Which type of wireless network is used to connect devices within a city to create a metropolitan-area network

2.4.3.3 Metropolitan Area Network

MANs are larger than LANs and generally are not owned by a single organization; instead their communications links and equipment are generally owned by either a consortium of users or by a single network provider who sells the service to the users. MANs typically cover transmission ranges between 5 and 10 km in diameter and may cover an area the size of a city, or even a group of small buildings. MANs often act as a high-speed network that allows sharing of regional resources and also provide a shared connection to other networks. MANs can also mean the interconnection of several LANs by bridging them with backbone lines. Wireless metropolitan area networks (WMANs) are used to establish wireless connections between multiple locations within a metropolitan area, such as between multiple office buildings in a city or on a university campus, without the high cost of laying fiber or leasing lines. For optical wires communications, infrared light can be used to transmit data. Broadband wireless access networks can provide high-speed access to the Internet. For example, point-to-point high data rates (100 Mb/s to tens of Gbp/s) can achieved over short distances using FSO communication with infrared lasers, unlicensed requiring line-of-sight (LOS), and is affected by weather (Fig. 2.5).

Q-in-Q

Within a metropolitan area network, a given service provider may carry traffic for a corporate client that is routed between different facilities in different parts of the city. These corporate clients want their networks to appear as one large corporate network even though they span multiple locations. In order to tunnel client data through their networks, service providers can add a second, outer, VLAN tag as shown in Figure 7.2. This was standardized by the IEEE as 802.1ad and is used in many provider networks today (sometimes called Provider Bridging). Because this is an extension to the 802.1Q standard, it has become more commonly known as Q-in-Q.

In order to protect the inner VLAN tag for customer use, an outer VLAN tag is added where the 12-bit VLAN ID field is used to identify a given customer. This tag is added and removed by the access switch and is used to isolate traffic in the provider network. All forwarding and learning in the provider network can be done with the MAC addresses and outer VLAN tag. In some cases, the VLAN 3-bit priority field is also used to assign class of service in the provider network. The inner VLAN remains untouched and can be used by the customer for their own purposes. One problem with this technique is that the VLAN ID is limited to 12-bits which can contain only up to 4096 customer IDs. Because of this, it is not very useful for tunneling in large cloud data center networks. In fact, many large telecom service providers use techniques such as MPLS Transport Profile (MPLS-TP) instead; this will be described in more detail below.

3.7.1 ATM Building Blocks and DS-N Rates

Typically, there are six building blocks: the hub, bridge, router, brouter, the LAN/MAN/WAN switch, and the transport media. Numerous varieties of terminal equipment are connected to the ATM network access node, which interconnects to the ATM hub and other nodes. The configuration and the interconnection depend upon the type of application (LAN/MAN/WAN) for which the ATM is being deployed.

The interface issue is generally handled by ATM interface/adapter cards. Currently, there are four interfaces: the high-performance parallel interface, the standard ATM, the standard SONET, and DS-3. The ATM bridge handles the connection between typically Ethernet and ATM networks, thus facilitating their coexistence in the same campus or LAN. The ATM router interfaces between two ATM networks.

6.8.1 Traditional Landline Applications

The wireless technologies have exerted a steady influence in traditional landline local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs). For example, in LAN applications, spread spectrum, infrared, CT2, CT3, personal communication networks, RF identification systems, and narrow-band radio can all carry voice/data to some extent or another. The spread spectrum technologies for LANs have been delivered up to 300 kbps in hilly terrain. For time division multiple access (TDMA), burst rates up to 8 Mbps may also be sustained in indoor settings over 60–100 ft. The bit rate for most LANs is at 4, 10, or 16 Mbps, or much higher rates for optical networks. For MAN applications, a variety of networks exists at very low rates compared to fiber rates, including cellular phones, conventional radio, trunked radio, FM sideband, TV vertical blanking interval, microwave, and paging networks. For WANs, two-way mobile satellite, very small aperture terminal (VSAT), pocket radio, and meteor-burst techniques exist. Various technologies have existed for use in voice and data communications. The following sections give an overview of each of them.

Kinoteket (Cinematheque)

In 2004, in collaboration with the Norwegian Film Institute, OPL initiated the Kinoteket project to establish a video-on-demand-based movie service to present the Norwegian film heritage to library users. The project was based on a commercial pay-per-view service via the internet, originally developed by the Norwegian Film Institute and Norgesfilm (Film Archive). The two organizations handled content, copyrights, and technical solutions. Participating libraries paid a set monthly fee to gain unlimited access to the archive. For end-users, the service functioned in the style of a DVD player. The central video-server was located at the main branch and supplied the same content to the four branches via a fiber-optical network. In addition to the five viewing locations, users could access content in the reception areas of the library branches, where plasmascreens and wireless headphones were provided.

The library experimented with promoting other library materials and pushing forward the development of the City of Oslo’s Metropolitan Area Network. According to Olav Celius, project coordinator, the goal of this project was to establish a framework for handling and distributing digital media within the library, and in a broader context than the film archive. The main challenges were issues such as copyright, standards, and commercial incentives, and for these reasons Celius and his staff did not limit themselves to any particular media, format or supplier, but wanted to expand the service as new media or technologies became available. With foresight, they kept their service strictly web-based, as this was where the most interesting developments were taking place, and where the most efforts have been made to work on standards and services.3

3.5 DQDB

The Distributed Queue Dual Bus (DQDB), illustrated in Figure 3.26, is the IEEE 802.6 standard for a MAN (metropolitan area network). The figure shows the topology of DQDB. Each station is attached to two unidirectional buses. The word bus is a misnomer, because the connections in each direction are implemented by a sequence of point-to-point links instead of a bus as in Ethernet or a token bus. (There are few DQDB vendors; given the popularity of FDDI and ATM, it is unlikely that DQDB will be widely deployed. Its main interest is that the DQDB protocol is used in the subscriber-network interface for SMDS.)

The DQDB MAC protocol is a clever way to regulate access to the medium as if all stations placed their packets in a single queue that is served on a first-come, first-served (FCFS) basis. Such a first-come, first-served protocol would be the fairest possible. However, it cannot be achieved perfectly because the queues are distributed in the different stations and no station knows exactly when the other stations got packets to transmit. We will see how the DQDB protocol approximates that FCFS behavior. A station wanting to transmit to another station situated to its right must use the upper bus, and it must use the lower bus to transmit to stations on its left. The operations of the two buses are identical; how the nodes transmit on the upper bus is shown in Figure 3.27.

Fifty-three byte frames are generated back to back by the head stations. Each frame has two special control bits: the busy bit B and the request bit R. The head stations generate idle frames: the frames from the left head station have R′ = B = 0, and frames from the right head station have R = B′ = 0. When a station wants to transmit and when the protocol described below allows it to transmit, the station uses the next idle frame to transmit its own frame, setting the busy bit to 1. Each station copies each frame and retains copies addressed to itself. The final head station removes the frame from the bus.

We now explain the MAC protocol. When a station S wants to transmit on the upper bus, it must first reserve a frame. To reserve a frame on the upper bus, S waits until it sees a frame on the lower bus with its request bit R = 0. S then sets bit R = 1. When that frame propagates, the stations to the left of S learn that one station to their right has a packet to transmit on the upper bus. By counting these requests, every station can keep track of the total number of packets that stations to its right want to transmit. More precisely, when station S gets a packet to transmit, it knows how many frames have been reserved by stations to its right. S stores that number in two counters: CD (count-down) and REQ (request). The DQDB protocol specifies that the stations must defer to their right. That is, station S cannot use an idle frame that comes by on its upper bus until all the CD reservations made by stations to its right have been serviced.

In order to implement this protocol, every time S sees an idle frame (indicated by B = 0) go by on its upper bus, it decrements REQ by one; and every time S sees a reservation (indicated by R = 1) on the lower bus, it increments REQ by one. So at each time, REQ is the number of outstanding requests from stations to the right of S. When S itself gets a packet to transmit, it loads the CD counter by the current value of REQ, CD :=REQ. (This is the number of outstanding requests from stations to the right of S at the time it received a packet.) It then decrements CD by one each time an idle frame goes by on the upper bus. As soon as CD reaches zero, S knows that all the reservations to its right that were placed before it got its packet to transmit have been serviced. Station S is then allowed to use the next idle frame to transmit its own packet.

The DQDB MAC protocol is very efficient. Unlike Ethernet, there is no loss of capacity due to collision. Unlike token ring, an idle frame is continuously generated by the head station. If there always are stations with packets to transmit in both directions, utilization will be 100%. However, the protocol is not perfectly fair because its topology is not symmetric. For instance, the leftmost station must transmit all its packets on the upper bus, and it must defer to all the other stations to its right. By contrast, a station in the middle transmits half its packets on each bus and defers to only half the stations on each bus. To correct this unfairness, the standard specifies that each station be allocated an individual parameter F. F specifies the number of successive frames that the station can use to transmit. The network manager can select these parameters so that the resulting utilizations of the buses by the different stations are comparable. F is called the bandwidth balancing parameter. The IEEE 802.6 standard specifies only the MAC protocol of DQDB. The standard also provides for different traffic priorities. Priorities are implemented by having distinct B and R bits and distinct counters for different priorities.

The networks considered above are used as local area networks that connect nearby computers or as campus networks that connect computers or LANs in nearby buildings. We now describe two wide area packet-switched networks, Frame Relay and SMDS. These networks are used to connect computers or LANs across a public switched network.

Metropolitan Area Network

While most people refer to a network in terms of being either a LAN or a WAN, an additional category that exists is called a metropolitan area network (MAN). A MAN will generally cover a metropolitan area like a city, but this isn't always the case. When LANs are connected together with high-speed solutions over a territory that is relatively close together (such as several buildings in a city, region, or county), it can be considered a MAN. A MAN is a group of LANs that are internetworked within a local geographic area, which IEEE defines as being 50 km or less in diameter.

4.2.2 Applications of SCM

SCM is widely used by cable operators today for transmitting multiple analog video signals using a single optical transmitter. SCM is also being used in metropolitan-area networks to combine the signals from various users using electronic FDM followed by SCM. This reduces the cost of the network since each user does not require an optical transmitter/laser. We will study these applications further in Chapter 11.

SCM is also used to combine a control data stream along with the actual data stream. For example, most WDM systems that are deployed carry some control information about each WDM channel along with the data that is being sent. This control information has a low rate and modulates a microwave carrier that lies above the data signal bandwidth. This modulated microwave carrier is called a pilot tone. We will discuss the use of pilot tones in Chapter 8.

Often it is necessary to receive the pilot tones from all the WDM channels for monitoring purposes, but not the data. This can be easily done if the pilot tones use different microwave frequencies. If this is the case, and the combined WDM signal is photodetected, the detector output will contain an electronic FDM signal consisting of all the pilot tones from which the control information can be extracted. The information from all the data channels will overlap with one another and be lost.

1.8.4 Beyond Transmission Links to Networks

The late 1980s also witnessed the emergence of a variety of first-generation optical networks. In the data communications world, we saw the deployment of metropolitan-area networks, such as the 100 Mb/s fiber distributed data interface (FDDI), and networks to interconnect mainframe computers, such as the 200 Mb/s enterprise serial connection (ESCON). Today we are seeing the proliferation of storage networks using the Fibre Channel standard, which has data rates in the multiples of gigabits per second, for similar applications. The telecommunications world saw the beginning of the standardization and mass deployment of SONET in North America and the similar SDH network in Europe and Japan. All these networks are now widely deployed. Today it is common to have high-speed optical interfaces on a variety of other devices such as IP routers and Ethernet switches.

As these first-generation networks were being deployed in the late 1980s and early 1990s, people started thinking about innovative network architectures that would use fiber for more than just transmission. Most of the early experimental efforts were focused on optical networks for local-area network applications, but the high cost of the technology for these applications has hindered the commercial viability of such networks. Research activity on optical packet-switched networks and local-area optical networks continues today. Meanwhile, wavelength-routing networks became a major focus area for several researchers in the early 1990s as people realized the benefits of having an optical layer. Optical add/drop multiplexers and crossconnects are now available as commercial products and are beginning to be introduced into telecommunications networks, stimulated by the fact that switching and routing high-capacity connections is much more economical at the optical layer than in the electrical layer. At the same time, the optical layer is evolving to provide additional functionality, including the ability to set up and take down lightpaths across the network in a dynamic fashion, and the ability to reroute lightpaths rapidly in case of a failure in the network. A combination of these factors is resulting in the introduction of intelligent optical ring and mesh networks, which provide lightpaths on demand and incorporate built-in restoration capabilities to deal with network failures.

There was also a major effort to promote the concept of fiber to the home (FTTH) and its many variants, such as fiber to the curb (FTTC), in the late 1980s and early 1990s. The problems with this concept were the high infrastructure cost and the questionable return on investment resulting from customers' reluctance to pay for a bevy of new services such as video to the home. However, telecommunications deregulation, coupled with the increasing demand for broadband services such as Internet access and video on demand, is accelerating the deployment of such networks by the major operators today. Both telecommunications carriers and cable operators are deploying fiber deeper into the access network and closer to the end user. Large businesses requiring very high capacities are being served by fiber-based SONET/SDH or Ethernet networks, while passive optical networks are emerging as possible candidates to provide high-speed services to homes and small businesses. This is the subject of Chapter 11.

LANs, WANs, MANs, and PANs

A LAN is a Local Area Network. A LAN is a comparatively small network, typically confined to a building or an area within one. A MAN is a Metropolitan Area Network, which is typically confined to a city, a zip code, a campus, or an office park. A WAN is a Wide Area Network, typically covering cities, states, or countries.

At the other end of the spectrum, the smallest of these networks are PANs: Personal Area Networks, with a range of 100 m or much less. Low-power wireless technologies such as Bluetooth are used to create PANs.