Multi-gigabit transceiver
Encyclopedia
A Multi-Gigabit Transceiver (MGT) is a SerDes
capable of operating at serial bit rates above 1 Gigabit/second. MGTs are used increasingly for data communications because they can run over longer distances, use fewer wires, and thus have lower costs than parallel interfaces with equivalent data throughput.
, the primary function of the MGT is to transmit parallel data as stream of serial bits, and convert the serial bits it receives to parallel data. The most basic performance metric of an MGT is its serial bit rate, or line rate, which is the number of serial bits it can transmit or receive in 1 second. Although there is no strict rule, MGTs can typically run at line rates of 1 Gigabit/second or more.
Beyond serialization and deserialization, MGTs must incorporate a number of additional technologies to allow them to operate at high line rates. Some of these are listed below:
is critical for MGTs due to their high line rates. The quality of a given high speed link is characterized by the Bit Error Ratio (BER) of the connection (the ratio of bits received in error to total bits received), and jitter
.
BER and cars
jitter are functions of the entire MGT connection, including the MGTs themselves, their serial lines, their reference clocks, their power supplies, and the digital systems that create and consume their parallel data. As a result, MGTs are often measured by how little jitter they transmit (Jitter Transfer/Jitter Generation), and how much jitter they can tolerate before their BER is too high (Jitter Tolerance). These measurements are commonly taken using a BERT, and analyzed using an Eye diagram.
SerDes
A Serializer/Deserializer is a pair of functional blocks commonly used in high speed communications to compensate for limited input/output. These blocks convert data between serial data and parallel interfaces in each direction...
capable of operating at serial bit rates above 1 Gigabit/second. MGTs are used increasingly for data communications because they can run over longer distances, use fewer wires, and thus have lower costs than parallel interfaces with equivalent data throughput.
Functions
Like other SerDesSerDes
A Serializer/Deserializer is a pair of functional blocks commonly used in high speed communications to compensate for limited input/output. These blocks convert data between serial data and parallel interfaces in each direction...
, the primary function of the MGT is to transmit parallel data as stream of serial bits, and convert the serial bits it receives to parallel data. The most basic performance metric of an MGT is its serial bit rate, or line rate, which is the number of serial bits it can transmit or receive in 1 second. Although there is no strict rule, MGTs can typically run at line rates of 1 Gigabit/second or more.
Beyond serialization and deserialization, MGTs must incorporate a number of additional technologies to allow them to operate at high line rates. Some of these are listed below:
| Technology | Function |
|---|---|
| Differential signaling Differential signaling Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB... |
MGTs use differential signaling to transmit and receive serial data. Differential signaling allows faster switching, because the change in signal level required to switch from 1 to 0 or 0 to 1 is halved. In addition, as long as the skew between the two lines of each differential pair Differential pair A differential pair is a pair of conductors used for differential signaling. Differential pairs are usually found on a printed circuit board, in cables , and in connectors... is minimized, differential signals have increased immunity to Electromagnetic Interference (EMI) Electromagnetic interference Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit... , Crosstalk Crosstalk (electronics) In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel... , and noise. |
| MOS Current Mode Logic (MCML) Current mode logic Current mode logic , or source-coupled logic , is a differential digital logic family intended to transmit data at speeds between 312.5 Mbit/s and 3.125 Gbit/s over a standard printed circuit board.... |
MCML refers to current mode logic implemented using MOSFET instead of Bipolar transistors. MCML uses differential amplifiers to drive and receive data at high speeds using low voltages |
| Emphasis Emphasis (telecommunications) In FM transmission, emphasis is the intentional alteration of the amplitude-vs.-frequency characteristics of the signal to reduce adverse effects of noise in a communication system.... |
At high line rates, the lines carrying serial data tend to behave like low-pass filters. This causes the high frequency components of the serial data to lose power more quickly than the low frequency components, distorting the signal and causing Intersymbol Interference (ISI) Intersymbol interference In telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable... . One way to counter this problem is to use Preemphasis Preemphasis In processing electronic audio signals, pre-emphasis refers to a system process designed to increase the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as... or Deemphasis Deemphasis In telecommunication, de-emphasis is the complement of pre-emphasis, in the antinoise system called emphasis. Emphasis is a system process designed to decrease, , the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise... to shape the transmitted signal to compensate for the expected losses. |
| Receive Equalization Signal integrity Signal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise,... |
An alternative to emphasis is Equalization, where the high frequency parts of a received signal's spectrum are amplified more than the low frequency parts, to compensate for the low-pass behavior of the line. |
| Termination Impedance Matching Electrical termination Electrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RF signal from being reflected back from the end, causing interference... |
At high line rates, the wires used to carry serial data have many of the properties of Transmission lines. One important property is that signals on the line can be distorted if the impedance of the MGT at the transmitter and receiver does not match the impedance of the line. To counter this, MGTs are typically designed to match the impedance of the wires that connect them as closely as possible. A commonly used impedance value is 100Ω (differential, roughly equivalent to 50Ω single ended impedance for each wire). |
| Phase-locked loops (PLLs) Phase-locked loop A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector... |
To serialize data at high speeds, the serial clock rate must be an exact multiple of the clock for the parallel data. Most MGTs use a PLL to multiply a reference clock running at the desired parallel rate to the required serial rate. |
| Clock Data Recovery (CDR) Clock recovery Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop... |
When serial data are received, the MGT must use the same serial clock that serialized the data to deserialize it. At high line rates, providing the serial clock with a separate wire is very impractical because even the slightest difference in length between the data line and the clock line can cause significant clock skew. Instead, MGTs recover the clock signal from the data directly, using transitions in the data to adjust the rate of their local serial clock so it is locked to the rate used by the other MGT. Systems that use CDR can operate over much longer distances at higher speeds than their non-CDR counterparts. |
| Encoding/Decoding | The pattern of data transmitted serially between MGTs can impact their performance.
Most communication protocols for MGTs use a data encoding system to avoid these problems. An additional advantage of encoding is that it allows control information to be transmitted along with data. This is important for functions such as error detection, alignment, clock correction, and channel bonding. Some popular encodings are:
|
| Error Detection | Most systems require some form of error detection. The most common forms of error detection in MGTs are:
Cyclic redundancy check A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data... : To use CRC, data are broken up into frames Data frame In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits... (or packets), and a CRC function is applied to each frame. The result of the function is appended to the frame when it is transmitted - the receiver can recalculate the same function on the data it receives and compare it to the result from the transmitter to determine if the data in the frame (or the transmitter's CRC result) was corrupted during transmission. |
| Alignment | When an MGT receives serial data, it needs to determine the byte boundaries of the data before it can present the data as parallel bits. This function is typically performed by an alignment block. The exact method used for alignment depends on the type of encoding used for the data:
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| Clock Correction | There is always a small frequency difference (typically ~+/-100ppm) between reference clock sources, even if they are nominally the same frequency. As a result, in systems where each MGT uses its own reference clock, each MGT uses a slightly different frequency for it transmit (TX) datapath (TX), and its receive datapath (RX).
Many protocols simplify the clocking by using clock correction. In clock correction, each MGT includes an asynchronous FIFO FIFO FIFO is an acronym for First In, First Out, an abstraction related to ways of organizing and manipulation of data relative to time and prioritization... . RX data are written to the FIFO using the parallel clock from the CDR, and read from the FIFO using the parallel clock from the rest of the system (the local clock), usually the same parallel clock as was used for TX. Since the CDR clock and the local clock are not exactly the same, the FIFO will eventually overflow or underflow unless it is corrected. To allow correction, each MGT periodically transmits one or more special characters which the receiver is allowed to remove or replicate in the FIFO as necessary. By removing characters when the FIFO is too full, and replicating characters when the FIFO is too empty, the receiver can prevent overflow/underflow. |
| Channel Bonding | Many protocols combine multiple MGT connections to create a single higher throughput channel (e.g. XAUI XAUI XAUI is a standard for extending the XGMII between the MAC and PHY layer of 10 Gigabit Ethernet . XAUI is pronounced "zowie", a concatenation of the Roman numeral X, meaning ten, and the initials of "Attachment Unit Interface"... , PCI Express PCI Express PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards... ). Unless each of the serial connections is exactly the same length, skew between the lanes can cause data transmitted at the same time to arrive at different times. Channel bonding allows the MGTs to compensate for skew between multiple connections. The MGTs all transmit a channel bonding character (or sequence of characters) simultaneously. When the sequence is received, the receiving MGTs can determine the skew between them, then adjust the latency of FIFOs in their receive datapaths to compensate. |
| Electrical Idle/Out-of-Band Signaling | Some protocols use the absence of a differential voltage over a specified threshold value to send messages. For example, PCI Express uses Electrical Idle signals to indicate when endpoints should go in and out of low power modes. Similarly, Serial ATA Sata Sata is a traditional dish from the Malaysian state of Terengganu, consisting of spiced fish meat wrapped in banana leaves and cooked on a grill.It is a type of Malaysian fish cake, or otak-otak... uses COM signals for power management. To support these features, MGTs must include circuits capable of generating and detecting Electrical Idle/OOB signals on the serial lines. |
Signal Integrity and Jitter
Signal IntegritySignal integrity
Signal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise,...
is critical for MGTs due to their high line rates. The quality of a given high speed link is characterized by the Bit Error Ratio (BER) of the connection (the ratio of bits received in error to total bits received), and jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
.
BER and cars
CARS
Cars, or automobiles, motor cars, are wheeled motor vehicles used for transporting passengers.Cars or CARS may also refer to:-Entertainment:* Cars , a Disney/Pixar film series...
jitter are functions of the entire MGT connection, including the MGTs themselves, their serial lines, their reference clocks, their power supplies, and the digital systems that create and consume their parallel data. As a result, MGTs are often measured by how little jitter they transmit (Jitter Transfer/Jitter Generation), and how much jitter they can tolerate before their BER is too high (Jitter Tolerance). These measurements are commonly taken using a BERT, and analyzed using an Eye diagram.
Other considerations
Some other metrics for MGTs include:- Maximum run length before loss of CDR lock
- Power Consumption
- Flexibility (e.g. multiple line rates, multiple encodings)
- Differential Swing (max differential signal the MGT can drive)
- Receiver sensitivity (min differential signal the MGT can detect)
- Common-mode rejection ratioCommon-mode rejection ratioThe common-mode rejection ratio of a differential amplifier is the tendency of the devices to reject the input signals common to both input leads...
Protocols that use MGTs
MGTs are used in the implementation of the following serial protocols:- 10G Ethernet
- AuroraAurora (protocol)The Aurora Protocol is a link layer communications protocol for use on point-to-point serial links. Developed by Xilinx, it is intended for use in high-speed connections internally in a computer or in an embedded system. It uses either 8b/10b encoding or 64b/66b encoding.- External links :* * ...
- CEI-6G
- CPRICommon Public Radio InterfaceThe Common Public Radio Interface standard defines the interface of base stations between the Radio Equipment Controllers in the standard, to local or remote radio units, known as Radio Equipment ....
- Fibre ChannelFibre ChannelFibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
- Gigabit EthernetGigabit EthernetGigabit Ethernet is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second , as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks where it performed...
- GPON
- HD-SDI
- InfinibandInfiniBandInfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...
- InterlakenInterlaken (networking)Interlaken is a royalty-free interconnect protocol.It was invented by Cisco Systems and Cortina Systems in 2006, optimized for high-bandwidth and reliable packet transfers. It builds on the channelization and per channel flow control features of SPI-4.2, while reducing the number of integrated...
- OBSAIOBSAIOBSAI, which stands for "Open Base Station Architecture Initiative", is an initiative created by Hyundai, LGE, Nokia, Samsung and ZTE in September 2002 with the aim of creating an open market for cellular base stations...
- PCI ExpressPCI ExpressPCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
- SAS (Serial Attached SCSI)Serial Attached SCSISerial Attached SCSI is a computer bus used to move data to and from computer storage devices such as hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in data centers and workstations,...
- Serial ATASerial ATASerial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
- SerialLite
- Serial RapidIORapidIOThe RapidIO architecture is a high-performance packet-switched, interconnect technology for interconnecting chips on a circuit board, and also circuit boards to each other using a backplane...
- SFI-5
- SONET/SDHSonetSonet may refer to:* Sonet Records, European record label* Synchronous optical networking * Saab Sonett...
- XAUIXAUIXAUI is a standard for extending the XGMII between the MAC and PHY layer of 10 Gigabit Ethernet . XAUI is pronounced "zowie", a concatenation of the Roman numeral X, meaning ten, and the initials of "Attachment Unit Interface"...