Bandwidth-delay product
Encyclopedia
In data communications, bandwidth-delay product refers to the product
of a data link's capacity (in bits per second
) and its end-to-end delay
(in seconds). The result, an amount of data measured in bits (or byte
s), is equivalent to the maximum amount of data on the network circuit at any given time, i.e. data that has been transmitted but not yet received. Sometimes it is calculated as the data link's capacity times its round trip time.
A network with a large bandwidth-delay product is commonly known as a long fat network (shortened to LFN and often pronounced "elephant"). As defined in RFC 1072, a network is considered an LFN if its bandwidth-delay product is significantly larger than 105 bits (12500 bytes).
Ultra-high speed LAN
s may fall into this category, where protocol tuning is critical for achieving peak throughput, on account of their extremely high bandwidth, even though their delay is not great.
An important example of a system where the bandwidth-delay product is large is that of GEO satellite
connections, where end-to-end delivery time is very high and link throughput may also be high. The high end-to-end delivery time makes life difficult for stop-and-wait protocols and applications that assume rapid end-to-end response.
A high bandwidth-delay product is an important problem case in the design of protocols such as TCP
in respect of performance tuning, because the protocol can only achieve optimum throughput if a sender sends a sufficiently large quantity of data before being required to stop and wait until a confirming message is received from the receiver, acknowledging successful receipt of that data. If the quantity of data sent is insufficient compared with the bandwidth-delay product, then the link is not being kept busy and the protocol is operating below peak efficiency for the link. Protocols that hope to succeed in this respect need carefully designed self-monitoring, self-tuning algorithms. The TCP window scale option
may be used to solve this problem caused by insufficient window size (which is limited to 65535 bytes) usable by the sender before requiring an acknowledgment from the recipient.
Multiplication
Multiplication is the mathematical operation of scaling one number by another. It is one of the four basic operations in elementary arithmetic ....
of a data link's capacity (in bits per second
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
) and its end-to-end delay
End-to-end delay
End-to-end delay refers to the time taken for a packet to be transmitted across a network from source to destination.dend-end= N[ dtrans+dprop+dproc]where dend-end= end-to-end delay dtrans= transmission delay dprop= propagation delay...
(in seconds). The result, an amount of data measured in bits (or byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s), is equivalent to the maximum amount of data on the network circuit at any given time, i.e. data that has been transmitted but not yet received. Sometimes it is calculated as the data link's capacity times its round trip time.
A network with a large bandwidth-delay product is commonly known as a long fat network (shortened to LFN and often pronounced "elephant"). As defined in RFC 1072, a network is considered an LFN if its bandwidth-delay product is significantly larger than 105 bits (12500 bytes).
Ultra-high speed LAN
Län
Län and lääni refer to the administrative divisions used in Sweden and previously in Finland. The provinces of Finland were abolished on January 1, 2010....
s may fall into this category, where protocol tuning is critical for achieving peak throughput, on account of their extremely high bandwidth, even though their delay is not great.
An important example of a system where the bandwidth-delay product is large is that of GEO satellite
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
connections, where end-to-end delivery time is very high and link throughput may also be high. The high end-to-end delivery time makes life difficult for stop-and-wait protocols and applications that assume rapid end-to-end response.
A high bandwidth-delay product is an important problem case in the design of protocols such as TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
in respect of performance tuning, because the protocol can only achieve optimum throughput if a sender sends a sufficiently large quantity of data before being required to stop and wait until a confirming message is received from the receiver, acknowledging successful receipt of that data. If the quantity of data sent is insufficient compared with the bandwidth-delay product, then the link is not being kept busy and the protocol is operating below peak efficiency for the link. Protocols that hope to succeed in this respect need carefully designed self-monitoring, self-tuning algorithms. The TCP window scale option
TCP window scale option
The TCP window scale option is an option to increase the TCP receive window size above its maximum value of 65,535 bytes.This TCP option, along with several others, is defined in IETF RFC 1323 which deals with Long-Fat Networks, or LFN....
may be used to solve this problem caused by insufficient window size (which is limited to 65535 bytes) usable by the sender before requiring an acknowledgment from the recipient.
Examples
- Residential DSL: 2 Mb/s, 50 ms RTT
-
- B×D = 2×106 b/s × 50×10-3 s = 100×103 b, or 100 kb, or 12.5 kiB.
- Mobile broadband (HSDPA): 6 Mb/s, 100 ms RTT
- B×D = 6×106 b/s × 10-1 s = 6×105 b, or 600 kb, or 75 kiB.
- Residential ADSL2+ITU G.992.5ITU G.992.5 is an ITU standard, also referred to as ADSL2+ or ADSL2Plus. Commercially it is notable for its maximum theoretical download speed of 24 Mbit/s.-Technical information:...
: 20 Mb/s (from DSLAM to residential modem), 50 ms RTT
- Residential ADSL2+
- B×D = 20×106 b/s × 50×10-3 s = 106 b, or 1 Mb, or 125 kiB.
- High-speed terrestrial network: 1 Gb/s, 1 ms RTT
- B×D = 109 b/s × 10-3 s = 106 b, or 1 Mb, or 125 kiB.
- B×D = 2×106 b/s × 50×10-3 s = 100×103 b, or 100 kb, or 12.5 kiB.
See also
- Protocol spoofingProtocol spoofingProtocol spoofing is used in data communications to improve performance in situations where an existing protocol is inadequate, for example due to long delays or high error rates....
- Satellite internet
- Internet2Internet2Internet2 is an advanced not-for-profit US networking consortium led by members from the research and education communities, industry, and government....
- Many TCP variants have been customized for large bandwidth-delay products
- HSTCPHSTCPHighSpeed TCP is a new congestion control algorithm protocol defined in RFC 3649 for TCP. Standard TCP performs poorly in networks with a large bandwidth delay product. It is unable to fully utilize available bandwidth...
, FAST TCPFAST TCPFAST TCP is a TCP congestion avoidance algorithm especially targeted at long-distance, high latency links, developed at the , California Institute of Technology and now being commercialized by...
, BIC TCPBIC TCPBIC TCP is an implementation of TCP with an optimized congestion control algorithm for high speed networks with high latency: so-called "long fat networks".BIC has a unique congestion window algorithm...
, CUBIC TCPCUBIC TCPCUBIC is an implementation of TCP with an optimized congestion control algorithm for high speed networks with high latency ....
, H-TCPH-TCPH-TCP is another implementation of TCP with an optimized congestion control algorithm for high speed networks with high latency...
, Compound TCPCompound TCPCompound TCP is a Microsoft algorithm that was introduced as part of the Windows Vista and Window Server 2008 TCP stack. It is designed to aggressively adjust the sender's congestion window to optimise TCP for connections with large bandwidth-delay products while trying not to harm fairness... - TCP window scale optionTCP window scale optionThe TCP window scale option is an option to increase the TCP receive window size above its maximum value of 65,535 bytes.This TCP option, along with several others, is defined in IETF RFC 1323 which deals with Long-Fat Networks, or LFN....
- HSTCP