| Key Issues |
The following are the key issues in regard to wireless ATM network.
The architecture proposed is composed of a large number of small transmission cells, called Pico cells. A base station serves each Pico cell. All the base stations in the network are connected via the wired ATM network. The use of ATM switching for intercell traffic also avoids the problem of developing a new backbone network with sufficient throughput to support intercommunication among a large number of small cells. To avoid hard boundaries between Pico-cells, the base stations can operate on the same frequency.
Reducing the size of the Pico-cells has major advantages in mitigating some of the major problems associated with in-building wireless LANs. The main difficulties encountered are the delay due to multipath effects and the lack of a line-of-sight path resulting in high signal attenuation. Pico-cells can also have some drawbacks as compared to larger cells. There are a small number of mobiles, on average, within range of any base-station, so base-station cost and connectivity is critical. As cell size is reduced, hand-over rate also increases. By using the same frequency, no hand-over will be required at the physical layer. The small cell size also gives us the flexibility of reusing the same frequency, thus avoiding the problem of running out of bandwidth. The mobile units in the cell communicate with only the base-station serving that particular cell, and not with other mobile units. The basic role of the base station is interconnection between the LAN or WAN and the wireless subnets, and also to transfer packets and converting them to the wired ATM network from the mobile units. In traditional mobile networks, transmission-cells are "coloured" using frequency division multiplexing or code division multiplexing to prevent interference between cells. Colouring is a wasteful of bandwidth since in order for it to be successful there must be areas between re-use of the colour in which it is idle. These inactive areas could potentially be used for transmission.
The ATM cell size (53 bytes) is designed for 64kbps or higher, which may be too big for some wireless LANs (due to low speed and high error rates), therefore wireless LANs may use 16 or 24 byte payload. The ATM header can also be compressed and be expanded to standard ATM at the base station. An example of ATM header compression is to use 2 bytes containing 12-bit VCI (virtual channel identifier) and 4 bit control (payload type, cell loss priority etc.). One of the cell formats proposed is to have a compatible pay-load size and addressing scheme, which should be different from the standard ATM cell format. Mobility should be as transparent as possible to the end-points and therefore the VCIs used by the end-points should not change during hand-over. The allocation of the VCI should remain valid as the mobile moves through different Pico-cells within the same domain. The translation of the VCIs should be as simple as possible due to movement between domains. This can be done by splitting the VCI space into a number of fields like Domain Identifier, Mobile Identifier, Base Station Identifier and Virtual Circuit number. A 16 bit CRC is also used to detect bit errors, due to high error rate of mobile networks.
Each processor that attaches to the ATM switch maintains a virtual connection to each other processor over which it passes data packets. In addition, the processors use a second, separate virtual circuit for routing updates. Therefore, each wireless base station will have two virtual circuits open to each other base station and to each router. As the packet arrives at the base station from the mobile wireless unit, it chooses the circuit that leads to the correct destination. Using two connections guarantees that routing information will not be confused with data because data packets never travel on the virtual circuits used for routing, and routing packets never travel on the circuits used for data. The circuits can also be assigned priority to guarantee that stations receive and process routing updates quickly.
The standard routing method for datagrams is not adequate to cope with wireless systems, because then every node of the wireless network must store the location of every mobile system to which a route exists, which represents a large amount of information and database. Secondly, it will become impossible to keep the routing information up to date and consistent throughout the network, since a very larger number of mobile users will exist.
Due to the above factors it a method was proposed by which mobile end-systems may be added to an internetwork with the minimum of disruption. A "mobile controller" node is required in every subnet of the wireless network. Each of these controllers has its network-layer software enhanced by an additional sublayer, which performs routing to mobile systems. Mobile systems are free to move between the subnets, and a network wide name server provides location information when communication with a mobile is to be initiated. A system of local caching of location information and forwarding of data allows movements to be hidden from the Transport Layer.
In a wide or local area network, virtual connection establishment is reasonably fast, but is likely to take longer time in wireless networks. Therefore it may not be practical to re-establish all virtual circuits whenever a mobile moves between Pico-cells. The solution is to isolate the small scale mobility of the mobile from the rest of the wired network latent virtual paths and the Mobile Switching Point (MSP) is used. The MSP provides a routing point through which all virtual circuits to the mobile are routed. From the mobile switching point there may be a number of potential base-stations which can be used to contact the mobile. Each of these routes for virtual circuits is termed a virtual path, which can be manipulated by a single signalling command at the MSP. Whenever a virtual path is active, all of the associated virtual circuits will also be active, other virtual paths are latent - the virtual circuits have been established but no traffic is flowing.
The basic design issue for next generation private communication network (PCN) is the selection of modulation methods, and a set of bit rates. A bit rates in the range of 5 -10 Mpbs can be achieved using the existing wireless technologies, in a picocellular environment. Thus, with the exception of HDTV, most other ATM applications can be supported. The preferred technique may actually vary with the specific PCN application, so that it is likely that both TDMA and CDMA solutions will co-exist .
CDMA provides an efficient integrated solution for frequency reuse and multiple access, and can typically achieve net bandwidth efficiency 2 -4 times that of comparable narrowband approaches. However, a major weakness of CDMA for multi-service PCN is that for a given system bandwidth, spectrum spreading limits the peak user data rate.
Narrow band (TDMA) can be used to achieve high bit rates, as the implementation is well understood, and has been with us for a long time. In a Pico-cellular environment, a bit rates in the range of 8 -16 Mb/s can be achieved, by using the narrow band approach. Overall, with a good physical level design, it should be possible for macro (5-10 km), micro (0.5 km), and Pico (100m) cells to support baud rates of the order 0.1-0.25 Msym/s, 0.5-1.5 Msym/s and 2-4 Msym/s . These rates should be sufficient enough to accommodate many of the broadband services.
One of the major problems of Wireless ATM is to find a suitable channel sharing /media access control technique at the data-link layer. Shared media access leads to poor performance in wireless networks. When spread spectrum modulation is used, CDMA is the main mode of operation. Narrow band modulation (TDMA) can also be used, and research suggests slotted ALOHA with exponential back off, as the protocol used for MAC. Slotted ALOHA has considerably better delay performance at low utilisation than a fixed allocation scheme and fits well with the statistical multiplexing of ATM.
CSMA/CD protocol gives the required performance on copper, but is not suitable for wireless, where all the systems in a cell are not in communication with each other. It is suggested that a reverse channel be used over which the base station echoes the incoming signal. This could be done at the expense of doubling the bandwidth required, and a busy/idle signal could be broadcast on a separate narrow-band channel. One of the problems with this kind of contention-based method is that in mixed-media applications access cannot be prioritised, though contention algorithms can be devised which increase the probability that a high-priority message will succeed in capturing the resources it requires.
One system proposed uses a form of TDMA (Time Division Multiple Access), in which the mobiles in a cell are polled to determine which of them have data to send. The mobiles are then allowed to transmit in turn on receipt of a token from the base station. After the polling, priority is then given to units, which have speech, or other continuous services, which can send it during the first part of the frame and the remainder of the frame can be used for data. When the traffic is light, the unit may be allowed to send data in most of the frame. A flag bit can also be used in the packet header, which is set, if the mobile has more data queued for transmission. This reduces the polling overhead. The challenge in designing the MAC protocol for Wireless ATM is to identify a wireless, multimedia capable MAC, which provides a sufficient degree of transparency for many ATM applications.
Wireless ATM needs a custom data link layer protocol, and it should be as transparent as possible. A custom data link protocol is needed due to high error rate and different packet size of Wireless ATM. Wireless ATM may use 16 or 24-byte payload, as 53 byte may be too long. The data link protocol may contain service type definition, error control, segmentation and reassemble, and handoff support.
A service type field is needed so as to indicate whether a packet is of type supervisory/control, CBR, VBR ABR etc. The service type field simplifies base station protocol processing.
Wireless ATM should provide an error control due to high noise interference and poor physical level characteristics of the wireless medium. This is achieved using a PCN packet sequence number filed (e.g. 10 bits) in the header along with a standard 2-byte CRC frame check sequence trailer. HDLC style retransmission can be used for connectionless data. Since wireless ATM may use 16 or 24 byte cells, segmentation and reassemble is required. This can be achieved with a segment counter that uses, for example, the two least significant bits of the error control sequence (PSN) number. Handoff is an important characteristic. Handoff occurs when the mobile unit leaves the area of one cell and enters the area of another. Therefore soft handoff without any data loss is important for any wireless network, and it should be transparent. This can be implemented by using bits in header, which indicates PDUs before and after the handoff.
In order to establish connections between the mobile unit and the base station, the mobile must be located. Searching and registration can pose problems.
Enquiries about the object are directed to this register using a static routing mechanism. When a mobile is within a domain, it is registered at the appropriate Domain Location Server (DLS) and this registers the mobile at its Home Register (HR). The HR keeps a record of the mobiles current DLS location. Each mobile has a statically bound home address that is mapped to the HR address.
One of the basic requirement of mobile units, is that the units should be allowed to roam freely from cell to cell. This process should not require any user intervention. If the signal falls below a pre-set limit, the switching centre looks for another base-station, which is receiving a stronger signal and transfers the call to that station. An interval can be set aside in each frame, during which newly arrived or activated mobiles attempt to inform the base-station of their presence. If the destination address is that of the mobile unit, then the local Domain Location Server (DLS) is contacted. If the local DLS have no knowledge of the unit, then the routing information is forwarded to the Home Registry (HR) of the mobile. The HR returns the address of the remote DLS, which in turn returns, the address of the mobiles MR (mobile registration). Once the address of the MR is known, then all the requests are directed to that particular MR. When the connection request arrives at the MR, it first consults the Mobile, which in turn decides whether to accept the call or not. If the mobile accepts the call then it allocates a virtual circuit number and returns it to the MR. The MR then creates the virtual circuits between the Mobile Switching Point (MSP) and the remote end-point and adds the new virtual circuit to the active and inactive virtual paths between the MSP and the base stations close to the mobile.
| Send comments to webmaster Copyright © 1997 Derek Mc Donnell. All Rights Reserved. Last updated 07-Apr-1998. |
 |