Multi-carrier adaptive bandwidth control maximizes capacity usage
This unabated demand for network capacity necessitates more efficient use of essential wireless links and is fueling the rush to technologies that make more efficient use of this valuable resource. One method is to switch from time-division multiplexing (TDM) to more effective packet-based technology. Multi-carrier adaptive bandwidth control (ABC) represents the next step in the evolution, boosting the efficiency, cost effectiveness, and resilience of wireless links while simultaneously helping operators achieve better quality of service.
Circuit switching and multiplexing
In circuit-switching technologies, a circuit is set up at the beginning of a connection and is maintained for its duration. Network resources at the sending and receiving end are reserved and dedicated; the source, information payload, capacity, path and destination are pre-determined. Over the circuit, the information that the sending and receiving stations will transmit is always at the pre-determined rate with no capability for change providing no flexibility in the face of fluctuating transmission conditions.
Circuit switching makes sense for traditional voice traffic where people make a connection, talk back and forth, and then close the connection when they are finished. For the purpose of transmitting data traffic, however, this is not an efficient method because the approach ties up network resources even when there is nothing to transmit; people stay online with the Internet, for example, even though they may not be accessing data.
Multiplexing increases the efficiency of circuit switching. In multiplexing, multiple communication streams are combined into one physical connection. Having a single circuit carry more than one stream more efficient use of network resources—if one stream is not transmitting at a given moment, perhaps other ones are.
TDM is the most widely used method of multiplexing for the transmission of digital signals in circuit-based networks. It is extensively deployed in legacy short- and long-haul wireless links.
In TDM, a transmission burst is divided into some pre-determined number of slots whereby each stream is allowed to insert its information into the appropriate slot. For example, consider two transmission streams—file downloads and voice conversations. To multiplex them over a single physical transmission circuit, we first create two fixed-length slots in each burst. Stream 1’s information (file download) will be placed into the first slot and Stream 2’s information (digitized voice) will be placed into the second slot. The ensuing transmission burst will send the information of both slots over the same circuit. At the receiving end, the information in the first slot will be separated from the information in the second slot and each will be sent onto the proper recipient.
Without multiplexing, two circuits would have to be dedicated for these two streams. If a stream had nothing to send in a given burst, the bandwidth of that circuit would be idle. With multiplexing, we can send both streams over the same circuit. If one of the streams has nothing to send while the other does, one of the slots would be wasted, but we would still be making use of the circuit to transmit the information of the other stream. This is highly simplified, of course. In actual multiplexing transmission schemes, eight or more streams can share slots so that bursts seldom go completely wasted.
Although multiplexing improves the overall efficiency, circuit-switching still suffers from inefficiency when one or more streams have nothing to include in their timeslots which remain empty for many transmission bursts (see figure 2). Overall, the circuit is being used, but timeslots are frequently wasted. If this happens often, the operator is not making good use of this critical resource.
Packet-switching technology
Packet-switching technology remedies TDM’s empty-timeslot inefficiency problem and adds capacity. With packet-switching, large amounts of information from multiple sources can be converted into smaller packets to be placed onto the same transmission stream. In contrast to TDM, packet-switching does not require fixed timeslots so any number of connections can use the same channel in any order at any time.
We can think of packet-switching as a kind of conveyor belt in a warehouse. Just as physical products can be selected from the warehouse and placed on a conveyor belt every time a customer makes an order (e.g., “Send this file,” “I want to receive my emails,” etc.), any data packet that is ready can be placed in the microwave transmission link and transmitted in the next burst. Just as each physical product carries order information so that it can be placed in the shopping cart of the correct customer, each data packet contains information such as the destination address, and sequence number so that it can be mixed with any other packet, transmitted in the next available burst, and then re-constructed at the receiving end and sent to the proper recipient (see figure 3). Packets can enter the stream in any order; as long as a sender has something to send, the information payload can be packetized and sent out as soon as there is available bandwidth to carry it—a place on the conveyor belt. Packets from different end-to-end connections can all share the same wireless link, making the most effective use of the bandwidth.
Resilient connections
Wireless networks can deteriorate or fail. Weather conditions can interfere with, or even terminate, communications for periods of time. In order to provide safeguards against failure, network operators employ backup schemes, for example dividing the capacity of a physical wireless link into two separate carriers where one carrier is on stand-by to back up the other automatically in case of failure. In this way, the network operator can keep the link operational virtually all of the time, but at the cost of 50% of the total capacity. Noticing that carriers rarely fail, operators can adopt more efficient backup schemes such as 7+1, in which the capacity of the wireless link is divided into eight carriers. Seven of the carriers are multiplexed circuits used to carry traffic, while the remaining carrier acts as a backup in case of failure of any one of the others. In this way, only one-eighth of the total capacity of the wireless link is sacrificed to backup. If two carriers fail simultaneously, however, there will be no backup available for one of them.
By their nature, packet-switched links are more resilient than circuit-switched links. For example, in a TDM network, if weather impacts a carrier even to a small degree, the carrier will fail completely and will have to rely on an available backup carrier to continue. In the more resilient packet-switched network, however, inclement weather might reduce the capacity of a carrier, but the transmission can step down automatically to a slower speed, boosting signal strength in order to maintain transmission leaving high-priority traffic unaffected.
Weather conditions are not the only differentiator between TDM and packet-based links. In TDM, all transmissions follow in sequence—a large file is sent in order. Packet technology has a better way: The file is broken up into packets in which each packet can follow a different network path and even arrive out of order. Equipment on the receiving end contains logic for re-constituting the packets in proper order to re-create the file as it was originally sent. In a case in which many carriers transmit over one physical wireless link, failure of one carrier does not stop packet transmission that can proceed on any of the other available carriers. In fact, packet technology is so flexible that, just like in our warehouse example, packets from different transmission streams can be placed on any available carrier in any order, maximizing the utilization of the capacity of the wireless link at any given moment.
Multi-carrier adaptive bandwidth control
As more and more wireless links are upgraded from TDM to packet technology, attention is focusing on how to further boost the capacity of packet-switched links. Moreover, network operators are keen on guaranteeing the quality of services they provide to their users and subscribers. Multi-carrier ABC technology further refines packet switching, providing operators with a way to utilize nearly 100% of available capacity.
Multi-carrier ABC technology optimizes the way traffic is distributed between multiple carriers over a single wireless link. In ABC, packets are further decomposed into bytes and each byte can be placed on any of the available carriers for transmission over the wireless link. At the receiving end of the link, ABC-aware equipment receives the bytes and re-constitutes the original packets.
ABC technology is always aware of the speed and congestion of each carrier at all times and distributes the bytes in the most optimal way for transmission given the current conditions of each carrier, for example congestion, current throughput rate, etc.
With ABC, all carriers can be used for transmission at all times; there is no need for a dedicated backup carrier. If a carrier fails, it is simply bypassed and bytes are placed on the remaining carriers in the most optimal way given current conditions. If a failed carrier comes back into operation, it is immediately re-included in the ABC byte-distribution optimization scheme. If a carrier deteriorates due to weather conditions, ABC adjusts by sending fewer bytes over that carrier according to the degree of deterioration. With multi-carrier ABC, the wireless link’s capacity is optimized at all times and in all situations.
ABC for quality of service
In addition to maximizing capacity, multi-carrier ABC provides another significant benefit—it allows operators to increase quality-of-service levels. Let’s look at a situation in which there are two carriers over one physical wireless link and where the operator supports two types of applications, each with a different class of service: streaming video (high) and email (low). ABC can give priority to the streaming video service by placing its bytes more frequently onto both carriers than the e-mail service. If there is deterioration in one of the carriers, ABC immediately adjusts accordingly, still prioritizing the video bytes over the e-mail bytes. If there is improvement in the transmission rate of one of the carriers, ABC immediately adjusts to that situation. In all cases, the transmission of the higher class-of-service streaming video is prioritized over EL-mail. In all cases, the capacity of the entire wireless link is maximized even as the traffic is allocated by priority.
Multi-carrier ABC makes the most efficient use of the total capacity of a wireless link taking into account the operator’s quality of service goals. Operators who deploy ABC over wireless links enjoy a higher level of network resiliency. They maximize the use of precious network resources by maximizing capacity while providing the best available service to their users.
About the author
Eirik Nesse is VP Product Strategy at Ceragon. He is responsible for the global strategy for Ceragon’s microwave product portfolio and associated products. Formerly, he was the Chief Technical Architect at Nera Networks where he worked in various positions and has more than 27 years of industry experience. Mr. Nesse holds a BSc degree in Electronic Engineering from University of Stavanger. Mr Nesse resides in Bergen, Norway.
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