Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-05-25
2002-09-10
Trost, William (Department: 2683)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S321000, C370S437000, C370S468000, C455S450000
Reexamination Certificate
active
06449267
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods of communicating information in packet-switched networks, and in particular to a method and system for communicating uplink data packets from an earth station to a satellite while minimizing transport delay.
2. Description of the Related Art
Satellite networks have become a popular means to disseminate information over a wide area Transception of data over satellite networks must comply with certain access protocols that are suitable for the type of data to be transmitted. The access protocol determines how channel bandwidth will be allocated among system users. One such access protocol is the medium access control (MAC) protocol. In the past, the dominant media form transmitted over such networks has been computer data. However, in recent years, there has been a need to provide interactive, real-time multimedia such as medium quality interactive video over such networks. To provide such information, the underlying networks must be capable of delivering communication services complying with a specified Quality of Service (QoS) criteria. At the MAC sub-layer, this QoS amounts to some statistical guarantees on packet delay, delay variance, and loss.
In satellite networks, MAC protocols must be applied to the uplink (earth terminal to satellite) channel. Since the uplink is shared between many users and is hence a shared channel, the MAC protocol can have a significant effect on the QoS the network is capable of delivering. In designing a MAC for bursty variable bit-rate (VBR) sources in a high latency system, there is a tradeoff between uplink utilization and achievable delay. For instance, a MAC protocol technique that delivers a high link utilization (and thus a high network capacity) will almost always produce poor packet delays. This is because the instantaneous bandwidth requirements of each node (earth terminal) must be determined in order to perform an optimal allocation of network resources. This can lead to excessive transmission delays. This is particularly so when used with high latency systems such as satellite networks operating in geosynchronous orbit (GEO). In GEO satellite systems, the distance between the ground station and the satellites itself is a significant source of data latency.
Currently, individuals can purchase a relatively small satellite dish capable of two-way communication with a GEO satellite. These can be used by individual households, companies, universities, and many other “users” who do not have access to a broadband wired infiastructure. This “personal earth terminal” model raises the specter of many low bit-rate terminals sharing a common uplink channel. Because there are many personal terminals, there are potentially few sources being aggregated at the uplink point. This can lead to source traffic that is highly bursty, which presents problems in efficient MAC protocol design.
There are several methods for gaining channel access in a shared channel system. These methods vary from random access (RA) to fixed bandwidth allocation (FBA) protocols. The QoS that these techniques can deliver varies as well. The simplest form of random access is an access protocol wherein the remote users (in this case, earth terminals) transmit packets in an uncoordinated manner. Since collision-free channel resources cannot be guaranteed with RA methods, QoS guarantees, in terms of packet loss and delay, are very weak. However, such random, uncoordinated transmission protocols do offer reduced control signaling and algorithmic overhead, and in ease of implementation. Random access MAC protocols are traditionally employed when the network traffic is unpredictable and bursty.
With fixed bandwidth allocation (FBA) protocols, medium access is accomplished when the connection is set up. A terminal acquires a fixed amount of channel resources and maintains this resource for the life of the connection. The only time the amount of channel resource may change is when the connection is preempted by another connection with higher priority. FBA protocols are capable of delivering much stronger QoS guarantees than RA protocols. However, this QoS improvement comes at the expense of system capacity. For example, in cases where the source has a varying bit-rate (VBR), simply acquiring channel bandwidth greater than or equal to its peak rate will provide a relatively firm upper bound on the delays of packets entering the network. However, since the source is VBR, uplink channel resources will be wasted when the source is producing packets at a rate less than the peak rate. This poor link utilization leads to low network capacity (i.e. the number of terminals that can be supported within a given amount of uplink bandwidth).
FIG. 1
is a diagram showing the operation of a communication system using a demand assigned multiple access (DAMA) protocol. DAMA techniques, which address the capacity issue by using instantaneous bandwidth demands to statistically multiplex many VBR sources on one channel, can be used to deliver predictable delays without the poor capacity of FBA
DAMA based MAC protocols comprise two primary elements: (1) a bandwidth request mechanism and (2) a mechanism for coordinated transmission. The bandwidth request mechanism normally consists of dedicated bandwidth for each terminal such as earth station
104
residing in a “request phase.” The transmission of data packets occurs in the “data phase”. The separation of these two phases is accomplished by a physical layer (PHY) protocol such as frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA). In the request phase, data bandwidth is reserved by the earth station (ES) by a resource request module
116
forming and transmitting a resource requesting having a resource metric that represents the current value of the earth station's
104
desired bandwidth. This resource request phase allows the ES to communicate their instantaneous bandwidth needs to an allocating agent (AA)
108
, which performs bandwidth allocation. In a satellite network
100
, the AA
108
resides either at the satellite (denoted
108
A) or at a terrestrial master control station (MCS)
106
(denoted
108
B). Once the AA
108
has received the bandwidth requests of all terminals and earth stations
104
, it decides how much channel resources (or resource units) to allocate to each terminal using an allocation algorithm (AAlg). Each earth station
104
is then informed, via a downlink channel, how many resource units (allocated frequencies in FDMA, time slot in CDMA, and codes in CDMA) it will receive and when to begin transmission. By informing each terminal when to transmit, the AA accomplishes coordinated transmission. This MAC scheme results in a system wherein the time varying bandwidth needs of any terminal can be accommodated. The only time insufficient bandwidth is allocated is when several stations
104
are producing traffic at close to their peak rates. When this occurs, some terminals may not get all of the channel bandwidth they requested. Because of this possibility, the QoS guarantees, in terms of delay bounds, are not as firm as the fixed bandwidth allocation case.
There are two main drawbacks to DAMA techniques, however. They are (1) bandwidth loss due to request signaling (2) and increased packet delay times. In a pure DAMA protocol, each data packet must wait a round trip time (i.e. twice the time required to transmit a packet between the earth station
104
and the allocating agent
108
A at the satellite) before it can begin transmission. Due to the high delay (latency) in satellite networks (particularly those in GEO), the process of passing DAMA request information between an earth terminal and the satellite can be very time consuming. For example, for GEO systems, the delay in a transmission from an earth terminal to a satellite (hop delay D) is on the order of 135 milliseconds. If the AA
108
is located at the satellite, the time between a packet
Duraiswamy V. D.
Hughes Electronics Corporation
Nguyen Simon
Sales M. W.
Trost William
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