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6. A number of OBS end devices are connected to a single OBS node in a star configuration. That is, there is a fiber from each end device to the OBS node, and from the OBS node there is a fiber to each end device. Assume that each fiber carries a single wavelength and that the OBS node has full conversion. Describe a simple scheduling algorithm that guarantees zero burst loss.
7. In Jumpstart, why are there two forwarding tables in a JITPAC?
8. In Jumpstart, the forwarding of a burst or a control message can be done by using either the next hop information (as in the IP network) or labels (as in the MPLS scheme). Explain how the scheme using MPLS labels can be implemented.
An access network is a packet-switching network that provides high-speed Internet connectivity to homes. It is anticipated that access networks 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 in place; one is provided over the telephone line and the other over the TV cable. New access networks, such as the ATM passive optical network (APON), and Ethernet-based and wireless-based access networks are also beginning to emerge.
Telephone operators provide high-speed access to the Internet over the telephone line in addition to basic telephone services. Video on demand and voice over IP or ATM will also be provided in the future. A family of modems known as x-type digital subscriber line (xDSL) has been developed to provide high-speed access to the Internet over the telephone line. Of the xDSL modems, the asymmetric DSL (ADSL) is the most popular one.
Cable operators provide access to the Internet over their cable network in addition to the distribution of TV channels. In addition, voice over IP and video on demand services over the cable have been introduced recently. The cable-based access network uses the data-over-cable service interface specification (DOCSIS).
APON is a cost-effective alternative to the telephone-based and cable-based access networks. An APON uses an optical distribution network, which consists of optical fibers and passive splitters. It can be used to provide high-speed Internet connection, voice over IP, voice over ATM, and video on demand services.
In this chapter, we describe the ADSL-based access network, the cable-based access network, and the APON. The ADSL-based access network and the APON have been designed to support ATM and consequently they are connection-oriented networks. The cable-based access network supports the IP network. Although the cable-based access network is not connection-oriented, it has been included in this chapter due to its importance in the access network market.
11.1 THE ADSL-BASED ACCESS NETWORKS
ADSL is one of the access technologies that can be used to convert the telephone line into a high-speed digital link. It is part of a family of technologies called the x-type digital subscriber line (xDSL), where x stands for one of several letters of the alphabet and it indicates a different transmission technique. Examples of the xDSL family technologies
Connection-oriented Networks Harry Perros © 2005 John Wiley & Sons, Ltd ISBN: 0-470-02163-2
are: asymmetric DSL (ADSL), high data rate DSL (HDSL), symmetric DSL (SDSL), ISDN DSL (IDSL), and very high data rate DSL (VDSL). Some of the xDSL technologies use analog signaling methods to transport analog or digital information over the twisted pair, while others use true digital signaling to transport digital information. A list of specifications for the xDSL family technologies is given in Table 11.1. In access networks, downstream means from the network to the user, and upstream means from the user to the network. These specifications are likely to change as the technology evolves.
The very high data rate DSL (VDSL), as its name implies, achieves very high data rates over the twisted pair. However, the distance over which such rates can be transported is limited. Currently, it can achieve a downstream data rate of 52 Mbps and an upstream data rate of 6 Mbps over a distance of up to 1000 feet. For the same distance, it can also provide symmetric rates of 26 Mbps downstream and 26 Mbps upstream. The longest distance it can be transported is currently 5000 feet, for which it can achieve 13 Mbps downstream and 1.6 Mbps upstream. VDSL can be used to deliver high quality video together with access to the Internet and regular telephone services. Because of the distance limitation, it is envisioned that it will be used to deliver information from a cabinet in the street which is connected to an APON.
The asymmetric digital subscriber line (ADSL) technology utilizes the existing twisted pair from the central office to the home to transport data in addition to the basic telephone services. It was originally designed to provide video on demand services transported over switched DS1 or E1 links. This type of traffic is referred to in the ADSL standard as the synchronous transfer mode (STM) traffic. In its current standard (ITU-T G.992.1) full rate ADSL has been defined to carry either ATM or STM traffic or both. ADSL is primarily used for ATM traffic, and there is a limited number of applications for STM traffic.