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Russian version of abstract

Ziyad T. Wadeea
   Faculty: Of computer information technologies and automation
   Speciality: Telecommunication systems and networks
   Theme of master's work: "The Methods of Access to the Internet"
   Lider of master`s work: Victoriya Yacovlevna
  

   ziayadk@yahoo.com

Introduction to Access Networks types

An access network is a packet – switching network that provides high-speed Internet connectivity to homes. It is anticipated that access network will also provide additional services, such as voice over IP or ATM, and video on demand. Access networks have different features and requirements than LANs, MANs, and WANs. Currently, there are two different access networks  is provided over telephone line and the other over TV cable. New access networks, such as ATM passive optical network (APON), and Ethernet-based and wireless-based access networks are also beginning to emerge.All the access methods need a chore network to connect different sites, and the most popular technology used for that is the ATM network as shown in the following diagram.

1.1   Digital Subscriber Line (DSL)

After traditional modems readied their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. DSL technology is one of the most promising for supporting high-speed digital communication over the existing local loops. DSL technology is as of technologies, cach differing in the first letter (AOSL, VDSL, HDSL and SDSL). The set is often referred to as xDSL, where x can be replaced by A, V, H, or S.

ADSL

The first technology in the is asymmetrical DSL (ADSL). ADSL like a 56 k modem, provides higher speed (bit rate) in the downstream direction (from the Internet to the resident) than in the upstream direction (from the resident to the Internet). That is the reason it is called asymmetric. Unlike the asymmetry in 56 k modems, the designers of ADSL specifically divided the available bandwidth of the local loop unevenly for the residential customer. The service is not suitable for business customers who need a large bandwidth in both directions.

ADSL2 and ADSL2+

ADSL2 and ADSL2+ add new features and functionality to ADSL. They were standardized by ITU-T, they achieve higher downstream and up stream, better reach and lower latency. They also have new features like VoIP and VoATM and other features.

SDSL

ADSL provides asymmetric communication. The downstream bit rate is much higher than the upstream bit rate. Although this feature meets the needs of most residential subscribers, it is not suitable for businesses that send and receive data in large volumes in both directions. The symmetric digital subscriber line (SDSL) is designed for these types of businesses. It divides the available bandwidth equally between the downstream and upstream directions.

HDSL

The high-bit-rate digital subscriber line (HDSL) was designed as an alternative to the T-1 line (1.544 Mbps). The T-1 line uses alternate mark inversion (AMI) encoding which is very susceptible to attenuation at high frequencies. This limits the length of aT-1 line to 1 km. For longer distances, a repeater is necessary, which means increased costs.

HDSL uses 2B1Q encoding, which is less susceptible to attenuation. A data rate of almost 2Mbps can be achieved without repeaters up to a distance of 3.6 Km. HDSL uses two twisted – pair wires to achieve full-duplex transmission.

IDSL

Integrated service digital network DSL (IDSL) supports a symmetrical (ISDN) user data rata of 144 Kbps over a single wire pair. It employs the same line code as ISDN but it does not use the D-channel to set up or monitor connections. It has the advantage over conventional ISDN of providing an “always on” service (i.e. a permanent virtual circuit), thus avoiding call set up delay. It does not support simultaneous telephone service. ISDL has an inherent range of between 5 and 6 km, but this can be increased with the use of repeaters. (Other xDSL technologies cannot generally use repeaters)

SHDSL

Single pair HDSL (SHDSL, also known as G. shdsl) is described in ITU-T D. 992.2. It supports symmetric upstream and downstream data rates between 192 kbps (ISDN primary access) and 2.3 mbps over a single twisted wire pair. It shares features with both ADSL and SDSL and can transport 2.048 mbps (or 1.544 mbps) primary multiplexes, allows it to trade lower data rate for longer local loop reach, which is a significant advantage over fixed rate DSL varieties. SHDSL employs trellis coded 16-level PAM without spectral shaping, the flatter spectrum of the transmitted signal allowing local loop lengths of up to 6 km. SHDSL is likely to become the preferred global standard for symmetric DSL technology.

VDSL

The very-high-bit-rate digital subscriber line (VDSL), an alternative approach that is similar to ADSL, uses coaxial, fiber-optic, or twisted pair cable for shor distances (300 to 1800m). Although VDSL potentially supports bit rates that owe an order of magnitude greater than ADSL, it is, ironically, a cheaper technology. This is, principally, because the shorter cope is much lower. The roll-out of VDSL relies on the availability of fiber to the kerb (or some equivalent technology), however, and the investment required to provide this will be a significant barrier to its early adoption. ADSL, in contrast, relies on existing infrastructure and therefore likely to be the dominant xDSL technology in the near and medium term.

1.2   The Cable-Based Access Network

Cable companies are now competing with telephone companies for the residential customer who wants high-speed access to the Internet. DSL technology provides high-data-rate connections for residential subscribers over the local loop. However DSL uses the existing unshielded twisted –pair cable, which is very susceptible to interference. This imposes an upper limit on the data rate. Another solution is the use of the cable TV network.

HFC

Hybrid fibre coax (HFC) is the cable system used in USA, it describes a tree network in which the trunk and main branches of the tree are implemented in fibre and the sub-branches are implemented in coaxial cable, figure 3. A master head-end unit typically receives its TV signals via satellite or terrestrial microwave links and rebroadcasts them to a subscriber base of perhaps a million households. It hoses the equipment which supports pay for view services, allows the cable operator to mix in local news programs and advertising, and gives subscriber access via routers to the Internet service providers.

Euro DOCSIS

Euro DOCSIS is a Eurocentric version of the North. American standard DOCSIS and essentially represents the DOCSIS specification with a (European) DVB specification for the downstream physical layer. Figure 4 illustrates the DOCSIS reference model.

 11.3 THE ATM PASSIVE OPTICAL NETWORK

An APON is a cost-effective alternative to the telephone-based and cable-based access networks. An APON consists of an optical line terminator (OLT), an optical distribution network (ODN) and optical network units (ONU). The OLT, which resides at the premises of the APON operator, is responsible for transmitting and receiving traffic to and from the ONUs, which reside at the customer site. Also, the OLT has interfaces to a packet-switching backbone network. The OLT is connected to multiple ONUs via an optical distribution network. An APON, as its name implies, was designed with a view to carrying ATM traffic.

FTTH and FTTB

FTTH describes the case where the fibre network extends right to the subscriber's premises. This avoids any bandwidth bottleneck in the last few hundred metres to the customer. The cost per subscriber of replacing the entire existing local loop with fibre is often prohibitive, however, especially in the case of domestic customers. FTTB might typically describe the access network for a business or a block of flats where copper is used to connect all subscribers in the building to a single fibre access point.
FTTCab and FTTC

FTTCab describes the situation where fibre extends to a distribution cabinet located close to the centre of a cluster of subscribers and either coaxial cable or twisted wire pair is used for last link to the customer. This is cheaper than FTTH but does restrict bandwidth below that which would otherwise be available, even with high bandwidth copper technologies. FTTC is similar to FTTCab but the fibre reaches closer to the customer's premises and fewer (typically four) copper 'drops' are supplied from a given fibre tap.
 

2. Comparison of access network technologies

 In this part we will discuss the different types of internet access methocls unsirning the following points:

2.1 Access medium

In this part echonomy palgecl. The biggest vole, for the case of telephone companies half of the their investments is in the connections between subscriber hand sets and their local exchange, also this part of the network generates the least revenue since local calls awe often cheap or, as in the USA, free. The length of these connections is 2 km on average and they seldom exceed 7 km. Advances in signal processing have resulted in an opportunity to use this existing (and expensive) local loop infrastructure in a new way. The result is the family of digital subscriber line xDSL technologies. The twisted-pair local loop that is used in local telephone network is actually capable of handling bandwidths up to 1.1 MHz but the filter installed at the end of the line by the telephone company limits the baud wioltu to uktlz (sufficient for voice communication). This was done to allow the multiplexing of large number of voice channals. If the filter is removed, however, the entire 1.1 MHz is available for data and voice communications. For the case of cable network cable companies are now competing with telephone companies for the residential customer who wants high-speed access to the Internet. DSL technology provides high-data rate connections for residential subscribers over the local loop. However DSL uses the exiting unshielded twisted-pair cable, which is very susceptible to interference. This imposes an upper limit on the date rate. Another solution is the use of the cable TV network. The coaxial cable which is used for cable TV 75 has a bandwidth that ranges from 5 to 75 MHz (proximately). The cable company has divided this bandwidth into three bands: video, downstream data, and upstream data. The third case is the broadband passive optical network (B-PON) is an alternative to the telephone-based to carry ATM traffic. The challenge of providing this service to all subscribers should not be underestimated. With 750 million telephones worldwide it would take more than 300 years for manufacturers to produce all the enquired cable at current production rates. The Ethernet is also used as an access network which also uses twisted pair as an access medium. Wireless access networks do not have the problem of the type of cable but problems of allocating the bandwidth, noise is so also limiting these technologies.

2.2 Down-up stream vs. Reach and Latency:

For this part we will discuss the different types of access methods suparatilly:

•  ADSL: maximum downstream 8.128 Mbps and upstream 800 Kbps with a reach of 2.7432 Km. Maximum reach is 4 Km with downstream of 2 Mbps, the latency is 20 ms.

•  ADSL2: maximum downstream is 26 Mbps with 3 Mbps upstream with a reach of 304 m. Maximum reach is 2.926 Km with 12 Mbps downstream and 1 Mbps upstream, latency is 20 ms.

•   SDSL: symmetrical bat stream of about 128 Kbps – 3 Mbps with maximum reach of 3.0488 Km, latency of 10 ms 13 ms.

•    IDSL: 144 Kbps symmetrical bit stream on single wire pair line code as ISDN, no telephone service simultaneous with a reach of 5 Km – 6 Km can be increased with repeaters (other xDSL types cannot), latency is about 10ms – 15 ms.

•    SHDSL: Symmetrical bit stream 192 K – 2.3 M, maximum reach up to 6 Km. It supports multi-rate allows it to trade lower data rate for longer distances. Latency less than 1.5 ms.

•    HDSL: Supports E1 or T1 bitrates (2.048 Mbps or 1.544 Mbps) the maximum reach is approximately 4 Km.

•    VDSL: Downstream 2 Mbps upstream 12 Mbps with a reach of 300 m – 1500 m. Latency of about 1 ms.

•    Cable internet: Downstream 56 Mbps, upstream 10 Mbps. Many tens of Km in trunk network pus a few Km in distribution network, the maximum reach is 160 Km, but it is limited to 16 Km to 24 Km.

•    APON: Downstream 622/155 Mbps upstream 155 Mbps, the maximum reach is 20 Km.

•    Ethernet: symmetrical bit stream of 100 Mbps, with a maximum reach of 100 m without a repetev. Latency is about 1 ms.

Table 1 Comparison of access technologies

2.3 Different Internet application vs. different access types:

•  Web hosting: With bitrates the lateen is not that important but if we compare two links with the same speed, latency would be an important factor then. For example if we compare two access links from different types but with the same speed like ADSL and Cable internet with a speed of 128 kbps to download a webpage of 17 Kbyte we will see the following:

we see that we have 80 ms time difference and will be worse with biger web pages that have graphics.

•  File transfer: The main thing here is how fast can we down loud or uptoud a file specially when we have big files, and then the bit rate will be the main thing an example of that if we like to down loud a file of 1 Gbyte using ADSL of 128 kbps and we can see the difference as follows:

We see that there is a big difference in download time and the latency here has no importance.

•  Games over the Internet: The interactive games over the internet is very popular, the game sends only tiny amounts of data, and delivered, they awe delivered quickly, or the interactivity of the game will be sluggish. So, the main thing here is the latency and not the high bit rate of the access link.

•  Voice over IP: The minimum bit rate speed needed for this protocol is 64 kbps, but with using data compression 32 kbps will be enough speed for this protocol, but still the maximum latency over the link that VoIP can take is < 100 ms. With biger values of latency there will be an echo in the voice, which makes conversation difficult.

• Vidoe on demand: The vidoe stream needs at least 6 mbps bitrate, kut with modern data compression ways 2 mbps will be enough for that, but the latency still will nake problems in synchronizing the vidoe steam over IP packets between the source and the user, and it will also will cause a jetter. 

Table 2 Comparing Access methods for different Internet applications

References

1.Digital Communications, IAN A. GLOVER.

2.Data Communications and Networking, BEHROUZ A. FOROUZAN.

3.Connection-Oriented Networks-SONET, SDH, ATM, MPLS and Optical Networks, HARRY G. PERROS.

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