to the Internet-backbone.
The DSLAM, functioning as a switch, collects the ADSL modem data (connected to it via twisted or non-twisted pair copper wire) and multiplexes this data via the gigabit link that physically plugs into the DSLAM itself, into the Telco's backbone.
DSLAM is not always located in the telco central office, but may also serve customers within a neighborhood Serving Area Interface (SAI), sometimes in association with a digital loop carrier. DSLAMs are also used by hotels, lodges, golfing estates, residential neighbourhoods and other corporations setting up their own private telephone exchange.
Besides being a data switch and multiplexer, DSLAM is also a large number of modems, each modem on the aggregation card communicating with a subscriber's DSL modem. This modem function being inside the DSLAM rather than separate hardware, and being wideband rather than voice band, it isn't often called a modem. Like voice band modems of standard v.32 and later, it has the ability to probe the line and train itself to compensate for echoes and other impairments, in order to move data at the maximum rate the line allows. This is also why twisted pair DSL services have a longer range than twisted pair (UTP) Ethernet.
1. Speed vs Distance
Balanced pair cable has higher attenuation at higher frequencies, hence the longer the wire between DSLAM and subscriber, the slower the maximum possible data rate. The following is a rough guide to the relation between wire distance and maximum data rate. Local conditions may vary, especially beyond 2 km, often necessitating a closer DSLAM to bring acceptable speeds:
• 25 Mbit/s at 1000 feet ~300 m
• 24 Mbit/s at 2000 feet ~600 m
• 23 Mbit/s at 3000 feet ~900 m
• 22 Mbit/s at 4000 feet ~1.2 km
• 21 Mbit/s at 5000 feet ~1.5 km
• 19 Mbit/s at 6000 feet ~1.8 km
• 16 Mbit/s at 15,000 feet~ 4.5 km
Additional features
A DSLAM may offer the ability to tag VLAN traffic as it passes from the subscribers to upstream routers. Though not a full stateful firewall, some DSLAMs also offer packet filtering facilities like dropping inter-port traffic and dropping certain protocols.
The DSLAM also supports quality of service (QoS) features like contention, differentiated services ("DiffServ") and priority queues.
Hardware details
Customers connect to the DSLAM through ADSL modems or DSL routers, which are connected to the PSTN network via typical unshielded twisted pair telephone lines. Each DSLAM has multiple aggregation cards, and each such card can have multiple ports to which the customers lines are connected. Typically a single DSLAM aggregation card has 24 ports, but this number can vary with each manufacturer. The most common DSLAMs are housed in a telco-grade chassis, which are supplied with (nominal) 48 volts DC. Hence a typical DSLAM setup may contain power converters, DSLAM chassis, aggregation cards, cabling, and upstream links. The most common upstream links in these DSLAMs use gigabit Ethernet or multi-l gigabit fiber optic links.