MAC SIMULATOR FOR HYBRID TDMA/CDMA ACCESS NETWORKS

Transcription

1 MAC SIMULATOR FOR HYBRID TDMA/CDMA ACCESS NETWORKS Danny Goderis, Rudy Hoebeke, Kristiaan Venken. Alcatel Corporate Research Center Department Access to Networks Francis Wellesplein 1, 2018 Antwerp, Belgium ABSTRACT In a TDMA/CDMA access network the transport channel is divided into sub-channels according to orthogonal codes. Each sub-channel is again divided into time-slots. CDMA spread spectrum techniques are used for wireless communication and HFC systems. Recently the European Telecommunication Standards Institute (ETSI) decided to retain a hybrid TD-CDMA interface as one of the two possible interfaces for the third generation mobile system UMTS. Dynamic MAC protocols for such communication systems allocate bandwidth to users in a twodimensional plane, taking into account Quality of Service (QoS) guarantees of individual connections, throughput efficiency and the typical 2D multi-user interference caused by simultaneous transmissions. The paper describes an OPNET MAC simulator with a generic two-dimensional (2D) channel interface. Channel characteristics such as multi-user interference are modelled as stand-alone modules. Simulation results are provided, allowing the comparison of alternative 2D- MAC protocols using TCP-, ON/OFF- and CBR sources for different media. TDMA ACCESS NETWORKS The main function of a shared-medium access network is to collect and multiplex traffic from several customers and offer it to the core network; and to distribute the traffic from the core network to the relevant customers. This involves the multiplexing and de-multiplexing of several connections. Typically the access network has a point to multi-point topology where all traffic between users (terminals or Network Terminations-NT) andcore network flows through the head-end (or Line Termination-LT) of the access network. Data-packets are broadcast in the downstream direction (from LT to NT), whereas multiple users share the upstream direction. Therefore a Medium Access Control (MAC) function is needed for controlling the upstream channel. Centrally controlled request-based MAC protocols for one-dimensional Time Division Multiple Access (TDMA) transport systems are very well known (fig 1). One time-slot corresponds with one packet or cell. Different timeslots are dynamically assigned to different terminals (NTs), under control of the MAC function implemented at the head-end (LT). Downstream Data Traffic LT MAC Controller Upstream D ata Traffic Traffic Requirements Traffic Profiles Connection Info Downstream M ac Control Info (permits) Network Span NT1 NT2 NTn Splitting Factor Data Traffic Figure 1: One-dimensional TDMA access network The allocation of time-slots is based on the traffic parameters, i.e. connection set-up information, and the Quality of Service requirements (QoS) of the connections. Dynamic bandwidth allocation also takes into account the instantaneous traffic-needs of the connections, e.g. the number of waiting cells at the terminals. The MAC-channel consists of the requests of the NTs sent to the LT and the permits broadcast from LT to NTs. In 1998 we reported at the OPNET conference how an OPNET MAC simulator was built for packet-based TDMA networks. This OPNET simulator enabled the study of advanced MAC protocols for e.g. ATM Passive Optical Networks (APON) and Hybrid Fibre Coax (HFC) access networks [1]. TWO-DIMENSIONAL MULTIPLEXING In this section we enlarge the one-dimensional model into a two-dimensional TD-CDMA framework enabling the study of generic MAC protocols for 2D-channels. In the next section we give an overview how the TDMA MAC simulator described in [1] is enhanced with 2Dmultiplexing functionality. 1

2 Code Division Multiple Access (CDMA) is a spread spectrum technique, which is mainly used for modulating signals in a bad communication medium with high noise characteristics and multi-path interference, e.g. the wireless communication channel. Recently the ETSI decided to retain a hybrid TD-CDMA interface as one of the two possible interfaces for the third generation mobile system UMTS (Universal Mobile Tele-communications System). Code axis One Sub-channel An important issue is the interdependency of the subchannels. Indeed, if the sub-channels were completely independent, i.e. any terminal can transmit on any subchannel independent of the events on the other subchannels, the problem simplifies into M-times a 1Dproblem. This is of course equivalent with the study of a one-dimensional TDMA system. However the subchannels are in general not independent due to the multiuser interference of the different sub-channels. For example in a CDMA system, distinct transmitting terminals (on different codes or sub-channels) are disturbing each other due to synchronisation problems on chip level. It means that the overall capacity of the channel decreases with an increasing number of distinct transmitting terminals (see fig. 3). Time axis Physical channel Fast performance loss channel capacity Slow performance loss Figure 2: Two-dimensional TD-CDMA access network The communication channel has a two-dimensional multiplexing structure (fig. 2). The channel is divided into M orthogonal sub-channels (with equal capacity C sub ) according to orthogonal codes. The sub-channel itself is divided into time-slots. The existence of sub-channels, i.e. of a 2D-channel, allows for the simultaneous, collision-free access or transmission of different users. This is the inherent multiple access property of CDMA. A physical channel is a 2D-point (slot, code) and corresponds with one packet or cell. A simple MAC protocol for such systems is the static allocation of sub-channels (or codes) to connections for the whole duration of this connection. The number of allocated sub-channels is proportional to the bandwidthneeds of the connection (e.g. the Peak Cell Rate) divided by the capacity of one sub-channel C sub. A dynamic request-based MAC protocol allocates physical channels based on the traffic parameters, the service requirements and the instantaneous bandwidthneeds of the connections. Contrary to one-dimensional systems, the MAC controller generates each timeslot a multi-permit, i.e. a series of single permits with the same x-coördinate (time-slot number). The multi-permit can either be a series of single permits to different terminals or it can be multiple permits to the same terminal; or a combination of both t# terminals Figure 3: Modelling multi-user interference As a general rule, the performance of the (M-code) 2Dchannel is less than the superposition of M independent 1D-TDMA channels. Modelling the influence of multiuser interference is however a very complicated physical layer issue. It depends on the physical medium, the number of terminals and the network topology, and the design of the physical layer. An optimal design should result in a minimal performance loss leading to almost independent sub-channels. MAC SIMULATOR OBJECT MODEL The OPNET MAC simulator for hybrid TDMA/CDMA access networks is an enhancement of our MAC simulator previously developed for networks with a onedimensional multiplexing structure [1]. This approach was possible because the 1D-TDMA simulator is a generic highly parameterised and modular OPNET program. Indeed, our strategy was to build further on the object-oriented modelling approach inherently present in the OPNET development tool. 2

3 The main building blocks (objects) in the MAC configuration are the bus transmission medium, the LT, the NT and the traffic sources (fig 4). The simulator supports CBR-, ON/OFF- and TCP traffic sources. The sources are implemented as independent nodes or as child processes of the main process model into the NTnode. Figure 4: The Network Model-view of the simulator For a detailed description of the bus model, the source model, the NT and LT state-machines and process models we refer to [1]. Here we limit ourselves to the typical 2D issues and enhancements necessary to support TDMA/CDMA access networks. Figure 5 gives the modular architecture of the MAC layer. It is a (simplified) high-level object model of the MAC OPNET simulator. CAC Static traffic parameters Dynamic traffic parameters Service Class Interface [MAC Transfer Capabilities] MAC Algorithm Permit Generator LT per time slot MAC LAYER Multi-Permit (nt_id, #codes) The NT and LT are the main static object classes. Naturally there are several instances of the NT-class while the LT-class is instantiated only once. The main sub-component of the LT is the MAC algorithm. The Interface of the MAC layer to higher layers is the Service Class Interface. It describes which services the network supports (e.g. POTS, TCP/IP traffic ) and gives a qualitative description of the traffic characteristics and requirements of the services. The static traffic parameters, such as the Peak Cell rate of a connection, serves as input for the MAC algorithm. Analogously to the well-known ATM Transfer Capabilities, the services are called MAC Transfer Capabilities. The interface to the physical layer is the Physical Channel Interface (PCI) module within the MAC Layer. This interface contains all physical layer related data such as the channel capacity, the number of sub-channels and timeslot length. A very important 2D function is the modelling of the multi-user interference. The interdependency of the sub-channels is thus expressed as a well-defined mathematical formula within the PCImodule. The MAC algorithm is responsible for the allocation of bandwidth amongst users in the 2D-plane. Based on the traffic parameters of the connections the MAC algorithm generates each time-slot a multi-permit. The PCIboundary conditions put constraints on this permitgeneration algorithm, reflecting the interdependency of the channels. Two object classes are modelling the dynamics of the system: the MAC_Credit and the MAC_Multi-permit class. A multi-permit object is created each time-slot by the MAC algorithm itself and is broadcasted from the LT to all NTs. An instantiation of MAC_Credit contains the dynamic traffic parameter of a connection, i.e. the number of cells at the NT waiting for transmission. A MAC_Credit object is instantiated by an NT and contains the NT_id and the number of waiting cells (requests). The MAC_Credit is transmitted from NT to LT either by polling or by piggybacking. It is the main dynamical input for the MAC algorithm. MAC Channel UP NT MAC Channel DOWN 2D-Physical Channel Interface For more implementation details about the MAC channel, especially the polling and piggybacking, we refer to [1]. Figure 5. MAC Layer Object Model Architecture 3

4 2D IMPLEMENTATION CONSIDERATIONS There are two main implementation differences between the 1D-TDMA and 2D-TDMA/CDMA access networks. First, the 2D-MAC algorithm generates each time slot a multi-permit object and - second the algorithm must take into account the PCI-boundary conditions Figure 6 illustrates the functional implementation of the MAC algorithm as a 2D-MAC permit-generator. timeslot n get_permit() OPNET LT multi-permit-list External MAC code (algorithm) multi-permit parallel datacell transmission get_pci () available codestructure OPNET NTs OPNET simulator body External PCI code Figure 6. OPNET simulator implementation The OPNET simulator body includes the implementation of the LT-node, the NT-nodes, the traffic sources and all communication aspects between nodes such as datapacket transmission, MAC polling and piggybacking. The OPNET simulator is also responsible for performance measuring and statistics monitoring such as the NT-queue lengths. All these functions are essentially the same for 1D and 2D access systems. The main implementation features and enhancements for networks with a two-dimensional multiplexing structure are the following: A basic timeslot is defined as the duration to send 1 cell with a single code (on one sub-channel). Each timeslot n the MAC simulator triggers the MAC algorithm with the function get_permit (). The algorithm is implemented by external C-code, enabling the simulation of different concrete algorithms. The interface between the simulator and the MAC algorithm is implemented as the argument of the function get_permit (). This argument is a pointer representing a list (or array) of multi-permits. Each entry in the multi-permit list represents one NT_id. A multi-permit is a structure with the following multi-permit (integer) attributes: NT_id QoS_class Number_of_Codes The latter indicates the number of single permits the NT receives during timeslot n, i.e. the number of orthogonal codes allocated to the terminal for the duration of one timeslot. The triggering of the MAC permit generator results in a number of transmission codes suitable for cell transmission by one or more NTs. These multipermits are broadcasted from LT to the NTs. As such each NT may receive from 0 up to M (number of sub-channels) codes. Upon reception of the multipermit the NT transmits simultaneously the appropriate number of cells indicated by the multipermit. No physical layer related stuff is implemented neither in the simulator body nor in the MAC external code. A separate external module represents the Physical Channel Interface modelling amongst other things the multi-user interference. The interface between the MAC permit generator and the PCI-module is a configuration of NTs which should be served during the same timeslot n. In response PCI returns the available code-structure, i.e. the maximum number of codes available per NT in the configuration. The code-structure is such that the multi-user interference caused by the simultaneous transmission is small enough for not causing any cell loss. A simple example is given in the next section. Based on a given available code-structure, the MAC algorithm decides which NTs will effectively be served, also taking into account the traffic parameters and the MAC strategy. Therefore PCI may be triggered several times - with different NTconfigurations - during the same timeslot. 4

5 SIMULATION EXAMPLES In this section we give as an illustration some simulation results for an ATM-based access network with a twodimensional multiplexing structure. Channel characteristics The network has a point to multi-point topology with 30 connected terminals with a maximum distance of 1000m, resulting in a round trip delay of about 10µs. The total channel upstream capacity is 1 Mega bit/s. The channel is divided into 15 sub-channels. The slot size of the TDMA sub-channels is 56 bytes: 1 ATM cell + 3 physical overhead bytes. The capacity reserved for fixed rate polling is 3%. The mutual NT-distances between terminals have no significant influence on the multi-user interference. The number of simultaneous transmitting terminals uniquely determines the interference-degree. In the example it is assumed that the capacity decreases with about 10% per extra transmitting terminal. number of transmitting terminals total number of available subchannels (codes) maximum number codes per terminal >6 0 0 Figure 7. 2D-interference simulation conditions (PCI) Simulations with ON/OFF sources We monitored the performance of several MAC protocols for different traffic scenarios and using different types of sources. A first series of simulation runs have been executed with ON/OFF sources representing best-effort, bursty data-traffic. Non of the connections has a bandwidth guarantee. Using the available OPNET statistics the queue-length is monitored at each terminal. Averaging these queuelength statistics results in a number of global network performance metrics such as the mean and maximum queue-length. The graph in figure 8 gives the mean NTqueue-length as a function of the network load. The latter is determined by the sum of the sustainable cell rate (SCR) of the ON/OFF sources. Mean Queue Size DAdvanced Protocol 1DTDMAStatic Priority 2D Static Priority Protocol 32% 42% 53% 63% 74% 85% 90% 94% Load Figure 8.Mean queue-length performance measurements Two 2D-MAC protocols are compared with the wellknown 1D-Static Priority protocol for TDMA channels (SP-MAC). Of course, the total capacity of the 2D channel is exactly the same as the capacity of the TDMA-channel. Also the packet size and the overhead bytes are taken to be the same in the 1D- and 2Dsimulation scenarios. Clearly the TDMA channel combined with the 1D SP- MAC protocol performs the best, because it does not suffer from multi-user interference. A simple extension of SP-MAC for 2D channels performs very badly (divergence at a load of 50%). This 2D SP-MAC allocates the codes as well as the time-slots in a Round Robin way to requesting terminals. This shows that a more advanced dynamic MAC protocol, taking into account the specific 2D-channel multi-userinterference, is really necessary. Indeed, figure 8 also illustrates that such an advanced dynamic 2D-MAC performs well up to a load of 90% Simulations with CBR sources Figure 9 illustrates some results for simulations with CBR-sources of 64 kbit/s, representing POTSconnections. The transport channel (1Mbit/s) supports in theory at most 15 connections. Simulation results show that the 1D-TDMA Static Priority MAC protocol (explained above) supports 14 connections. 5

6 The 2D-static allocation of sub-channels supports 4 simultaneous connections. This minimum can be calculated taking into account the performance loss given by figure 7. The number of connections supported by dynamic 2D- MAC protocols should be between 4 and 14. Dynamic protocols can support more connections than static ones based on the mechanism of cell clumping. The MAC protocol induces an extra cell-delay allowing the simultaneous transmission of cells by the same terminal. # connections D Advanced Protocol + time-mechanism 2D Static Priority Protocol Static Sub-Channel Allocation induced cell-delay in timeslot units Figure 9. Supported POTS-connections for a given (allowed) cell delay ThecellsarebufferedattheNTandthentransmittedas a group by using several sub-channels simultaneously for the same connection. This mechanism yields less multi-user interference, but at the cost of extra-induced jitter or delay. A supplementary time-delay mechanism is also needed in order to guarantee delay-bounds for all active POTS connections. An advanced 2D protocol can support the same number of connections as the 1D TDMA SP protocol if the allowed jitter is large enough. The 2D-SP protocol however can never support more than 7 connections. This behaviour corresponds with the divergence of the queue-length at a load of 50% measured for ON/OFF sources (fig. 8). Simulations with TCP sources Figure 10 shows the results of simulations with 5 TCP sources. Each source has a (different) minimum bandwidth guarantee (guaranteed frame rate GFRservice). The same MAC protocols are evaluated as in figure 8. However, each of the protocols is now exhibited with an extra mechanism, known as Weighted Round Robin. This mechanism takes into account the minimum bandwidth guarantee of connections and distributes in a fair way the available bandwidth amongst the NTs according to their relative weights. In the simulation example (1,2,4,8,16) gives the relative weight distribution. kbit/s TCP Goodput 1D TDMA Static Priority 2D Static Priority Protocol 2D Advanced Protocol terminal Id Figure 10. TCP-traffic with bandwidth guarantees The 1D-SP-protocol and the 2D-advanced MAC protocol almost perform equally good. The 2D-MAC protocol for example obtains a total TCP-goodput of 707 kbit/s. The maximum obtainable TCP goodput is 714 kbit/s. This theoretical number can be calculated based on the header overheads of the protocol stack TCP/IP/AAL5/ATM. The 2D-SP protocol again performs very badly. The obtained TCP-goodput is only 284 kbit/s, which is even less than 50%. CONCLUSIONS In this paper we described an OPNET simulator suited for the evaluation of MAC protocols for access networks with a two-dimensional multiplexing structure. This has been realised by an upgrade of a simulator for TDMA networks. This approach was possible based on our object-oriented development approach and the inherent object-oriented characteristics of the OPNET simulation tool. REFERENCES [1] MAC Simulator for ATM-based Shared-Medium Access Networks. Kristiaan Venken, Rudy Hoebeke, Frank Ploumen. OPNETWORK Conference