Internet Engineering Task Force Sally Floyd INTERNET DRAFT Jamshid Mahdavi draft-floyd-sack-00.txt Matt Mathis Matthew Podolsky Allyn Romanow August 1999 Expires: February 2000
An Extension to the Selective Acknowledgement (SACK) Option for TCP
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
https://www.doczj.com/doc/3914644328.html,/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
https://www.doczj.com/doc/3914644328.html,/shadow.html.
Abstract
This note defines an extension of the Selective Acknowledgement
(SACK) Option [RFC2018] for TCP. RFC 2018 specified the use of the
SACK option for acknowledging out-of-sequence data not covered by
TCP’s cumulative acknowledgement field. This note extends RFC 2018
by specifying the use of the SACK option for acknowledging duplicate packets. This note suggests that when duplicate packets are
received, the first block of the SACK option field can be used to
report the sequence numbers of the packet that triggered the
acknowledgement. This extension to the SACK option allows the TCP
sender to infer the order of packets received at the receiver,
allowing the sender to infer when it has unnecessarily retransmitted a packet. A TCP sender could then use this information for more
robust operation in an environment of reordered packets, ACK loss,
packet replication, and/or early retransmit timeouts.
1. Conventions and Acronyms
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [B97].
2. Introduction
The Selective Acknowledgement (SACK) option defined in RFC 2018 is
used by the TCP data receiver to acknowledge non-contiguous blocks of data not covered by the Cumulative Acknowledgement field. However,
RFC 2018 does not specify the use of the SACK option when duplicate
segments are received. This note specifies the use of the SACK
option when acknowledging the receipt of a duplicate packet [F99].
We use the term D-SACK (for duplicate-SACK) to refer to a SACK block that reports a duplicate segment.
This document does not make any changes to TCP’s use of the
cumulative acknowledgement field, or to the TCP receiver’s decision
of *when* to send an acknowledgement packet. This document only
concerns the contents of the SACK option when an acknowledgement is
sent.
This extension is compatible with current implementations of the SACK option in TCP. That is, if one of the TCP end-nodes does not
implement this D-SACK extension and the other TCP end-node does, we
believe that this use of the D-SACK extension by one of the end nodes will not introduce problems.
The use of D-SACK does not require separate negotiation between a TCP sender and receiver that have already negotiated SACK capability.
The absence of separate negotiation for D-SACK means that the TCP
receiver could send D-SACK blocks when the TCP sender does not
understand this extension to SACK. In this case, the TCP sender will simply discard any D-SACK blocks, and process the other SACK blocks
in the SACK option field as it normally would.
3. The Sack Option Format as defined in RFC 2018
The SACK option as defined in RFC 2018 is as follows:
+--------+--------+
| Kind=5 | Length |
+--------+--------+--------+--------+
| Left Edge of 1st Block |
+--------+--------+--------+--------+
| Right Edge of 1st Block |
+--------+--------+--------+--------+
| |
/ . . . /
| |
+--------+--------+--------+--------+
| Left Edge of nth Block |
+--------+--------+--------+--------+
| Right Edge of nth Block |
+--------+--------+--------+--------+
The Selective Acknowledgement (SACK) option in the TCP header
contains a number of SACK blocks, where each block specifies the left and right edge of a block of data received at the TCP receiver. In
particular, a block represents a contiguous sequence space of data
received and queued at the receiver, where the ‘‘left edge’’ of the
block is the first sequence number of the block, and the ‘‘right
edge’’ is the sequence number immediately following the last sequence number of the block.
RFC 2018 implies that the first SACK block specify the segment that
triggered the acknowledgement. From RFC 2018, when the data receiver chooses to send a SACK option, ‘‘the first SACK block ... MUST
specify the contiguous block of data containing the segment which
triggered this ACK, unless that segment advanced the Acknowledgment
Number field in the header.’’
However, RFC 2018 does not address the use of the SACK option when
acknowledging a duplicate segment. For example, RFC 2018 specifies
that ‘‘each block represents received bytes of data that are
contiguous and isolated’’. RFC 2018 further specifies that ‘‘if sent at all, SACK options SHOULD be included in all ACKs which do not ACK the highest sequence number in the data receiver’s queue.’’ RFC 2018 does not specify the use of the SACK option when a duplicate segment is received, and the cumulative acknowledgement field in the ACK
acknowledges all of the data in the data receiver’s queue.
4. Use of the SACK option for reporting a duplicate segment
This section specifies the use of SACK blocks when the SACK option is used in reporting a duplicate segment. When D-SACK is used, the
first block of the SACK option should be a D-SACK block specifying
the sequence numbers for the duplicate segment that triggers the
acknowledgement. If the duplicate segment is part of a larger block of non-contiguous data in the receiver’s data queue, then the
following SACK block should be used to specify this larger block.
Additional SACK blocks can be used to specify additional non-
contiguous blocks of data, as specified in RFC 2018.
The guidelines for reporting duplicate segments are summarized below: (1) A D-SACK block is only used to report a duplicate contiguous
sequence of data received by the receiver in the most recent packet.
(2) Each duplicate contiguous sequence of data received is reported
in at most one D-SACK block. (I.e., the receiver sends two identical D-SACK blocks in subsequent packets only if the receiver receives two duplicate segments.)
(3) The left edge of the D-SACK block specifies the first sequence
number of the duplicate contiguous sequence, and the right edge of
the D-SACK block specifies the sequence number immediately following the last sequence in the duplicate contiguous sequence.
(4) If the D-SACK block reports a duplicate contiguous sequence from
a (possibly larger) block of data in the receiver’s data queue above the cumulative acknowledgement, then the second SACK block in that
SACK option should specify that (possibly larger) block of data.
(5) Following the SACK blocks described above for reporting duplicate segments, additional SACK blocks can be used for reporting additional blocks of data, as specified in RFC 2018.
Note that because each duplicate segment is reported in only one ACK packet, information about that duplicate segment will be lost if that ACK packet is dropped in the network.
4.1 Reporting Full Duplicate Segments
We illustrate these guidelines with three examples. In each example, we assume that the data receiver has first received eight segments of 500 bytes each, and has sent an acknowledgement with the cumulative
acknowledgement field set to 4000 (assuming the first sequence number is zero). The D-SACK block is underlined in each example.
4.1.1: Example 1: Reporting a duplicate segment.
Because several ACK packets are lost, the data sender retransmits
packet 3000-3499, and the data receiver subsequently receives a
duplicate segment with sequence numbers 3000-3499. The receiver
sends an acknowledgement with the cumulative acknowledgement field
set to 4000, and the first, D-SACK block specifying sequence numbers 3000-3500.
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
3000-3499 3000-3499 3500 (ACK dropped)
3500-3999 3500-3999 4000 (ACK dropped)
3000-3499 3000-3499 4000, SACK=3000-3500
---------
4.1.2. Example 2: Reporting an out-of-order segment and a duplicate segment.
Following a lost data packet, the receiver receives an out-of-order
data segment, which triggers the SACK option as specified in RFC
2018. Because of several lost ACK packets, the sender then
retransmits a data packet. The receiver receives the duplicate
packet, and reports it in the first, D-SACK block:
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
3000-3499 3000-3499 3500 (ACK dropped)
3500-3999 3500-3999 4000 (ACK dropped)
4000-4499 (data packet dropped)
4500-4999 4500-4999 4000, SACK=4500-5000 (ACK dropped)
3000-3499 3000-3499 4000, SACK=3000-3500, 4500-5000
---------
4.1.3. Example 3: Reporting a duplicate of an out-of-order segment.
Because of a lost data packet, the receiver receives two out-of-order segments. The receiver next receives a duplicate segment for one of these out-of-order segments:
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
3500-3999 3500-3999 4000
4000-4499 (data packet dropped)
4500-4999 4500-4999 4000, SACK=4500-5000
5000-5499 5000-5499 4000, SACK=4500-5500
(duplicated packet)
5000-5499 4000, SACK=5000-5500, 4500-5500
---------
4.2. Reporting Partial Duplicate Segments
It may be possible that a sender transmits a packet that includes one or more duplicate sub-segments--that is, only part but not all of the transmitted packet has already arrived at the receiver. This can
occur when the size of the sender’s transmitted segments increases,
which can occur when the PMTU increases in the middle of a TCP
session, for example. The guidelines in Section 4 above apply to
reporting partial as well as full duplicate segments. This section
gives examples of these guidelines when reporting partial duplicate
segments.
When the SACK option is used for reporting partial duplicate
segments, the first D-SACK block reports the first duplicate sub-
segment. If the data packet being acknowledged contains multiple
partial duplicate sub-segments, then only the first such duplicate
sub-segment is reported in the SACK option. We illustrate this with the examples below.
4.2.1. Example 4: Reporting a single duplicate subsegment.
The sender increases the packet size from 500 bytes to 1000 bytes.
The receiver subsequently receives a 1000-byte packet containing one 500-byte subsegment that has already been received and one which has not. The receiver reports only the already received subsegment using a single D-SACK block.
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 500-999 1000
1000-1499 (delayed)
1500-1999 (data packet dropped)
2000-2499 2000-2499 1000, SACK=2000-2500
1000-2000 1000-1499 1500, SACK=2000-2500
1000-2000 2500, SACK=1000-1500
---------
4.2.2. Example 5: Two non-contiguous duplicate subsegments covered by the cumulative acknowledgement.
After the sender increases its packet size from 500 bytes to 1500
bytes, the receiver receives a packet containing two non-contiguous
duplicate 500-byte subsegments which are less than the cumulative
acknowledgement field. The receiver reports the first such duplicate segment in a single D-SACK block.
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 500-999 1000
1000-1499 (delayed)
1500-1999 (data packet dropped)
2000-2499 (delayed)
2500-2999 (data packet dropped)
3000-3499 3000-3499 1000, SACK=3000-3500
1000-2499 1000-1499 1500, SACK=3000-3500
2000-2499 1500, SACK=2000-2500, 3000-3500
1000-2499 2500, SACK=1000-1500, 3000-3500
---------
4.2.3. Example 6: Two non-contiguous duplicate subsegments not covered by the cumulative acknowledgement.
This example is similar to Example 5, except that after the sender
increases the packet size, the receiver receives a packet containing two non-contiguous duplicate subsegments which are above the
cumulative acknowledgement field, rather than below. The first, D-
SACK block reports the first duplicate subsegment, and the second,
SACK block reports the larger block of non-contiguous data that it
belongs to.
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 500-999 1000
1000-1499 (data packet dropped)
1500-1999 (delayed)
2000-2499 (data packet dropped)
2500-2999 (delayed)
3000-3499 (data packet dropped)
3500-3999 3500-3999 1000, SACK=3500-4000
1000-1499 (data packet dropped)
1500-2999 1500-1999 1000, SACK=1500-2000, 3500-4000
2000-2499 1000, SACK=2000-2500, 1500-2000,
3500-4000
1500-2999 1000, SACK=1500-2000, 1500-3000,
---------
3500-4000
4.3. Interaction Between D-SACK and PAWS
RFC 1323 [RFC1323] specifies an algorithm for Protection Against
Wrapped Sequence Numbers (PAWS). PAWS gives a method for
distinguishing between sequence numbers for new data, and sequence
numbers from a previous cycle through the sequence number space.
Duplicate segments might be detected by PAWS as belonging to a
previous cycle through the sequence number space.
RFC 1323 specifies that for such packets, the receiver should do the following:
Send an acknowledgement in reply as specified in RFC 793 page 69, and drop the segment.
Since PAWS still requires sending an ACK, there is no harmful
interaction between PAWS and the use of D-SACK. The D-SACK block can be included in the SACK option of the ACK, as outlined in Section 4, independently of the use of PAWS by the TCP receiver, and
independently of the determination by PAWS of the validity or
invalidity of the data segment.
TCP senders receiving D-SACK blocks should be aware that a segment
reported as a duplicate segment could possibly have been from a prior cycle through the sequence number space. This is independent of the use of PAWS by the TCP data receiver. We do not anticipate that this will present significant problems for senders using D-SACK
information.
5. Detection of Duplicate Packets
This extension to the SACK option enables the receiver to accurately report the reception of duplicate data. Because each receipt of a
duplicate packet is reported in only one ACK packet, the loss of a
single ACK can prevent this information from reaching the sender. In addition, we note that the sender can not necessarily trust the
receiver to send it accurate information [SCWA99].
In order for the sender to check that the first (D)SACK block of an
acknowledgement in fact acknowledges duplicate data, the sender
should compare the sequence space in the first SACK block to the
cumulative ACK which is carried IN THE SAME PACKET. If the SACK
sequence space is less than this cumulative ACK, it is an indication that the segment identified by the SACK block has been received more than once by the receiver. An implementation MUST NOT compare the
sequence space in the SACK block to the TCP state variable snd.una
(which carries the total cumulative ACK), as this may result in the
wrong conclusion if ACK packets are reordered.
If the sequence space in the first SACK block is greater than the
cumulative ACK, then the sender next compares the sequence space in
the first SACK block with the sequence space in the second SACK
block, if there is one. This comparison can determine if the first
SACK block is reporting duplicate data that lies above the cumulative ACK.
TCP implementations which follow RFC 2581 [RFC2581] could see
duplicate packets in each of the following four situations. This
document does not specify what action a TCP implementation should
take in these cases. The extension to the SACK option simply enables the sender to detect each of these cases. Note that these four
conditions are not an exhaustive list of possible cases for duplicate packets, but are representative of the most common/likely cases.
Subsequent documents will describe experimental proposals for sender responses to the detection of unnecessary retransmits due to
reordering, lost ACKS, or early retransmit timeouts.
5.1. Replication by the network
If a packet is replicated in the network, this extension to the SACK option can identify this. For example:
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 500-999 1000
1000-1499 1000-1499 1500
(replicated)
1000-1499 1500, SACK=1000-1500
---------
In this case, the second packet was replicated in the network. An
ACK containing a D-SACK block which is lower than its ACK field and
is not identical to a previously retransmitted segment is indicative of a replication by the network.
WITHOUT D-SACK:
If D-SACK was not used and the last ACK was piggybacked on a data
packet, the sender would not know that a packet had been replicated
in the network. If D-SACK was not used and neither of the last two
ACKs was piggybacked on a data packet, then the sender could
reasonably infer that either some data packet *or* the final ACK
packet had been replicated in the network. The receipt of the D-SACK packet gives the sender positive knowledge that this data packet was replicated in the network (assuming that the receiver is not lying).
RESEARCH ISSUES:
The current SACK option already allows the sender to identify
duplicate ACKs that do not acknowledge new data, but the D-SACK
option gives the sender a stronger basis for inferring that a
duplicate ACK does not acknowledge new data. The knowledge that a
duplicate ACK does not acknowledge new data allows the sender to
refrain from using that duplicate ACKs to infer packet loss (e.g.,
Fast Retransmit) or to send more data (e.g., Fast Recovery).
5.2. False retransmit due to reordering
If packets are reordered in the network such that a segment arrives
more than 3 packets out of order, TCP’s Fast Retransmit algorithm
will retransmit the out-of-order packet. An example of this is shown below:
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 500-999 1000
1000-1499 (delayed)
1500-1999 1500-1999 1000, SACK=1500-2000
2000-2499 2000-2499 1000, SACK=1500-2500
2500-2999 2500-2999 1000, SACK=1500-3000
1000-1499 1000-1499 3000
1000-1499 3000, SACK=1000-1500
---------
In this case, an ACK containing a SACK block which is lower than its ACK field and identical to a previously retransmitted segment is
indicative of a significant reordering followed by a false
(unnecessary) retransmission.
WITHOUT D-SACK:
With the use of D-SACK illustrated above, the sender knows that
either the first transmission of segment 1000-1499 was delayed in the network, or the first transmission of segment 1000-1499 was dropped
and the second transmission of segment 1000-1499 was duplicated.
Given that no other segments have been duplicated in the network,
this second option can be considered unlikely.
Without the use of D-SACK, the sender would only know that either the first transmission of segment 1000-1499 was delayed in the network,
or that either one of the data segments or the final ACK was
duplicated in the network. Thus, the use of D-SACK allows the sender
to more reliably infer that the first transmission of segment
1000-1499 was not dropped.
[AP99] and [L99] both note that the sender could unambiguously detect an unnecessary retransmit with the use of the timestamp option. In
addition, [AP99] proposes a heuristic for detecting an unnecessary
retransmit in an environment with neither timestamps nor SACK. [L99] also proposes a two-bit field as an alternate to the timestamp option for unambiguously marking the first three retransmissions of a
packet. A similar idea was proposed in [ISO8073].
RESEARCH ISSUES:
The use of D-SACK allows the sender to detect some cases (e.g., when no ACK packets have been lost) when a a Fast Retransmit was due to
packet reordering instead of packet loss. This allows the TCP sender to adjust the duplicate acknowledment threshold, to prevent such
unnecessary Fast Retransmits in the future. Coupled with this, when the sender determines, after the fact, that it has made an
unnecessary window reduction, the sender has the option of
‘‘undoing’’ that reduction in the congestion window by resetting
ssthresh to the value of the old congestion window, and slow-starting until the congestion window has reached that point.
Any proposal for ‘‘undoing’’ a reduction in the congestion window
would have to address the possibility that the TCP receiver could be lying in its reports of received packets [SCWA99].
5.3. Retransmit Timeout Due to ACK Loss
If an entire window of ACKs is lost, a timeout will result. An
example of this is given below:
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 500-999 1000 (ACK dropped)
1000-1499 1000-1499 1500 (ACK dropped)
1500-1999 1500-1999 2000 (ACK dropped)
2000-2499 2000-2499 2500 (ACK dropped)
(timeout)
500-999 500-999 2500, SACK=500-1000
--------
In this case, all of the ACKs are dropped, resulting in a timeout.
This condition can be identified because the first ACK received
following the timeout carries a D-SACK block indicating duplicate
data was received.
WITHOUT D-SACK:
Without the use of D-SACK, the sender in this case would be unable to decide that no data packets has been dropped.
RESEARCH ISSUES:
For a TCP that implements some form of ACK congestion control
[BPK97], this ability to distinguish between dropped data packets and dropped ACK packets would be particularly useful. In this case, the connection could implement congestion control for the return (ACK)
path independently from the congestion control on the forward (data) path.
5.4. Early Retransmit Timeout
If the sender’s RTO is too short, an early retransmission timeout can occur when no packets have in fact been dropped in the network. An
example of this is given below:
Transmitted Received ACK Sent
Segment Segment (Including SACK Blocks)
500-999 (delayed)
1000-1499 (delayed)
1500-1999 (delayed)
2000-2499 (delayed)
(timeout)
500-999 (delayed)
500-999 1000
1000-1499 (delayed)
1000-1499 1500
...
1500-1999 2000
2000-2499 2500
500-999 2500, SACK=500-1000
--------
1000-1499 2500, SACK=1000-1500
---------
...
In this case, the first packet is retransmitted following the
timeout. Subsequently, the original window of packets arrives at the receiver, resulting in ACKs for these segments. Following this, the retransmissions of these segments arrive, resulting in ACKs carrying SACK blocks which identify the duplicate segments.
This can be identified as an early retransmission timeout because the ACK for byte 1000 is received after the timeout with no SACK
information, followed by an ACK which carries SACK information
(500-999) indicating that the retransmitted segment had already been received.
WITHOUT D-SACK:
If D-SACK was not used and one of the duplicate ACKs was piggybacked on a data packet, the sender would not know how many duplicate
packets had been received. If D-SACK was not used and none of the
duplicate ACKs were piggybacked on a data packet, then the sender
would have sent N duplicate packets, for some N, and received N
duplicate ACKs. In this case, the sender could reasonably infer that some data or ACK packet had been replicated in the network, or that
an early retransmission timeout had occurred (or that the receiver is lying).
RESEARCH ISSUES:
After the sender determines that an unnecessary (i.e., early)
retransmit timeout has occurred, the sender could adjust parameters
for setting the RTO, to prevent more unnecessary retransmit timeouts. Coupled with this, when the sender determines, after the fact, that
it has made an unnecessary window reduction, the sender has the
option of ‘‘undoing’’ that reduction in the congestion window.
6. Security Considerations
This document neither strengthens nor weakens TCP’s current security properties.
7. Acknowledgements
We would like to thank Mark Handley, Reiner Ludwig, and Venkat
Padmanabhan for conversations on these issues, and to thank Mark
Allman for helpful feedback on this document.
8. References
[AP99] Mark Allman and Vern Paxson, On Estimating End-to-End Network Path Properties, SIGCOMM 99, August 1999. URL
‘‘https://www.doczj.com/doc/3914644328.html,/sigcomm/sigcomm99/papers/session7-3.html’’.
[BPK97] Hari Balakrishnan, Venkata Padmanabhan, and Randy H. Katz,
The Effects of Asymmetry on TCP Performance, Third ACM/IEEE Mobicom
Conference, Budapest, Hungary, Sep 1997. URL
‘‘https://www.doczj.com/doc/3914644328.html,/?padmanab/index.html#Publications’’.
[F99] Floyd, S., Re: TCP and out-of-order delivery, Message ID
<199902030027.QAA06775@https://www.doczj.com/doc/3914644328.html,> to the end-to-end-interest
mailing list, February 1999. URL
‘‘https://www.doczj.com/doc/3914644328.html,/floyd/notes/TCP_Feb99.email’’.
[ISO8073] ISO/IEC, Information-processing systems - Open Systems
Interconnection - Connection Oriented Transport Protocol
Specification, Internation Standard ISO/IEC 8073, December 1988.
[L99] Reiner Ludwig, A Case for Flow Adaptive Wireless links,
Technical Report UCB//CSD-99-1053, May 1999. URL
‘‘https://www.doczj.com/doc/3914644328.html,/papers/Ludwig-FlowAdaptive/’’.
[RFC1323] V. Jacobson, R. Braden, and D. Borman, TCP Extensions for
High Performance, RFC 1323, May 1992. URL
‘‘https://www.doczj.com/doc/3914644328.html,/rfc/rfc1323.txt’’.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and Romanow, A., TCP
Selective Acknowledgement Options. RFC 2018, Proposed Standard, April 1996. URL ‘‘ftp://https://www.doczj.com/doc/3914644328.html,/in-notes/rfc2018.txt’’.
[RFC2581] M. Allman, V. Paxson, W. Stevens, TCP Congestion Control,
RFC 2581, Proposed Standard, April 1999. URL ‘‘ftp://https://www.doczj.com/doc/3914644328.html,/in- notes/rfc2581.txt’’.
[SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, Tom Anderson, TCP Congestion Control with a Misbehaving Receiver, draft paper,
April 1999.
AUTHORS’ ADDRESSES
Sally Floyd
AT&T Center for Internet Research at ICSI (ACIRI)
Phone: +1 510-642-4274 x189
EMail: floyd@https://www.doczj.com/doc/3914644328.html,
URL: https://www.doczj.com/doc/3914644328.html,/floyd/
Jamshid Mahdavi
Novell
Phone: 1-408-967-3806
Email: mahdavi@https://www.doczj.com/doc/3914644328.html,
Matt Mathis
Pittsburgh Supercomputing Center
Phone: 412 268-3319
Email: mathis@https://www.doczj.com/doc/3914644328.html,
URL: https://www.doczj.com/doc/3914644328.html,/?mathis/
Matthew Podolsky
UC Berkeley Computer Science Dept.
Phone: 510-649-8914
Email: podolsky@https://www.doczj.com/doc/3914644328.html,
URL: https://www.doczj.com/doc/3914644328.html,/?podolsky Allyn Romanow
Cisco Systems
Phone: 408-525-8836
Email: allyn@https://www.doczj.com/doc/3914644328.html,
This draft was created in August 1999.
It expires February 2000.
集成电路专业学年论文 论文题目：MEMS光开关的研究及市场分析学院：电子工程学院 年级：2008级 专业：集成电路设计与集成系统 姓名：刘欣 学号：20083410 指导教师：窦雁巍 2011年7月8日
摘要 光开关是光通信网络的重要功能器件,MEMS光开关是最具发展前景的光开关之一。在简介不同种类光开关原理特点的基础上，详细分析了当前主要的MEMS光开关的分类、结构、工艺与性能特点，并给出了研究与发展情况和采用MEMS体硅工艺制作的三种结构的微机械光开关。它们的工作原理都基于硅数字微镜技术。这三种光开关采用了静电力驱动，具有较低的驱动电压。在硅基上制作了光纤自对准耦合槽，并对光开关的开关特性进行了计算机模拟与分析，并进行结果分析。 关键词 微机械；光开关；开关阵列；微镜；硅-玻璃键合；光纤通信
Abstract Optical switch is an important functional device in optical fibre communication networks, MEMS optical switch is one of the most promiseful optical switches. This paper introduces basic principles and characters of several kinds of optical switches, and illustrates the classification, structures, fabrication methods and functional characters of current MEMS optical switch in details. And recent development and progress on this research area are presented and three kinds of MEMS optical switches with different mechanical structures are produced by the bulk-micromachining processes. Their principles of operation are all based on silicon digital micro mirrors technology. The electrostatic actuators with low driving voltage are used in the three kinds of optical switch. The grooves used for optical fibers being self-aligned coupling are made on silicon substrate for device. Computer simulation and analysis of on-off characteristic show that the second and the third optical switches have switching time. Key words MEMS; optical switch; switch array; micro mirror; silicon-on-glass bonding; ptical fiber communication
一、简介 1.概述 DMP300型微机变压器差动保护测控装置，适用于110KV及以下电压等级的三圈变或两圈变，具有开入采集、脉冲电度量采集、遥控输出、通讯功能。其中DMP321适用于三圈变，DMP322适用于两圈变。 保护功能：a）差电流速断保护 b）二次谐波制动的比率差动保护 c）CT断线识别和闭锁功能 d）过负荷告警 e）过载启动风冷 f）过载闭锁有载调压 遥信量采集：a）本体轻、重瓦斯信号 有载轻、重瓦斯信号 压力释放信号 变压器超温告警 b）主变一侧开关的弹簧未储能、压力异常闭锁、报警 c）从主变一侧开关操作箱中采集开关跳、合位，手跳、手合开关量脉冲电量：一路有功脉冲电度、一路无功脉冲电度 遥控：遥控主变一侧开关 2.特点： 1）差动保护中各侧电流平衡补偿由软件完成，中低压侧电流不平衡系数均以高压侧为基准。变压器各侧CT二次电流相位也由软件自动校正，即变压器各侧CT二次回路可接成丫型（也可选择常规接线），这样简化了CT二次接线，增加了可靠性。 1)变压器保护的差动保护与后备保护完全独立，各侧后备也完全独立，独立的工作电 源、CPU实现真正意义上的主、后备保护，极大地提高了主变保护的可靠性。 2)通过菜单可直接查看主变各侧电流值的大小、相位关系，差电流大小，方便用户调 试与主变投运。 3)选用高性能、高可靠性的80C196单片机，高度集成的PSD可编程外围芯片；宽温军 用、工业级芯片；高精度阻容元件；进口密封继电器。 4)抗干扰、抗震动的结构设计
全封闭金属单元机箱，箱内插板间加装隔离金属屏蔽板；高可靠性的进口接插件，加装固定挡条。 5)独到的多重抗干扰设计 单元装置采取了隔离、软硬件滤波、看门狗电路、智能诊断各种开放闭锁控制，ALL IN ONE的主板电路设计原则，新型结构设计等多种抗干扰措施，取得了良好的效果。 6)体积小、模块化，既可安装于开关柜，构成分散式系统，又可集中组屏。 7)大屏幕液晶汉字显示运行参数、菜单，具有极好的人机界面，操作简单、直观、易 学、易用。 8)所有保护功能均可根据需要直接投退，操作简单。 9)软件实现交流通道的模拟量精度调整，取消了传统的采保通道的误差补偿电位器， 不但简化了硬件，更方便了现场调试、校验，还提高了精度。 10)独到的远动试验菜单功能。装置中设有“远动试验”菜单,通过菜单按钮进行远动信息 传输试验,如“差动速断动作”、“高压侧CT断线告警”等，无需试验接点真正闭合，可在线试验，方便了远动调试。 11)多层次的PASSWORD：运行人员口令、保护人员口令、远动人员口令。 12)事件记录分类记录32条故障信息，32条预告信息，8条自检信息，并具掉电保持功 能。
RCS-9611C 线路保护测控装置技术使用说明书 V1.00 南瑞继保电气 2005年1月
RCS-9611C线路保护测控装置 1基本配置及规格： 1．1基本配置 RCS-9611C适用于110KV以下电压等级的非直接接地系统或小电阻接地系统中的线路保护及测控装置，可在开关柜就地安装。 保护方面的主要功能有：1）三段可经复压闭锁的方向过流保护；2）三段零序过流保护；3）三相一次重合闸；4）过负荷功能；5﹚独立过流和零序过流加速保护；6）低周减载功能；7）小电流接地选线；8）独立的操作回路。 测控方面的主要功能有：1）25路遥信开入采集；2）正常断路器遥控分合、小电流接地选线；3）IA、IC、I0、UA、UB、UC、UAB、UBC、UCA、U0、F、P、Q、COSф共14个模拟量的遥测；4）事件SOE等； 保护信息方面的主要功能：1）装置描述的远方查看；2）装置参数的远方查看；3）保护定值和区号的远方查看、修改功能；4）软压板状态的远方查看、投退；5）装置保护开入状态的远方查看；6）装置运行状态（包括保护动作元件的状态、运行告警和装置的自检信息）的远方查看；7）远方对装置实现信号复归；8）故障录波上送功能。 支持电力行业标准DL/T667-1999（IEC60870-5-103标准）的通讯规约，配有以太网，双网，100Mbps，超五类线或光纤通讯接口。 1．2技术数据 1．2．1额定数据 直流电压：220V，110V 允许偏差+15％，-20％ 交流电压：100/3V（相电压），100V（线电压） 交流电流：5A，1A 频率：50Hz 1．2．2功耗 交流电压：< 0.5VA/相 交流电流：< 1.0VA/相(In =5A) < 0.5VA/相(In =1A) 直流：正常 < 15W 跳闸 < 25W 1．2．3主要技术指标 1>定时限过流： 电流定值：0.1In～20In 定值误差： < 5％ 时间定值：0～100S 2>零序过流保护 电流定值：0.1A～12A 定值误差： < 5％ 时间定值：0～100S 3>低周减载 频率定值：45～50Hz
红外线光控开关电路图及工作原理一、特点 该装置采用锁相环单音检测电路ＬＭ５６７构成自发射自接收的闭环控制形式。就是说，把ＬＭ５６７产生的方波电信号调制在红外线光信号上并发射出去，红外线光敏二极管接收该信号，并把其变为电信号，经放大，又被该ＬＭ５６７自身检测。这样，ＬＭ５６７自身的振荡频率与要接收的信号频率永远相同，即使由于某种原因使ＬＭ５６７的振荡频率发生了变化。在一定的频带宽度内，由于ＬＭ５６７只对与自身振荡频率非常接近的信号产生响应，而对其他频率的干扰信号不响应，所以，该装置具有可靠性高、抗干扰性强、安装调试简单的特点。该装置可应用于自动门、自动水龙头、防盗报警、危险区域误入报警、警戒区域侵入报警等控制。 二、工作原理 电路原理图见图１。红外线光敏二极管ＰＨ检测到由红外线发射二极管ＬＥ发出的红外线光信号，并将其转换成电信号。该信号经由ＩＣ１Ａ构成有源高通滤波器，滤除外界低频干扰信号；再经ＩＣ１Ｂ、ＩＣ１Ｃ两级固定增益放大器的放大、以及ＩＣ１Ｄ可调增益限幅放大器的放大，进入锁相环单音检测电路 ＩＣ２的第③脚。ＩＣ２检测到与自身振荡频率相同的信号后，其第⑧脚输出低电平，使继电器ＤＬ吸合，触点Ｓ１、Ｓ２接通，控制其他设备。ＩＣ２第⑧脚的最大吸入电流为１００ｍＡ。ＩＣ２第⑤脚输出的方波信号，经Ｃ８、Ｒ１６组成的微分电路和Ｎ１、Ｎ２驱动电路，使红外线发射二极管发出该频率调制的红外线光信号。微分电路使正方波信号变为低占空比的方波信号。用低占空比方波调制红外线发射管，可提高红外线发射管的工作效率，即其峰值电流很大，而平均工作电流却很小。这样，有利于红外线光敏二极管的接收。电阻Ｒ１２、Ｒ１３和电解电容Ｅ３是集成电路ＩＣ１的中点电位偏置电路，使ＩＣ１工作于单电源方式。该装置有两种工作方式。一种是：红外线发射二极管和红外线光敏二极管都在同一侧，构成反射检测方式，见图２。另一种是：红外线发射二极管在一侧，而红外线光敏二极管在另一侧，构成对射式检测方式，见图３。一般情况下，反射式控制距离可达两米，对射式控制距离可达五米。控制距离的远近可由调节电位器Ｗ来控制，Ｗ的阻值越大，ＩＣ１Ｄ放大器的增益越大，控制距离越远。反之，控制距离越近。如果给红外线发射二极管或光敏二极管一方加上光学透镜，可增加控制距离；给双方都加上光学透镜，更可增加控制距离。红外线发射二极管的外面要套上长度为５０ｍｍ左右的金属管，以防止其散射光干扰红外接收管。 电解电容Ｅ５的容量越大，抗干扰性越好，但响应的时间也越长，一般Ｅ５的选取范围是１０μＦ～１００μＦ。由于该装置工作在闭环状态，所以对ＩＣ２工作频率的稳定度要求不严格，并且可在很宽的范围内设定频率值，范围可达５ｋＨｚ～４０ｋＨｚ，频率由电
名师支招：新托福口语Task 3-6 完全攻略：征服新托福口语Task 3 “To succeed academically in English-speaking colleges and universities，students need to be able to combine all their English-language skills- in reading，listening，and speaking…”，所以在新托福口语task3中考生们首先遭遇了先读，再听，后说相结合的考试形式，虽然内容为较易理解的校园生活场景，但环环相扣的考试环节加上严格紧张的时间限制，不免给备考带来了种种压力。在本文中，笔者将根据Task 3 各个环节中经常出现的误区和问题进行探讨,并给出相应的解决方案，以助托福准考生们一臂之力。 一．怎样“读”? 误区：阅读部分考察“记忆”能力 相对于托福考试的阅读部分，Task3中只有75-100字的简短阅读材料简直是小巫见大巫。可正是因为这样，却偏偏激发了好多考生的“完美主义”心态，想要把所有内容统统记下，结果因小失大，笔记中过多的细节淹没了重点信息。其实Task3中阅读资料的真正作用是为听力提供背景，唯一的价值是告诉考生接下来的对话中两人谈论的政策或计划等是什么，所以不应浪费时间记过多细节。 支招：笔记中只需要notice/message/proposal (key words) + reasons(keywords)。其中关于政策的关键词多半出现在题目以及文章的前两行，而通过一些明显的“信号”也可以轻松找到给出理由的关键词。例如：(Official Guide P213)announcement 关键词位于第一行“increase tuition”，而通过“but itis necessary to increase them now for several reasons”和“we havealso…”可以找到并列关系的两组理由关键词“more students，additionalprofessors”和“laboratoryfacilities”。所以笔记可记做：学费（tuition）↑，学生（students）+，老师（professors）+，实验设备（labfacilities）+(可根据习惯中英符号相结合)。二．怎样“听”? 误区1：对话中的两个人都听 我们都知道在最终的口语表达中只需陈述其中一个人的态度及理由，所以两个说话者的信息都听无疑只会分散精力，并且另一个人所描述的信息也可能会被考生误引入到最后的口语表达中,违背题目要求给出错误信息。 支招：找出主角（leading speaker），只听一人 在听力对话一开始的时候不要着急记笔记，这样很容易无法分清主次。先把两个人的分别一句话听完，基本就可以分清主角和次角，因为，主角往往会持有很强烈的支持或反对态度，确定下主次以后，就可以专注只听一个人并记笔记了。误区2：听力对话考察“听写”能力 有的考生曾经抱怨说自己的笔记都快赶上听力原文了，可是分数却不尽如人意。其实这又是“完美主义”心态在作怪了。考生总是不放心，对话中的每一个细节列举都不放过，结果在口语表达时内容冗长，却依然会遗漏重点信息，而这正是Task3丢分的主要原因之一。托福口语考察的是抓住重点的能力，并不是考生的书写速度。 支招：笔记只需要记主角所给出的态度（赞成/反对）+reasons（key words） 在听力对话中寻找关键信息，快速识别信号词或信号句就显得更为重要，这是听力的必备技巧之一。例如在(Official GuideP214-215)，我们可以看到“But I can see why”，“ And the other thing is ”，“I’mdoing OK, but the facilities here are so limited.”这些明显的信号句所引出的关键信息，所以笔记可记为：many students（班级+），hard get personalattention （关注?），not job（工作难），facilities?equipment ?（设备?）out of date（过时） 三．怎样“说”? 误区：语速要快，尤其是在时间快结束时，这样才会多提供信息防止丢分。 考生在时间来不及时慌慌张张讲得飞快，是因为想把笔记中的内容讲完，但在托福考官看来“how clearly and coherentlyyou convey information is as important as how much information youconvey”（表达的清晰与连贯和表达内容的多少是一样重要的），仓促作答一定顾不得清晰与连贯，所以这样的表现往往会失分。 支招：备考时采用“计时录音”的方法进行练习。 如果平时练习时就有时间的概念，那么考试时对口语表达的内容长短就比较有把握了，练习越多，就越胸有成竹。这个
iPACS-5711线路保护测控装置 技术说明书 版本：V2.01 江苏金智科技股份有限公司
目录 1 概述 (1) 1.1应用范围 (1) 1.2保护配置和功能 (1) 1.2.1 保护配置 (1) 1.2.2 测控功能 (1) 1.2.3 保护信息功能 (1) 2 技术参数 (2) 2.1机械及环境参数 (2) 2.1.1 工作环境 (2) 2.1.2 机械性能 (2) 2.2电气参数 (2) 2.2.1 额定数据 (2) 2.2.2 功率消耗 (2) 2.2.3 过载能力 (3) 2.3主要技术指标 (3) 2.3.1 过流保护 (3) 2.3.2 零序保护 (3) 2.3.3 低频保护 (3) 2.3.4 重合闸 (3) 2.3.5 遥信开入 (4) 2.3.6 遥测量计量等级 (4) 2.3.7 电磁兼容 (4) 2.3.8 绝缘试验 (4) 2.3.9 输出接点容量 (4) 3 软件工作原理 (5) 3.1保护程序结构 (5) 3.2装置起动元件 (5) 3.2.1 过电流起动 (5)
3.2.2零序电流起动 (6) 3.2.3低频起动 (6) 3.2.4位置不对应起动 (6) 3.3过流保护 (7) 3.4零序保护（接地保护） (8) 3.5过负荷保护 (9) 3.6加速保护 (9) 3.7低频保护 (9) 3.8重合闸 (9) 3.9装置自检 (10) 3.10装置运行告警 (10) 3.10.1 TWJ异常判别 (10) 3.10.2 交流电压断线 (11) 3.10.3 线路电压断线 (11) 3.10.4 频率异常判别 (11) 3.11遥控、遥测、遥信功能 (11) 3.12对时功能 (11) 3.13逻辑框图 (12) 4 定值内容及整定说明 (13) 4.1系统定值 (13) 4.2保护定值 (13) 4.3通讯参数 (15) 4.4辅助参数 (16) 4.5软压板 (17) 5装置接线端子与说明 (18) 5.1模拟量输入 (19) 5.2背板接线说明 (19) 5.3跳线说明 (21)
一、前言 光纤通信技术的问世和发展给通信业带来了革命性的变革，目前世界大约85％的通信业务经光纤传输，长途干线网和本地中继网也已广泛使用光纤。同时，密集波分复用(DWDM) 技术的发展和成熟为充分应用光纤传输的带宽和容量开拓了广阔的空间，具有高速率、大带宽明显优势的DWDM 光通信网络已经成为目前通信网络发展的趋势。特别是近几年，以IP 为主的Internet 业务呈现爆炸性增长，这种增长趋势不仅改变了IP 网络层与底层传输网络的关系，而且对整个网络的组网方式、节点设计、管理和控制提出了新的要求。一种智能化网络体系结构—自动交换光网络(ASON ：automatic switched optical networks) 成为当今系统研究的热点，它的核心节点由光交叉连接(OXC ：optical cross connect) 设备构成，通过OXC ，可实现动态波长选路和对光网络灵活、有效的管理。光交叉互连(OXC) 技术在日益复杂的DWDM 网中是关键技术之一，而光开关作为切换光路的功能器件，则是OXC 中的关键部分。光开关矩阵是OXC 的核心部分，它可实现动态光路径管理、光网络的故障保护、波长动态分配等功能，对解决目前复杂网络中的波长争用，提高波长重用率，进行网络灵活配置均有重要的意义。 光开关不仅是OXC 中的核心器件，它还广泛应用于以下领域。 (1)光网络的保护倒换系统，实际的光缆传输系统中都留有备用光纤，当工作通道传输中断或性能劣化到一定程度，光开关将主信号自动转至备用光纤系统传输，从而使接收端能接收到正常信号而感觉不到网路已出了故障，其会将网络节点连成环形以进一步改善网络的生存性。 (2)网络性能的实时监控系统，在远端光纤测试点，通过1XN多路光开关把多根 光纤接到光时域反射仪上，进行实时网络监控，通过计算机控制光开关倒换顺序和时间，实现对所有光纤的检测，并将检测结果传回网络控制中心，一旦发现某一路出现问题，可在网管中心直接进行处理。 ( 3)光开关还应用在光纤通信器件测试系统以及城域网、接入网的差／分复用和交 换设备中。光开关的引入使未来全光网络更具灵活性、智能性、生存性。光开关技术已经成为未来光联网、光交换的关键技术，在通信、自动控制等领域发挥着越来越重要的作用。 在众多种类的光开关中，微机械(MEMS) 光开关被认为最有可能成为光开关的主流器件。本文在概述多种光开关原理特点的基础上，重点分析了几种主要的MEMS 光开关，并阐述了各自的结构与性能特点。 二、光开关的原理及种类 光开关性能参数有多种，如：快切换速度、高隔离度、小插入损耗、对偏振不敏感及可靠性，不同领域对它的要求也各不相同。其种类有保护、切换系统中常用的传统光机械开关，也有这几年飞速发展的新型光开关，如：热光开关、液晶开关、电光开关、声光开关、微光机电系统光开关(MOEMS ，micro optic electro mechanical systems) 、 气泡开关等。在超高速光通信领域，还有马赫-曾德尔(Maeh-Zehnder) 干涉型光开关、非线性环路镜(NOLM ，nonlinear optical fiber loop mirror) 光开关等光控开关。 1、机械光开关 传统机械光开关的工作原理：通过热、静电等动力，旋转微反射镜，将光直接送到或反射到
2019年TPO20托福口语Task3原文及参考答案 TPO20口语Task3题目： Energy costs reach sky-high levels Heating prices, and the university’s heating bills, reached record-high levels this year. In an effort to reduce heating costs, the university plans to lower the temperature in all classroom buildings on weekends and on weekdays after five P.M., when the day’s classes have officially ended. Because classes will not be in session at these times, the reduced hear should have no negative impact on students and their studies. The university considers it essential that the cost-saving measures not interfere with student activities. ConversationNow listen to two students discussing the article. Woman： What on earth is the university thinking? Man： Huh? What do you mean? Woman：Well, those buildings aren’t just used for classes. That’s where my evening st udy group usually meets. Man：Oh, of course, that’s true. But you could go to the library instead. Woman： Yeah, but the library only has a few rooms that people can use for group meetings which is why classrooms are so convenient. But now, if it’s wint er and cold, where can we go?
BDF100系列低压馈线保护 技术资料 北京北斗银河科技有限公司 版本号:V2.7 技术不断创新,请随时联系,证实本版资料目录 1 概述 (1 2 产品选型 (2 2.1功能详表 (2 2.2产品选型表 (3 2.3订货须知 (3 3 产品系列 (4 3.1BDF100-C系列 (5 3.1.1操作面板 (5 3.1.2端子示意 (5 3.1.3端子定义 (5 3.2BDF100-M系列 (6 3.2.1操作面板 (6 3.2.2端子示意 (6
3.2.3端子定义 (7 3.3BDF100-T+系列 (8 3.3.1操作面板 (8 3.3.2端子示意 (8 3.3.3端子定义 (8 3.4外形尺寸 (9 3.4.1BDF100-C系列外形尺寸 (9 3.4.2BDF100-M/M+内置外形尺寸 (9 3.4.3BDF100-T+、BDF100-M系列外形尺寸 (9 4 应用说明 (10 4.1专用外置互感器 (10 4.1.1BDCTAD-00外置式电流互感器 (10 4.1.2BDCTAD-01外置式电流电压互感器 (11 4.1.3BDCTL外置式漏电流互感器 (12 4.2模拟量 (13 4.2.1电流输入方式 (13 4.2.2电流输入接线图 (13 4.2.3零序电流与漏电流 (14 4.2.4电压输入方式 (14
4.2.5模拟量输出 (14 4.3开关量 (15 4.3.1开关量输入 (15 4.3.2开关量输出 (15 4.4事件记录 (15 4.5面板控制功能 (15 4.6通信与系统 (16 5技术说明 (18 5.1线路保护功能 (18 5.1.1速断保护 (18 5.1.2过流一段保护 (18 5.1.3过流二段保护 (18 5.1.4反时限过流保护 (19 5.1.5零序过流一段保护 (19 5.1.6零序过流二段保护 (20 5.1.7零序过流三段保护 (20 5.1.8漏电流保护 (21 5.1.9低电压保护 (22 5.1.10过电压保护 (22
光开关,光开关的分类,光开关的工作原理是 什么? 2010 年03 月20 日 17:30 www.elecfans.co 作者：佚名用户评论（0） 关键字：光开关(7) 光开关,光开关的分类,光开关的工作原理是什么? 光开关是一种具有一个或多个可选择的传输窗口，可对光传输线路或集成光路中的光信号进行相互转换或逻辑操作的器件。 机械式光开关：插入损耗低；隔离度高；不受偏振和波长影响；开关时间长(ms)，重复性较差。 其它光开关：开关时间短(ms)；体积小；插入损耗大；隔离度低。 光开关的特性参数 1.插入损耗(Insertion loss) 2.回波损耗（Return loss） 从输入端返回的光功率与输入光功率的比值。
3.隔离度 两个相隔离的输出端口光功率的比值。 4. 串扰 输入光功率与从非导通端口输出的光功率的比值。 5.消光比 两个端口处于导通和非导通状态的插入损耗之差。 ER＝IL－IL0 6.开关时间 开关端口从某一初状态转为通或者断所需的时间。从在开关上施加或撤去能量的时刻算起。 光开关的工作原理： 1. 机械式光开关
移动光纤式光开关 移动反射镜式光开关 以上两种体积大，难实现集成化的开关网络。近年正大力发展一种集成的微机电系统(MEMS)开关，在硅片上用微加工技术做出大量可移动的微型镜片构成的开关阵列。 用16 个移动反射镜光开关构成的两组4 4MEMS开关阵列 2 电光开关
电光开关的原理一般是利用材料的电光效应或电吸收效应，在电场作用下改变材料的折射率和光的相位，再利用光的干涉或偏振等使光强突变或光路转变。 电光开关一般利用泡克耳斯(Pockels) 效应，即折射率 n随光场E而变化的电光效应。 折射率变化与光场的变化关系为： 而光波传输距离L相应的相位变化为： 定向耦合型光开关 定向耦合器中两耦合波导光功率周期性相互转换
2019年TPO22托福口语Task3原文及参考答案 TPO22口语Task3题目： University Announces Change in Campus Tour Guides Traditionally, staff members from the admissions office have led the campus tours for secondary-school students who are considering attending the university. Beginningnext semester, however, current students will lead these guided walks around the campus instead. “The staff of the admissions office no longer have time to lead the campus tours. We need to focus our resources on our other responsibilities,” explain Christine Tanner, the director of admissions.“Furthermore, we feel that current students will offer unique and valuable insight to our visitors during these campus tours.” Conversation Now listen to two students discussing the article. Man： So, what do you think of this? Woman：Well, I think it’s a good idea, necessary, too. Man： Why is that? Woman：Well, you know, I’ve actually been up to the admissions office a couple of times, like, to pick up some information for a friend, you know, and every time I’ve ever been there, wow, you can tell they’re really busy. Man： Oh, really.
托福综合口语TASK2各类实用模板分 享 托福口语和写作一样，都是属于需要考生自己组织语言来进行表达输出的考试题型。今天给大家带来了托福综合口语TASK2各类实用模板分享,希望能够帮助到大家，下面就和大家分享，来欣赏一下吧。 【精选素材】托福综合口语TASK2各类实用模板分享 托福综合口语TASK2备考模版 1、“The school has implemented a new policy that ... due to ... . And the man/woman holds a positive/negative view towards the announcement. The first reason s/he gives is that ... . And the second one is based on the fact that ... .” 2、From the reading material, we know that (the college) is going to ... Obviously, the man/woman in the conversation thinks that this is a great/bad idea, due to the following reasons..
One reason is that……. Another is,……. (万一有时间)So thats all the reasons s/he has to form that opinion. (尽量记，听为主) 3、The man/woman’s opinion about STH is that……. (选择方案型：听选择的原因。1.Firstly, the man/woman states that…….2.And then he/she states that…….) (提出建议型：听建议的利弊。1.Firstly, the man/woman talks about the positive aspect of this issue. He/she states that…….2.On the other hand, the man/man also talks about the negative aspect of this question. He/she points out that…….) 4、阅读引用部分的模版 (1) In the readingmaterial, (2) There is a/anannouncement/message/notice/proposal about ..记下的关键词) (3) The university/college is going to..(稍稍展开下) (4) In the listeningmaterial, two students discussabout the..填入关键词) 听力中人物表达观点的模版:
第二章技术说明 DA-R711线路保护测控装置 1 功能 ●三段式过流保护（可经电压启动，可带方向） ●三段式零序过流保护（可带方向） ●过负荷告警 ●重合闸 ●合闸加速保护 ●低频减载 ●低压减载 ●手动检同期功能 ●小电流接地选线 ●I，U，P，Q，Cosφ，有功电度，无功电度，14路开关量采集 ●GPS对时（分脉冲，秒脉冲或IRIG-B方式） 2 原理说明 2.1 三段式过流保护 当任一相电流大于定值，经延时，装置跳闸。 三段过流保护均可由控制字独立选择投入或退出，是否需要经电压启动，是否带方向。 当选择经电压启动时：A相电流经Uab、Uca电压启动，B相电流经Uab、Ubc电压启动，C相电流经Ubc、Uca电压启动。 当选择带方向时：Ia与Ubc组成A相方向元件，Ib与Uca组成B相方向元件，Ic与Uab组成C相方向元件。当电流相对与电压的相角为（－30°～＋90°）时，为正方向。方向元件带有记忆功能以消除近端三相短路时方向元件的死区。 当装置检测出母线PT断线时，装置将根据控制字选择经低压启动或带方向的过流保护退出或改为纯过流保护，对于没有经电压启动和未带方向的纯过流保护不受PT断线影响。 三段过流控制字定值取值含义为： 0：退出，1～4：投入--1：单纯过流，2：方向过流，3：低压闭锁过流，4：低压闭锁方向过流保护的动作条件见逻辑图。
2.2 三段式零序过流保护（可带方向） 当一次系统采用小电阻接地方式时，如3I0大于定值，经延时，零序I段和II段跳闸，零序Ⅲ段可经控制字选择跳闸或告警。 如考虑带方向，则成为三段零序方向过流保护。 当3U0相对与3I0相角为（－75°～－195°）时，认为是正方向。 零序电压3U0由保护自产，即3U0＝Ua+Ub+Uc。当3U0<2V时，自动闭锁零序方向过流保护。 零序电流3I0由保护自产，取三相保护电流之和，即3I0＝Ia+Ib+Ic。 当装置检测出母线PT断线时，装置自动闭锁零序方向过流保护。对于未带方向的零序流保护不受PT 断线影响。 三段零序过流保护控制字定值取值含义为： 0：退出，1～2：投入--1：单纯零序过流，2：零序方向过流 保护的动作条件见逻辑图。
光开关,光开关得分类,光开关得工作原理就 是什么？ 2010年03月２0日 17:３0 作者:佚名用户评论(0） 关键字：光开关（7） 光开关,光开关得分类，光开关得工作原理就是什么? 光开关就是一种具有一个或多个可选择得传输窗口,可对光传输线路或集成光路中得光信号进行相互转换或逻辑操作得器件。 机械式光开关：插入损耗低；隔离度高;不受偏振与波长影响;开关时间长（ｍs），重复性较差。 其它光开关：开关时间短(mｓ）;体积小；插入损耗大;隔离度低。 光开关得特性参数 1、插入损耗（Insｅrtion lｏss） ２、回波损耗（Return loss) 从输入端返回得光功率与输入光功率得比值。 3、隔离度 两个相隔离得输出端口光功率得比值. 4、串扰 输入光功率与从非导通端口输出得光功率得比值。
5、消光比 两个端口处于导通与非导通状态得插入损耗之差。 ER=ＩＬ-IＬ0 6、开关时间 开关端口从某一初状态转为通或者断所需得时间.从在开关上施加或撤去能量得时刻算起。 光开关得工作原理: 1、机械式光开关 移动光纤式光开关 移动反射镜式光开关
以上两种体积大,难实现集成化得开关网络.近年正大力发展一种集成得微机电系统(MEMS）开关,在硅片上用微加工技术做出大量可移动得微型镜片构成得开关阵列. 用16 个移动反射镜光开关构成得两组４ 4MＥMS开关阵列 2 电光开关 电光开关得原理一般就是利用材料得电光效应或电吸收效应,在电场作用下改变材料 得折射率与光得相位，再利用光得干涉或偏振等使光强突变或光路转变。 电光开关一般利用泡克耳斯(Ｐockels）效应，即折射率n随光场E而变化得电光效应。折射率变化与光场得变化关系为: 而光波传输距离L相应得相位变化为： 定向耦合型光开关
新托福口语考试Task 1模版 Personally, I would have to say that, um, the person that I admire most / a valuable possession I want to talk about / the place I would most like to go is ______________. And there are a couple of reasons to name. The most important thing is that, you know, ____________________.What’s more, ____________________. So that’s why ____________________. 新托福口语考试Task 2模版 Well, in my opinion, I would definitely agree with the point that, um, ____________________.The first reason that I wanna say is that____________________. More importantly, ____________________. So, uh, that’s why I choose ____________________ for the two reasons listed above. 新托福口语考试Task 3模版 The school has implemented a new policy that________________due to ____________________. And the man/woman holds a positive / negative view towards the announcement. The first reason s/he gives is that____________________. And the second one is based on the fact that____________________. Therefore, s/he agrees/disagrees with that opinion. 新托福口语考试Task 4模版 In the lecture, the professor mainly talked about the theory that____________________. To reinforce the theory, the professor gave two examples / reasons in his speech. The first one is that____________________. The other one is that____________________.
YH-B2100PDB型 微机配电变压器保护测控装置技术说明书 Ver 1.0
第一章概述 YH-B2100PDB型微机配电变压器保护测控单元作为新一代数字式变电站线路保护测控装置，适用于110KV及以下电压等级的各种箱式变压器、出线变压器等配电变压器，对其进行控制、保护、测量、远动通信、故障录波、事件记录等功能。该装置既可单独供货，亦可与YH-B2100型系统其他装置及监控系统一起组成变电站综合自动化系统。 该装置具有以下特点： ★采样元件采用精密电压电流互感器，体积小，重量轻，负载小。 ★测量采用12位A/D转换器，精度达0.5级，并计及11次谐波，符合国家对RTU 的要求。 ★人机界面采用128×64全汉化大屏幕液晶显示器，界面友好直观，操作简单，使用方便。 ﹡就地及远方定值整定功能（定值、时限独立整定）； ﹡实时显示运行参数及保护定值功能； ﹡远方就地位置显示。 ★软硬件均采用标准化模块设计，适用于各种配置要求，能方便的实现用户不同要求的保护组合，硬件通用性好，互换性强。 ★提供一路CANBUS接口和一路232接口，支持与变电站综合自动化系统相关单元通信。 ★ CANBUS波特率为33333 BPS，报文具有优先级，可主动上发报警，大大提高了故障响应时间。 ★可显示测量和保护电量参数的幅值及相位，便于用户检查接线。 ★可记录保护动作前3个周波、故障后5个周波，跳闸前8个周波的电压电流波形，通过CANBUS总线传至后台机或录波显示及打印单元打印或显示。 ★事件顺序记录。 ★可通过就地的分合按钮或后台机遥控命令，控制一台断路器。 ★断路器压力异常闭锁功能。 ★双层印制电路板布线技术及SMT表面贴装工艺，抗干扰能力强。 ★体积小的全密封装置，安装方便，外形美观，内部结构紧凑合理。 第二章工作原理 本单元集远动通信、保护、控制、测量、故障录波、事件记录为一体，采用交流采样技术，采样频率为1200HZ，保护电量的基波及谐波通过数字滤波，用差分和付氏算法求得，做出保护逻辑判断，驱动或闭锁单元上的出口继电器，该算法有利于消除模拟通道的零漂，具有较高的可靠性。测量的电量参数通过模拟量有效值算法求得（计及11次谐波），在大屏幕液晶显示器上显示，精度较高。为保证开关拒动时使继电器不损坏，单元设有完善的防跳回路，同时还具有可靠的自保持回路及断路器控制电路。