question: How does an ISP control my internet speed?
comment: When Cable modems boot up they pull a configuration file from the ISPs controller that dictates what speed your modem operates at. Which configuration file you get is determined based on a database in their provisioning and billing system. Changing your speed is not as easy as manipulating the file though, the system has a bunch of checks and balances in it to prevent people from doing exactly that. Higher end internet connections have speed controls done on the switch level. Fiber internet is really nothing more than a fiber line running into a switch on the ISP side. They can either hard set the port to a specific speed (10mb/s, 100mb/s, 1gb/s) or they can set the switch port to operate at a specific speed using a service policy. We do this sort of thing quite frequently in the networking world.
passages: Section::::Models/modalities.:Group-based.
   BULLET::::- OIF CEI 6+ Gbit/s for lane speeds of 5.0 and 6.25 GBd (4 and 5 Gbit/s)
  
  BULLET::::- 10GBASE-KR 802.3-ap (long reach) and 802.3-ba (short reach) for lane speeds of 10.3125 GBd (9.85 Gbit/s)
  
  The RapidIO PCS/PMA layer supports two forms of encoding/framing:
  
  BULLET::::- 8b/10b for lane speeds up to 6.25 GBd
  
  BULLET::::- 64b/67b, similar to that used by Interlaken for lane speeds over 6.25 GBd
  
  Every RapidIO processing element transmits and receives three kinds of information: Packets, control symbols, and an idle sequence.
  
  Section::::Protocol overview.:Physical layer.:Packets.
   The term "at wire speed", or the adjective "wire speed", describes any computer system or hardware device that is able to achieve a throughput equal to the maximum throughput of the communication standard. This requires that the CPU capacity, bus capacity, network switching capacity, etc., be sufficient. Network switches, routers, and similar devices are sometimes described as operating at wire speed. Data encryption and decryption and hardware emulation are software functions that might run at wire speed (or close to it) when embedded in a microchip.
   The technique involves multiple methods:
  
  BULLET::::- Continuously monitoring the status of interfering signal levels using current bit-loading compared to maximum achievable bit rate, number of errored seconds, number of severely errored seconds, number of forward error corrections (FEC) and making decisions about the underperforming scenario's cause and forcing the link to train in a specific way.
  
  BULLET::::- Identifying the neighborhood cables in the binders that may be causing unwanted cross-talk and lowering their upstream transmission power until bit-rates are optimized for the network.
   Section::::Description.:Speeds.
   Section::::Types.:Ack filtering/decimation.
  
  Ack filtering or decimation is used on highly asymmetric links. In asymmetric links the upstream and downstream rates vary widely. A common example is satellite broadband where a downstream satellite link provides significantly greater bandwidths than the upstream dialup modem link. In this scenario, the speed at which the modem can return TCP acknowledgements can be a limiting factor. As TCP acknowledgements are cumulatively acknowledged some can be decimated or filtered to improve performance.
  
  Section::::Types.:Snoop.
   Section::::Metrics for ISPs.
   Network hardware, software and specifications, as well as the expertise of network management personnel are important in ensuring that data follows the most efficient route, and upstream connections work reliably. A tradeoff between cost and efficiency is possible.
  
  Section::::Law enforcement and intelligence assistance.
   RADSL allows rate-adaptation while the connection is in operation — rate-adaptation during connection setup is possible in many other DSL variants, including G.dmt and its successors. Rate-adaptation while the connection is in operation is specified as an option in ADSL2, ADSL2+, and VDSL2, under the name "seamless rate adaptation" (SRA).
  
  Section::::Technology.
   Some DSL modems have filtering circuitry built-in, to which the telephones and fax machines can be connected.
  
  Section::::Modulation techniques and specifications.
  
  BULLET::::- ITU G.992.1 (G.dmt)
  
  BULLET::::- ITU G.992.2 (G.lite)
  
  BULLET::::- ITU G.992.3 (ADSL2)
  
  BULLET::::- ITU G.992.4 (G.lite.bis)
  
  BULLET::::- ITU G.992.5 (ADSL2+)
  
  Section::::Maintenance and failure.
   As with all DSL connections, the further the distance from the DSLAM (usually located at the telephone exchange) the customer site is, the slower the line speed will be. Sky uses DLM (Dynamic Line Management) over the first 10 days of a new connection to set the line at an acceptable downstream and upstream speed in order for the connection to remain stable.
  
  Lines are initially connected at 4Mbit/s and gradually increased over the 10-day "training period" until the line shows signs of instability, this allows Sky to know what speeds the line can handle whilst remaining stable.
   Section::::Protocols and configurations.
  
  Many DSL technologies implement an asynchronous transfer mode (ATM) layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.
   Many "modems" provide the additional functionality to host a LAN so most Internet access today is through a LAN, often a very small LAN with just one or two devices attached. And while LANs are an important form of Internet access, this raises the question of how and at what data rate the LAN itself is connected to the rest of the global Internet. The technologies described below are used to make these connections.
  
  Section::::Technologies.:Hardwired broadband access.
   Note that BITS does not necessarily measure the actual bandwidth. BITS versions 3.0 and up will use Internet Gateway Device counters, if available, to more accurately calculate available bandwidth. Otherwise, BITS will use the speed as reported by the NIC to calculate bandwidth. This can lead to bandwidth calculation errors, for example when a fast network adapter (10 Mbit/s) is connected to the network via a slow link (56 kbit/s).
  
  Section::::Technology.:Jobs.
   Section::::Equipment settings.:New path.
   The definition of the Fibre Channel signalling voltage is complex. Eye-diagrams are defined for both the transmitter and receiver. There are many eye-diagram parameters which must all be met to be compliant with the standard. In simple terms, the transmitter circuit must output a signal with a minimum of 600 mV peak-to-peak differential, maximum 2000 mV peak-to-peak differential. A good signal looks rather like a sine-wave with a fundamental frequency of half the data rate, so 1 GHz for a typical system running at 2 gigabits per second.
   In order to use Broadband Internet services simultaneously with voice telephony, it is necessary to use a DSL filter. This is a low pass filter in line with the phone outlet. This prevents high frequency data noise from affecting the lower frequency voice bandwidth and it also prevents the low impedance of the connected phone from attenuating or modulating the high speed DSL data Path. Enough bandwidth is retained for voice telephony and the majority is used for high speed data. 
   Through the use of plugin modules, connection tracking can be given knowledge of application-layer protocols and thus understand that two or more distinct connections are "related". For example, consider the FTP protocol. A control connection is established, but whenever data is transferred, a separate connection is established to transfer it. When the codice_42 module is loaded, the first packet of an FTP data connection will be classified as "related" instead of "new", as it is logically part of an existing connection.
   Section::::Important elements of a transport stream.:Packet identifier (PID).
  
  Each table or elementary stream in a transport stream is identified by a 13-bit packet identifier (PID). A demultiplexer extracts elementary streams from the transport stream in part by looking for packets identified by the same PID. In most applications, time-division multiplexing will be used to decide how often a particular PID appears in the transport stream.
  
  Section::::Important elements of a transport stream.:Programs.
   At the PoP, the customer's aggregated information is typically directed into an Add-drop multiplexer or an MSPP (multiservice provisioning platform), which are complex and expensive devices that sit between customers and the core network. They manage traffic streams coming from customers and forward those streams to the PSTN (voice) or appropriate wide area networks (ATM, frame relay, or the Internet).
   Section::::Algorithms.:TCP Westwood+.
   Section::::Equipment settings.:Old path.
   The final main aspect of TCP is congestion control. TCP uses a number of mechanisms to achieve high performance and avoid congestion collapse, where network performance can fall by several orders of magnitude. These mechanisms control the rate of data entering the network, keeping the data flow below a rate that would trigger collapse. They also yield an approximately max-min fair allocation between flows.
   Fibre Channel network protocols
  
  Communication between devices in a fibre channel network uses different elements of Fibre Channel standards.
  
  Section::::Transmission words and ordered sets.
  
  All Fibre Channel communication is done in units of four 10-bit codes. This group of 4 codes is called a transmission word.
  
  An ordered set is a transmission word that includes some combination of control (K) codes and data (D) codes
  
  Section::::AL_PAs.
   A comprehensive comparison of various network traffic classifiers, which depend on Deep Packet Inspection (PACE, OpenDPI, 4 different configurations of L7-filter, NDPI, Libprotoident, and Cisco NBAR), is shown in the Independent Comparison of Popular DPI Tools for Traffic Classification.
  
  Section::::Hardware.
  