Aims. The aims of this unit are to:

Transcription

1 Aims The aims of this unit are to: Outlines the usage of mobile networks and how these may integrate into the Internet. Understand the key elements of GSM networking. Understands the key parameters used in wireless networking. Unit 13: Mobile Networks 189

2 Introduction The structure of networks and the Internet is changing as more devices connect to it in different ways. At one time it was basically a collection of workstations connected over networks. Networks were then structured in a fixed way, where cabling and the location of networking devices defined the layout of networks. Thus, networks have evolved in a rather random way, as was intended when the Internet was created. Figure 10.1 shows how networks have generally evolved. Initially they were created in a centralised way, where a central processor routed traffic between nodes. As with a star network, this type of topology causes problems when the central node (in this case, node E) become inoperatable, or is overburdened. Thus networks become decentralised, where extra routes were added so that traffic could be taken away from the central part of the network. In the example in Figure 10.2, the decentralised network allows nodes G, F and D to intercommunicate without requiring to go through E. This network reduces the dependency on the central part, but it is still required to connect disjoint parts of the network. If this was a military network, then it can be seen that node E is still a weak point of the whole network. Thus DARPA created a distributed model for the Internet, where the complete network could withstand a military strike on any part of the Internet, and it would not affect the connectivity of the whole network. The Internet has thus evolved into a totally distributed system, where a fault in any of the connections does not stop any of the other nodes from intercommunicating. Obviously as the network size increases in a distributed system the number of connections required increase. It has been shown, though, that a robust and reliable network requires only between 3 or 4 connections from each node. F F F F A A EE DD AA EE DD BB B B CC CC Centralised Decentralised F F F F A A EE DD A A EE DD B B B B CC CC Distributed More fully distributed Figure 10.1 From centralisation to distributed 190 Computer Networks CO33006

3 As the Internet has increased its size at an amazing rate, it would have been impossible to create an Internet with multiple connects to billions of nodes, as the routing between nodes would have been too complex. Thus, the Internet now uses layered approach where the network is split into a number of Autonomous Systems (ASs). At the top level of the Internet, routers simply route between ASs, as illustrated in Figure We now thus have a model which is distributed and decentralised. The core of the Internet is similar to the core level of the three-layered model presented in Unit 4. It can be seen that there could not be multiple connections between ASs, thus some of the ASs must allow traffic to be routed through themselves. It can be seen in Figure 10.2 that a node connected to the BT AS would require routing through the Layer 3 AS in order to communicate with a node which is connected to the US military AS. A special routing protocol, named an exterior routing protocol, is used to define the route that data packets take between ASs. The evolution of ASs is only part of the future, as the next major change will come in the connection of other devices, especially mobile ones. It is thus possible, with IP Version 6 and NAT, to allow many computers to break their physical links to a network, and expand the number of devices which connect to the Internet. It will also be possible for many non-computing devices, such as engine management systems, and central heating systems to be assigned network addresses, and be connected to the Internet. The mobile devices will typically connect through intermittent and lower-speed connections, which will be pushed towards the peripheral areas of the Internet. Figure 10.3 illustrates this where the core of the Internet runs at speeds of many tens Gbps (using OC- and STM- data streams), while organisation backbones will run at several Gbps (using technologies such as Gigabit Ethernet). Most of the user devices which connect to organisational networks will form the next layer, where 10/100Mbps communication gives the required network bandwidth to run networked applications. The next two layers will be made by mobile and wireless networks, where lower speeds are typically used. This is likely to involve GPRS devices which have permanent connections to the Internet, but have lower transmission rates than workstation devices. Mobile devices have evolved over several iterations. For mobile phones,the main generations have been: First generation (1G). First generation mobile phones (1G) had very low transmission rates (typically just a few KB/s), Second generation (2G and 2.5G). These are devices improved this to give several hundred KB/s. Third generation (3G). These devices give almost workstation network bandwidths (several MBps), which allows for full multimedia transmissions. The Internet is thus becoming an exciting and flexible place, where connections depend less on physical connection, and more on mobile ones. The applications of the Internet will thus increase as more devices connect in this way. Unit 13: Mobile Networks 191

4 AOL customers AS AS --AOL AOL AS AS Layer3 Layer3 Napier Napier HW HW UMIST UMIST AS AS -- SuperJanet AS AS --BT BT Internet core AS AS US US Military Military Possible routing through an AS The Internet Figure 10.2 Internet based around ASs Low/medium speed mobile Devices (<1Mbps) Host Connection (1Mbps-10Mbps) High-speed mobile access (GPRS) Organisational connection (10/100Mbps) Organisational backbone (>1Gbps) Internet Core/Backbone (>10Gbps) Figure 10.3 The evolving Internet At the core of the Internet is Synchronous Digital Hierarchy (SDH - in Europe) or SONET (in the USA). SDH defines the data rate and the standard for communication over fibre optic cable. The basic rate of SDH is known at STM-1 (Synchronous Transport Mode - 1) which gives a data rate of Mbps. SONET uses OC-x, which stands for Optical Carrier, and is defined in a series of physical protocols (OC-1, OC-2, OC-3, and so on). The base rate is Mbps (OC- 1); each signal level thereafter operates at a speed divisible by that number (thus, OC-3 runs at Mbps). A summary is: 192 Computer Networks CO33006

5 OC-standard SDH Equivalent Mbps OC OC-3 STM OC-9 STM OC-12 STM OC-18 STM OC-36 STM OC-48 STM OC-96 STM OC-192 STM GSM networking A key element of a mobile network is the availability of antennas. One of the most widely established mobile networks is GSM. This network links mobile phones directly on the telephone network. GSM uses a cellular approach where the phone connects to an antenna which gives the strongest signal strength. The phone can then move and can reconnect to other antennas as the signal strength varies, as Figure The advantage of the GSM network is that it is widely available, and covers most of the areas of the world. It has many disadvantages, such as being costly, especially as it is charged for the time of the connection, rather than for the amount of data sent and received. POTS POTS (Plain (Plain Old Old Telephone Telephone System) System) GSM GSM network GSM GSM gateway Internet Figure 10.4 Connections to the cellular network Unit 13: Mobile Networks 193

6 Wireless Networking Wireless networks use high-frequency radio waves to transmit from node to node. The 11Mbps IEEE b standard has been designed so that nodes can connect using a peer-to-peer connection (known as an ad-hoc connection) or connect to a wireless hub (known as an infrastructure connection). The bit rate is equivalent to a base-rate Ethernet connection, and is thus able to easily integrate with existing Ethernet networks. The advantages of IEEE b include: Integrates well with existing Ethernet networks. Is supported by most network operating systems. Provides a range of up to 800 feet, in an open environment. Provides increased mobility. Reduces the cost of wiring. Supports 1, 2, 5.5 and 11 Mbps bit rates. Supports either a point-to-point (ad-hoc) and point-to-multipoint (infrastructure) access. Supports Plug and Play, and is easy to install. Uses strong encryption using WEP encryption (64-bit and 128-bit) Uses Direct Sequence Spread Spectrum (DSSS) which is a robust, interferenceresistant and secure wireless connection. The applications of wireless technology is likely to increase over the forthcoming year, especially with the increasing processing power of mobile devices, but typical applications include: Environments which have frequent changes, such as in a retail environment, or in workplaces which are continually rearranged. High security networks. Ethernet has suffered from security problems, thus wireless networks with encryption can overcome this. Providing remote access for a corporate network. Providing temporary LANs which could be used for special projects. Remote access to databases in mobile applications, such as for medical practitioners, or office staff. Supporting networks in environments where cable runs are difficult, such as in old buildings, hazardous areas, and in open spaces. Support for users who use SOHO (Small Office and Home Office), as it provides a quick access to networks. Basic specification IEEE b uses a number of channels in frequency range around 2.4 GHz to 2.45 GHz. This high frequency allows the radio wave to propagate fairly well through building and air. At 11Mbps, the maximum range is around 140 meters, but this reduces when there are obstacles in the way. At 1Mbps, the range increases to 400 meters. The frequencies are split into a number of channels. In Northern America, there are Computer Networks CO33006

7 channels, in Japan, there are 14, and in Europe, there are 13 channels (as shown in Figure 10.5). Operating Channels: 11 for N. America, 14 Japan, 13 Europe (ETSI), 2 Spain, 4 France Operating Frequency: GHz (North America), GHz (Japan), GHz (Europe ETSI), GHz (Spain), GHz (France) Data Rate: 1, 2, 5.5 or 11Mbps Media Access Protocol: CSMA/CA, Compliant Range: 11Mbps: 140m (460 feet) 5.5Mbps: 200m (656 feet) 2Mbps: 270m (885 feet) 1Mbps: 400m (1311 feet) RF Technology: Direct Sequence Spread Spectrum Modulation: CCK (11Mps, 5.5Mbps), DQPSK (2Mbps), DBPSK (1Mbps) Output Power: 13 dbm Sensitivity: 11Mbps < -83 dbm 5.5Mbps < -86dBm 2Mbps < -89dBm 1Mbps < -91dBm The wireless adapter will typically connect to a node using one of a number of ways, such as through the USB port, PCI card, PCMCIA card, and so on. The wireless protocol corresponds to a network adapter, and can thus support most higher layer protocols, such as TCP/IP, NetBEUI, and IPX/SPX, as shown in Figure Figure 10.5 IEEE channel setting for Europe Unit 13: Mobile Networks 195

8 Figure 10.6 Setting for protocols and network Wireless network connections IEEE b can be either connected as an infrastructure network or as an ad-hoc network. Figure 10.7 shows an infrastructure network where the wireless nodes connect to an access point. The access point defines the domain of the wireless network. These domains can then interconnect through an Ethernet backbone. Figure 10.8 shows the usage of SSID, which is a unique ID given to the access point. All the clients which connect to a certain access point must define the correct name (otherwise they may connect to another access point). If the access point ID is not known then ANY can be used (although this is not recommended for security reasons). An ad-hoc network uses channels to define different networks, as illustrated in Figure In this example, LAN 1 uses channel 3, and LAN 2 uses channel 7. In Europe, for example, it would be possible to create up to 13 different ad-hoc networks, within a certain range (between 100 and 400 meters, depending on the environment, and the bit rate). If an ad-hoc network has a range of L meters, then an infrastructure network will have a diameter range of 2L, as illustrated in Figure Computer Networks CO33006

9 Server Ethernet backbone Access point Access point LAN01 LAN02 Figure 10.7 Infrastructure network SSID = group 1 SSID = group 1 SSID = group 1 SSID = group 1 Access point Et her net Figure 10.8 SSID for a wireless network Unit 13: Mobile Networks 197

10 Ad-hoc wireless LAN 1 Ad-hoc wireless LAN 2 Channel is identical such as channel = 3 Channel = 5 Figure 10.9 Ad-hoc networks Ad-hoc L Radius of coverage =2L L Access point L Infrastructure IEEE b settings Figure Span of networks The IEEE b device must be set up properly as the communications can be pickedup by other users, who, if the connection is not setup properly, could read all the communications sent and received. They may also be able to connect to the resources of the node connected to the wireless adapter. Figure shows some of the settings which must be set-up on the adapter. The settings are: Authentication algorithm. This sets whether the adapter uses an open system (where other nodes can listen to the communications), or uses encryption (using either a WEP key, or a shared key). Channel. If an ad-hoc network is used, then the nodes which communicate must use the same channel. Fragmentation threshold. This can be used to split large data frames into smaller fragements. The value can range from 64 to 1500 bytes. This is used to improve the 198 Computer Networks CO33006

11 efficiency when there is a high amount of traffic on the wireless network, as smaller frames make more efficient usage of the network. Network type. This can either be set to an infrastructure network (which use access points, or wireless hubs) or Ad-hoc, which allows nodes to interconnect without the need for an access point. Preamble mode. This can either be set to Long (which is the default) or short. A long preamble allows for interoperatively with 1Mbps and 2Mbps DSSS specifications. The shorter allows for faster operations (as the preamble is kept to a minimum) and can be used where the transmission parameters must be maximized, and that there are no interoperatablity problems. RTS/CTS threshold. The RTS Threshold prevents the Hidden Node problem, where two wireless nodes are within range of the same access point, but are not within range of each other. As they do not know that they both exist on the network, they may try to communicate with the access point at the same time. When they do, their data frames may collide when arriving simultaneously at the Access Point, which causes a loss of data frames from the nodes. The RTS threshold tries to overcome this by enabling the handshaking signals of Ready To Send (RTS) and Clear To Send (CTS). When a node wishes to communicate with the access point it sends an RTS signal to the access point. Once the access point defines that it can then communicate, the access point sends a CTS message. The node can then send its data. Encryption Figure Setting for IEEE b adaptor Figure shows that IEEE b has three encryption operations. These are: Disable. No encryption used. 64-bit WEP. Data encryption with an access point using a 64-bit key. Unit 13: Mobile Networks 199

12 128-bit WEP. Data encryption with an access point using a 128-bit key. Figure Setting encryption mode Figure shows that that it is possible to set the encryption key as a pass phase or manually. For 64-bit encryption, 5 alphanumeric characters or 10 hexadecimal values is used to define the encryption key, or for 128-bits encryption, the key is specified with 13 alphanumeric values or a 26 hexadecimal characters. The system will only use one of the four keys for its encryption. All the stations and connected access point, if connected, must use the same encryption key. For example a 64-bit key could be: Edin1 Whereas 128-bit encryption could use: Edinburgh Network 11 This encryption can be optional (only use, if necessary) or mandatory (where it will only ever use encryption). 200 Computer Networks CO33006

13 Figure Setting encryption key Unit 13: Mobile Networks 201

14 Activity 10.1: Test The end of unit test contains questions on the material in this unit. 202 Computer Networks CO33006