Logical Channel
Related terms:
Multiplexing, Control Channel, Control Plane, Motion Compensation, Transport
Block, Transport Channel
View all Topics
Scheduling
Erik Dahlman, ... Johan Sköld, in 5G NR: the Next Generation Wireless Access
Technology, 2018
14.2.1 Uplink Priority Handling
Multiple logical channels of different priorities can be multiplexed into the same
transport block using the MAC multiplexing functionality. Except for the case when
the uplink scheduling grant provides resources sufficient to transmit all data on all
logical channels, the multiplexing needs to prioritize between the logical channels.
However, unlike the downlink case, where the prioritization is up to the scheduler
implementation, the uplink multiplexing is done according to a set of well-defined
rules in the device with parameters set by the network. The reason for this is that
a scheduling grant applies to a specific uplink carrier of a device, not explicitly to a
specific logical channel within the carrier.
A simple approach would be to serve the logical channels in strict priority order.
However, this could result in starvation of lower-priority channels—all resources
would go to the high-priority channel until the buffer is empty. Typically, an operator
would instead like to provide at least some throughput for low-priority services as
well. Furthermore, as NR is designed to handle a mix of a wide range of traffic types,
a more elaborate scheme is needed. For example, traffic due to a file upload should
not necessarily exploit a grant intended for a latency-critical service.
The starvation problem is present already in LTE where it is handled by assigning
a guaranteed data rate to each channel. The logical channels are then served in
decreasing priority order up to their guaranteed data rate, which avoids starvation
as long as the scheduled data rate is at least as large as the sum of the guaranteed
data rates. Beyond the guaranteed data rates, channels are served in strict priority
order until the grant is fully exploited, or the buffer is empty.
NR applies a similar approach. However, given the large flexibility of NR in terms of
different transmission durations and a wider range of traffic types supported, a more
advanced scheme is needed. One possibility would be to define different profiles,
each outlining an allowed combination of logical channels, and explicitly signal the
profile to use in the grant. However, in NR the profile to use is implicitly derived
from other information available in the grant rather than explicitly signaled.
Upon reception of an uplink grant, two steps are performed. First, the device
determines which logical channels are eligible for multiplexing using this grant.
Second, the device determines the fraction of the resources that should be given
to each of the logical channels.
The first step determines the logical channels from which data can be transmitted
with the given grant. This can be seen as an implicitly derived profile. For each logical
channel, the device can be configured with:
•
The set of allowed subcarrier spacings this logical channel is allowed to use;
•
The maximum PUSCH duration which is possible to schedule for this logical
channel; and
The set of serving cell, that is, the set of uplink component carriers the logical
channel is allowed to be transmitted upon.
•
Only the logical channels for which the scheduling grant meets the restrictions
configured are allowed to be transmitted using this grant, that is, are eligible
for multiplexing at this particular time instant. In addition, the logical channel
multiplexing can also be restricted for transmission without a dynamic grant.
Coupling the multiplexing rule to the PUSCH duration is in 3GPP motivated by the
possibility to control whether latency-critical data should be allowed to exploit a grant
intended for less time-critical data.
As an example, assume there are two data flows, each on a different logical channel.
One logical channel carries latency-critical data and is given a high priority, while
the other logical channel carries non-latency-critical data and is given a low priority.
The gNB takes scheduling decisions based on, among other aspects, information
about the buffer status in the device provided by the device. Assume that the gNB
scheduled a relatively long PUSCH duration based on information that there is only
nontime-critical information in the buffers. During the reception of the scheduling
grant, time-critical information arrives to the device. Without the restriction on the
maximum PUSCH duration, the device would transmit the latency-critical data, pos-
sibly multiplexed with other data, over a relatively long transmission duration and
potentially not meeting the latency requirements set up for the particular service.
Instead, a better approach would be to separately request a transmission during a
short PUSCH duration for the latency critical data, something which is possible by
configuring the maximum PUSCH duration appropriately. Since the logical channel
carrying the latency-critical traffic has been configured with a higher priority than
the channel carrying the non-latency-critical service, the noncritical service will not
block transmission of the latency-critical data during the short PUSCH duration.
The reason to also include the subcarrier spacing is similar to the duration. In the
case of multiple subcarrier spacings configured for a single device, a lower subcarrier
spacing implies a longer slot duration and the reasoning above can also be applied
in this case.
Restricting the uplink carriers allowed for a certain logical channel is motivated
by the possibly different propagation conditions for different carriers and by dual
connectivity. Two uplink carriers at vastly different carrier frequencies can have
different reliability. Data which are critical to receive might be better to transmit
on a lower carrier frequency to ensure good coverage, while less-sensitive data can
be transmitted on a carrier with a higher carrier frequency and possibly spottier
coverage. Another motivation is duplication, that is, the same data transmitted
on multiple logical channels, to obtain diversity as mentioned in Section 6.4.2. If
both logical channels would be transmitted on the same uplink carrier, the original
motivation for duplication—to obtain a diversity effect—would be gone.
At this point in the process, the set of logical channels from which data are allowed to
be transmitted given the current grant is established, based on the mapping-related
parameters configured. Multiplexing of the different logical channels also needs to
answer the question of how to distribute resources between the logical channels
having data to transmit and eligible for transmission. This is done based on a set of
priority-related parameters configured for each local channel:
•
Priority;
•
Prioritized bit rate (PBR); and
•
Bucket size duration (BSD).
The prioritized bit rate and the bucket size duration together serve a similar purpose
as the guaranteed bit rate in LTE but can account for the different transmission
durations possible in NR. The product of the prioritized bit rate and the bucket size
duration is in essence a bucket of bits that at a minimum should be transmitted
for the given logical channel during a certain time. At each transmission instant,
the logical channels are served in decreasing priority order, while trying to fulfill the
requirement on the minimum number of bits to transmit. Excess capacity when all
the logical channels are served up to the bucket size is distributed in strict priority
order.
Priority handling and logical channel multiplexing are illustrated in Fig. 14.6.
Figure 14.6. Example of logical channel prioritization for four different scheduled
data rates and two different PUSCH durations.
> Read full chapter
The IEEE 802.16m Medium Access Control Common Part Sub-layer (Part II)
Sassan Ahmadi, in Mobile WiMAX, 2011
7.4.2 Logical and Transport Channels
The different logical and transport channels in LTE are illustrated in Figures 7-19 and
7-20, respectively. Each logical channel type is defined by what type of information is
transferred. The logical channels are generally classified into two groups: (1) Control
Channels (for the transfer of control-plane information) and (2) Traffic Channels (for
the transfer of user-plane information), as shown in Figure 7-19 [9].
FIGURE 7-19. Classification of 3GPP LTE logical channels
FIGURE 7-20. Classification of 3GPP LTE transport channels
The control channels are exclusively used for transfer of control-plane information.
The control channels supported by MAC can be classified as follows (see Figure 7-19):
•
•
•
•
•
Broadcast Control Channel (BCCH): a downlink channel for broadcasting
system control information.
Paging Control Channel (PCCH): a downlink channel that transfers paging
information and system information change notifications. This channel is used
for paging when the network does not know the location of the UE.
Common Control Channel (CCCH): a channel for transmitting control information between UEs and eNBs. This channel is used for UEs having no RRC
connection with the network.
Multicast Control Channel (MCCH): a point-to-multipoint downlink channel
used for transmitting MBMS control information from the network to the UE,
for one or several MTCHs. This channel is only used by UEs that receive MBMS.
Dedicated Control Channel (DCCH): a point-to-point bi-directional channel
that transmits dedicated control information between a UE and the network.
It is used by UEs that have RRC connection.
The traffic channels are exclusively used for the transfer of user-plane information.
The traffic channels supported by MAC can be classified as follows (as shown in
Figure 7-20):
•
•
Dedicated Traffic Channel (DTCH): a point-to-point bi-directional channel
dedicated to a single UE for the transfer of user information.
Multicast Traffic Channel (MTCH): a point-to-multipoint downlink channel for
transmitting traffic data from the network to the UE. This channel is only used
by UEs that receive MBMS.
The physical layer provides information transfer services to MAC and higher layers.
The physical layer transport services are described by how and with what characteristics data are transferred over the radio interface. This should be clearly separated
from the classification of what is transported, which relates to the concept of
logical channels at the MAC sub-layer. As shown in Figure 7-20, downlink transport
channels can be classified as follows:
•
•
Broadcast Channel (BCH) characterized by fixed, pre-defined transport format
and required to be broadcast in the entire coverage area of the cell;
Downlink Shared Channel (DL-SCH) characterized by support for HARQ,
support for dynamic link adaptation by varying the modulation, coding and
transmit power, possibility for broadcast in the entire cell, possibility to use
beamforming, support for both dynamic and semi-static resource allocation,
support for UE discontinuous reception to enable power saving, support for •
MBMS transmission;
Paging Channel (PCH) characterized by support for UE discontinuous recep- •
tion in order to enable power saving, requirement for broadcast in the entire
coverage area of the cell, mapped to physical resources which can be used
dynamically also for traffic or other control channels;
Multicast Channel (MCH) characterized by a requirement to be broadcast in
the entire coverage area of the cell, support for macro-diversity combining
of MBMS transmission on multiple cells, support for semi-static resource
allocation;
The uplink transport channels are classified as follows (see Figure 7-20):
•
•
Uplink Shared Channel (UL-SCH) characterized by the possibility to use beamforming, support for dynamic link adaptation by varying the transmit power
and modulation and coding schemes, support for HARQ, support for both
dynamic and semi-static resource allocation;
Random Access Channel (RACH) characterized by limited control information
and collision risk.
The mapping of the logical channels to the transport channels in the downlink and
uplink is shown in Figure 7-21. As shown in Figures 7-16 and 7-17, the main services
and functions provided by the RLC sub-layer include transfer of upper layer PDUs
supporting AM or UM, TM data transfer, error correction through ARQ (since CRC
check is provided by the physical layer, no CRC is needed at RLC level), segmentation
according to the size of the transport block, re-segmentation of PDUs that need to
be re-transmitted, concatenation of SDUs for the same radio bearer, in-sequence
delivery of upper layer PDUs except during handover, duplicate detection, and
protocol error detection and recovery.
FIGURE 7-21. Mapping of logical to transport channels in the downlink and uplink
[9]
The users in the 3GPP LTE system are assigned temporary identifiers to protect user
privacy and confidentiality. Depending on the state of the UE, different types of
temporary identifiers are used. Table 7-1 summarizes the Radio Network Temporary
Identifiers (RNTI) and their usage in 3GPP LTE. For example, the Random Access
RNTI (RA-RNTI) is used on the PDCCH when random access response messages are
transmitted. It unambiguously identifies which time-frequency resource was utilized
by the UE to transmit the random access preamble. The Msg3 acronym in Table 7-1
denotes the message transmitted on UL-SCH containing a C-RNTI MAC Control
Element (CE) or CCCH SDU, submitted from an upper layer and associated with
the UE Contention Resolution Identity, as part of a random access procedure [10].
The various temporary identifiers in 3GPP LTE are conceptually similar to station
identifiers, as well as MAP information elements in IEEE 802.16m systems that
are used to identify the users and their active connections and allocations in the
downlink and uplink.
Table 7-1. Various Radio Network Temporary Identifiers and their Usage in 3GPP
LTE [10]
Radio Network Tempo- Usage
rary Identifier
Transport Channel
Logical Channel
Paging RNTI (P-RNTI)
Paging and system in- PCH
formation change notification
PCCH
System Information
RNTI (SI-RNTI)
Broadcast of system in- DL-SCH
formation
BCCH
Random Access RNTI
(RA-RNTI)
Random access response
N/A
Temporary C-RNTI
Contention resolution DL-SCH
(when no valid C-RNTI
Is available)
CCCH
Temporary C-RNTI
Msg3 transmission
CCCH, DCCH, DTCH
Cell RNTI (C-RNTI)
Dynamically scheduled DL-SCH, UL-SCH
unicast transmission
DCCH, DTCH
C-RNTI
Triggering of PDCCH-ordered random
access
N/A
N/A
Semi-Persistent Sched- Semi-persistently
uling C-RNTI
scheduled unicast
transmission
(activation,
reactivation, and
re-transmission)
DL-SCH, UL-SCH
DCCH, DTCH
Semi-Persistent Sched- Semi-persistently
uling C-RNTI
scheduled unicast
transmission
(deactivation)
N/A
N/A
Transmit Power Control-Physical Uplink
Control Channel-RNTI
(TPC-PUCCH-RNTI)
Physical layer uplink
power control
N/A
N/A
Transmit Power Control-Physical Uplink
Shared Channel-RNTI
(TPC-PUSCH-RNTI)
Physical layer uplink
power control
N/A
N/A
> Read full chapter
DL-SCH
UL-SCH
Wireless and mobile technologies and
protocols and their performance evaluation
Salima Samaoui, ... Wahida Mansouri, in Modeling and Simulation of Computer
Networks and Systems, 2015
3.3.2 Medium access layer (MAC)
The MAC layer is responsible for mapping between logical channels and transport
channels, multiplexing and demultiplexing of upper layer PDUs, scheduling air
interface resources in both uplink and downlink, error correction through HARQ,
priority handling between UEs by means of dynamic scheduling, and priority handling between logical channels of one UE. The MAC layer is located below the RLC
layer and it provides services to the RLC by offering logical channels. According to
the 3GPP standards [14], the logical channel types are: Broadcast Control Channel
(BCCH), Paging Control Channel (PCCH), Common Control Channel (CCCH), Dedicated Control Channel (DCCH), Multi-cast Control Channel (MCCH), Dedicated
Traffic Channel (DTCH), and Multicast Traffic Channel (MTCH). The MAC layer uses
the services offered by the physical layer in terms of using the transport channels. The
LTE transport channels are: Broadcast Channel (BCH), Downlink Shared Channel
(DL-SCH), Paging Channel (PCH), Multicast Channel (MCH), Uplink Shared Channel (UL-SCH), and Random Access Channel (RACH).
> Read full chapter
Radio-Interface Architecture
Erik Dahlman, ... Johan Sköld, in 5G NR: the Next Generation Wireless Access
Technology, 2018
6.4.4.1 Logical Channels and Transport Channels
The MAC provides services to the RLC in the form of logical channels. A logical
channel is defined by the type of information it carries and is generally classified as
a control channel, used for transmission of control and configuration information
necessary for operating an NR system, or as a traffic channel, used for the user data.
The set of logical-channel types specified for NR includes:
•
The Broadcast Control Channel (BCCH), used for transmission of system information from the network to all devices in a cell. Prior to accessing the system,
a device needs to acquire the system information to find out how the system is•
configured and, in general, how to behave properly within a cell. Note that, in
the case of non-standalone operation, system information is provided by the
LTE system and there is no BCCH.
The Paging Control Channel (PCCH), used for paging of devices whose location•
on a cell level is not known to the network. The paging message therefore
needs to be transmitted in multiple cells. Note that, in the case of non-standalone operation, paging is provided by the LTE system and there is no PCCH.
The Common Control Channel (CCCH), used for transmission of control
•
information in conjunction with random access.
The Dedicated Control Channel (DCCH), used for transmission of control infor-•
mation to/from a device. This channel is used for individual configuration of
devices such as setting various parameters in devices.
The Dedicated Traffic Channel (DTCH), used for transmission of user data
to/from a device. This is the logical channel type used for transmission of all
unicast uplink and downlink user data.
The above logical channels are in general present also in an LTE system and used for
similar functionality. However, LTE provides additional logical channels for features
not yet supported by NR (but likely to be introduced in upcoming releases).
From the physical layer, the MAC layer uses services in the form of transport channels.
A transport channel is defined by how and with what characteristics the information
is transmitted over the radio interface. Data on a transport channel are organized
into transport blocks. In each Transmission Time Interval (TTI), at most one transport
block of dynamic size is transmitted over the radio interface to/from a device (in the
case of spatial multiplexing of more than four layers, there are two transport blocks
per TTI).
Associated with each transport block is a Transport Format (TF), specifying how the
transport block is to be transmitted over the radio interface. The transport format
includes information about the transport-block size, the modulation-and-coding
scheme, and the antenna mapping. By varying the transport format, the MAC layer
can thus realize different data rates, a process known as transport-format selection.
The following transport-channel types are defined for NR:
•
•
The Broadcast Channel (BCH) has a fixed transport format, provided by the
specifications. It is used for transmission of parts of the BCCH system information, more specifically the so-called Master Information Block (MIB), as
described in Chapter 16.
The Paging Channel (PCH) is used for transmission of paging information from
the PCCH logical channel. The PCH supports discontinuous reception (DRX) to
allow the device to save battery power by waking up to receive the PCH only •
at predefined time instants.
The Downlink Shared Channel (DL-SCH) is the main transport channel used •
for transmission of downlink data in NR. It supports key NR features such as
dynamic rate adaptation and channel-dependent scheduling in the time and
frequency domains, hybrid ARQ with soft combining, and spatial multiplexing.
It also supports DRX to reduce device power consumption while still providing
an always-on experience. The DL-SCH is also used for transmission of the
parts of the BCCH system information not mapped to the BCH. Each device
has a DL-SCH per cell it is connected to. In slots where system information is
received there is one additional DL-SCH from the device perspective.
The Uplink Shared Channel (UL-SCH) is the uplink counterpart to the
DL-SCH—that is, the uplink transport channel used for transmission of uplink
data.
In addition, the Random-Access Channel (RACH) is also defined as a transport
channel, although it does not carry transport blocks.
Part of the MAC functionality is multiplexing of different logical channels and
mapping of the logical channels to the appropriate transport channels. The mapping
between logical-channel types and transport-channel types is given in Fig. 6.11.
This figure clearly indicates how DL-SCH and UL-SCH are the main downlink and
uplink transport channels, respectively. In the figures, the corresponding physical
channels, described further below, are also included and the mapping between
transport channels and physical channels is illustrated.
Figure 6.11. Mapping between logical, transport, and physical channels.
To support priority handling, multiple logical channels, where each logical channel
has its own RLC entity, can be multiplexed into one transport channel by the MAC
layer. At the receiver, the MAC layer handles the corresponding demultiplexing and
forwards the RLC PDUs to their respective RLC entity. To support the demultiplexing
at the receiver, a MAC header is used. The placement of the MAC headers has
been improved compared to LTE, again with low-latency operation in mind. Instead
of locating all the MAC header information at the beginning of a MAC PDU,
which implies that assembly of the MAC PDU cannot start until the scheduling
decision is available, the subheader corresponding to a certain MAC SDU is placed
immediately before the SDU, as shown in Fig. 6.12. This allows the PDUs to be
preprocessed before having received the scheduling decision. If necessary, padding
can be appended to align the transport block size with those supported in NR.
Figure 6.12. MAC SDU multiplexing and header insertion (uplink case).
The subheader contains the identity of the logical channel (LCID) from which the RLC
PDU originated and the length of the PDU in bytes. There is also a flag indicating
the size of the length indicator, as well as a reserved bit for future use.
In addition to multiplexing of different logical channels, the MAC layer can also
insert MAC control elements into the transport blocks to be transmitted over the
transport channels. A MAC control element is used for inband control signaling and
identified with reserved values in the LCID field, where the LCID value indicates the
type of control information. Both fixed- and variable-length MAC control elements
are supported, depending on their usage. For downlink transmissions, MAC control
elements are located at the beginning of the MAC PDU, while for uplink transmissions the MAC control elements are located at the end, immediately before the
padding (if present). Again, the placement is chosen in order to facilitate low-latency
operation in the device.
MAC control elements are, as mentioned above, used for inband control signaling.
It provides a faster way to send control signaling than RLC, without having to resort
to the restrictions in terms of payload sizes and reliability offered by physical-layer
L1/L2 control signaling (PDCCH or PUCCH). There are multiple MAC control elements, used for various purposes, for example:
•
•
Scheduling-related MAC control elements, such as buffer status reports and
power headroom reports used to assist uplink scheduling as described in
Chapter 14, and the configured grant confirmation MAC control element used
when configuring semipersistent scheduling;
Random-access-related MAC control elements such as the C-RNTI and contention-resolution MAC control elements;
Timing-advance MAC control elements to handle timing advance as described
in Chapter 15;
Activation and deactivation of previously configured components;
•
DRX-related MAC control elements;
•
Activation/deactivation of PDCP duplication detection; and
•
•
•
Activation/deactivation of CSI reporting and SRS transmission (see Chapter 8).
The MAC entity is also responsible for distributing data from each flow across the
different component carriers, or cells, in the case of carrier aggregation. The basic
principle for carrier aggregation is independent processing of the component carriers in the physical layer, including control signaling, scheduling, and hybrid-ARQ
retransmissions, while carrier aggregation is invisible above the MAC layer. Carrier
aggregation is therefore mainly seen in the MAC layer, as illustrated in Fig. 6.13,
where logical channels, including any MAC control elements, are multiplexed to
form transport blocks per component carrier with each component carrier having
its own hybrid-ARQ entity.
Figure 6.13. Carrier aggregation.
Both carrier aggregation and dual connectivity result in the device being connected
to more than one cell. Despite this similarity, there are fundamental differences,
primarily related to how tightly the different cells are coordinated and whether they
reside in the same or in different gNBs.
Carrier aggregation implies very tight coordination, with all the cells belonging to
the same gNB. Scheduling decisions are taken jointly for all the cells the device is
connected to by one joint scheduler.
Dual connectivity, on the other hand, allows for a much looser coordination between
the cells. The cells can belong to different gNBs, and they may even belong to
different radio-access technologies as is the case for NR-LTE dual connectivity in
case of non-standalone operation.
Carrier aggregation and dual connectivity can also be combined. This is the reason
for the terms master cell group and secondary cell group. Within each of the cell
groups, carrier aggregation can be used.
> Read full chapter
End-to-End Protocols
Larry L. Peterson, Bruce S. Davie, in Computer Networks (Fifth Edition), 2012
(a)
Explain the bottleneck we might expect, even with infinite bandwidth, if the
client sends all its write requests through a single logical channel, and explain
why using a pool of channels could help. Hint: You will need to know a little
about disk controllers.
(b) Suppose the server's reply means only that the data has been placed in the disk
queue. Explain how this could lead to data loss that wouldn't occur with a local
disk. Note that a system crash immediately after data was enqueued doesn't
count, because that would cause data loss on a local disk as well.
(c) An alternative would be for the server to respond immediately to acknowledge
the write request and to send its own separate request later to confirm the
physical write. Propose different RPC semantics to achieve the same effect, but
with a single logical request and reply.
> Read full chapter
A single-cycle router with wing channels†
In Networks-On-Chip, 2015
2.3.1 Channel dispensers
Figure 2.6 shows the microarchitecture of the channel dispenser, which is mainly
composed of the VC assigner, VC tracker, and VC table. The VC table forms the core
of the dispenser logic. It is a compact table indexed by the logical channel ID, and
holds the physical channel ID and tail pointers at each entry. With the VC table, the
VC tracker simply provides the next available channels by keeping track of all the
normal ones.
Figure 2.6. The channel dispenser.
When receiving the information from the VC tracker, the VC assigner decides
whether to grant a wing channel or a normal channel to the new packet on the basis
of the generated wing flag. Once any channel has been granted to the incoming
packets, the VC assigner needs to provide the dispensed channel ID to the VC table to
change the status of the available channels. Here, the VC assigner is also responsible
for improving the throughput by fully utilizing all VCs at high rates. When all the
normal VCs at the local input are exhausted, the VC assigner is forced to allocate
the wing channel to the new packet. In this way, when the normal VCs are occupied
and the generated wing flag is false, the wing channel will serve as a normal one,
and the packet allocated the wing channel will apply for the channel and switch like
those in the normal ones. Our experiments described in Section 2.4.3 validate the
effectiveness of this technique under different routing schemes.
This channel dispenser also guarantees protocol- and network-level deadlock-free
operations by adopting relatively low-cost methods. Toward that goal, we use different VC sets for request and response message types, respectively, to avoid protocoland network-level deadlock situations. The proposed router includes two VC sets
per port: the first VC set is composed of channel 0 (i.e., wing channel), channel 1,
and channel 2, and is used by the request packets; the second VC set, comprising
channels 0, 1, and 3, is used by the response packets.
Note that since channels 2 and 3 are used by request and response packets exclusively, we can provide deadlock-free operations in two aspects. First, in order to
provide deadlock recovery in adaptive routing algorithms, channels 2 and 3 employ
a deterministic routing algorithm to break the network deadlock. For any packet
at the wing channel or channel 1, once it has been checked for possible deadlock
situations, the channel dispenser unit of the neighborhood will grant channels 2
and 3 to this packet with higher priority, thereby using deterministic routing to
break the network deadlock situation. Second, we introduce separate channels 2 and
3 for two different message types to break the dependencies between the request
and response messages. When both the shared channel 0 and channel 1 at the
neighborhood are holding other messages of different types, the current message
can be transferred through the exclusive VC on the basis of its own type, thereby
breaking the dependency cycle between messages of different classes.
To satisfy the tight timing constraint, the real-time generation of a wing flag is very
important, as shown in Figure 2.6, where we adopt a favorable tradeoff between
the limited timing overhead and the fast packet transfer. In the dispenser unit, the
inputs of the second-phase switch arbitration are used to inspect the output state
of the next cycle; however, the inspection of the input state using the results of the
original SA pipeline would prove to be intolerable because it influences seriously the
timing of the critical path. Instead, we consider the previous outputs of the SA to
be the default input state, thereby reducing the critical path. In such a scenario, the
winning request from the SA of the current cycle influences the single-cycle transfer
of wing channels but does not harm the network performance at high loads, since
the winning packet will be transferred in the next cycle. As illustrated in Figure 2.6,
the nrc information from the header flit controls the multiplexer (MUX) to select the
inputs of the second-phase switch arbitration on the basis of its output direction.
Only when both the MUX output and the previous output of the SA are false is the
wing channel granted to the new packets.
Using the technology-independent model proposed by Peh and Dally [17], we
compare the delay of channel dispensation indicated by the thick line with that
between the input of the second-phase switch arbitration and VSA stage results, as
shown below:
(2.1)
(2.2)
In the typical case of five ports and four channels, the former equation equals 9.3
fan-outs of 4 (FO4), which is close to the 9.9 FO4 of the latter one. In addition,
it also introduces some timing overheads to generate the buffer pointer based on
the LTs. With the same model, a delay of 3 FO4 is added when compared with the
v-input MUX of the original router. Here, the delay of 1 mm wire after placement
and routing in a 65 nm process is calculated be 8.2 FO4, which is far less than the 18
FO4 of the original critical path. Hence, this overhead would not harm the network
frequency.
> Read full chapter
Introduction
WuZhijun , in Information Hiding in Speech Signal for Secure Communication,
2015
1.1.2.2 Basic Principles and Classifications
Information hiding technology comprises a large number of research areas. It
may be divided into several aspects in accordance with Fabien A. P. Petitcolas’s
classification [21], as shown in Figure 1.1.
Figure 1.1. Classification of information hiding technology.
The classification of information hiding is explained next [21,22]2122.
Covert Channel
Built on the common channel, the covert channel is a logical channel for sending
covert messages. Actually, this channel does not exist in reality, it only utilizes
information hiding technology to build a logical channel over a public channel. The
establishment of the covert channel is key for an information hiding system. Once
the covert channel is found or destroyed, then the whole information hiding system
will be paralyzed.
Steganography
Steganography is the general name of secure communication methods, and refers to
the technology that embeds secret messages into public information. The so-called
public information is a kind of information that does not cause any attention from
others. The hiding method usually depends on the hypothesis that the third party
cannot sense the existence of the covert communication. This method is used mainly
in point-to-point covert communication between two parties that trust each other.
Therefore, steganography is not robust. For example, the covered information could
not be restored after data change.
Anonymity
Valid Internet users may employ the anonymous communication mechanism to ask
for help or to vote secretly in an online election. But no one is willing to provide
such a mechanism, because either intentional or unintentional attackers can easily
overload it.
Copyright Marking
The main impetus for the development of information hiding is the copyright
concerns. The audio, video, image, or other works can be in digitized form and
it is very easy to obtain a perfect duplication. Publishers of music, movies, books,
software, and other media are most concerned with the emergence of various unauthorized duplicates. Recently, the most significant study includes digital watermarks
(hidden copyright information) and digital fingerprints (hidden serial number). The
fingerprint is used to identify the copyright violators and the watermark is used to
prosecute them.
These four categories are interrelated. The present research focuses on steganography and copyright marks. Specifically, technology-based image steganography and
digital image watermarking are the hot points [21,22]2122.
Digital Watermarking
Embedding special information in digital works (such as still images, video, audio,
etc.) can certify copyright ascription and trace copyright infringement. The special
information may be the author’s serial number, company logo, meaningful text,
and so on. In contrast with the camouflage, the hidden information in watermark
is robust enough to resist attacks. Even if the watermarking algorithm is public
and the hidden information is known by attackers, it is still difficult for them to
devastate the hidden watermark (impossible in the ideal situation). In cryptography,
the well-known Kerkhoffs principle [16] goes like this: the encryption system is still
safe even under the condition that the attacker knows the theory and algorithm
but does not know the corresponding key. Robustness requires that watermarking
algorithms embed less information in public data than camouflage. Watermarking
and steganography are complementary rather than competing.
Data Hiding and Data Embedding
Data hiding and data embedding usually are used in different contexts, and generally
refer to camouflage or applications between camouflage and watermarking. In these
applications, there is no need to protect the embedded data that are open to the
public. For example, embedded data may be auxiliary information or service; they
are obtained for free, and have nothing to do with copyright protection and access
control functions.
Fingerprinting and Labeling
Fingerprinting and labeling refer to the particular purpose of a watermark. The
watermark consists of information about a digital work that belongs to a creator or
purchaser, which is embedded into the media. Each watermark is a unique code in a
set of codes. The information in the watermark identifies a unique copy of a digital
product.
Steganography
Steganography is the art and science of encoding hidden secret information in a way
that no one, apart from the sender and intended recipient, suspects the existence of
this information. It is a form of security through obscurity.
Steganography includes the concealment of secret information within media files,
such as image and speech, and communication protocol, for example session
initiation protocol (SIP).
Copyright Protection for Digital Media
This technique of information hiding provides a method of digital media copy
protection by embedding copyright information into digital media files. The method
includes a process of hiding digital media data with a public key, using a hybrid cryptographic technique, a process of watermarking the media data, and a measurement
compliance testing process in an effective way.
> Read full chapter
Getting Connected
Larry L. Peterson, Bruce S. Davie, in Computer Networks (Fifth Edition), 2012
2.5.3 Concurrent Logical Channels
The data link protocol used in the ARPANET provides an interesting alternative to
the sliding window protocol, in that it is able to keep the pipe full while still using
the simple stop-and-wait algorithm. One important consequence of this approach
is that the frames sent over a given link are not kept in any particular order. The
protocol also implies nothing about flow control.
The idea underlying the ARPANET protocol, which we refer to as concurrent logical
channels, is to multiplex several logical channels onto a single point-to-point link
and to run the stop-and-wait algorithm on each of these logical channels. There is no
relationship maintained among the frames sent on any of the logical channels, yet
because a different frame can be outstanding on each of the several logical channels
the sender can keep the link full.
More precisely, the sender keeps 3 bits of state for each channel: a boolean, saying
whether the channel is currently busy; the 1-bit sequence number to use the next
time a frame is sent on this logical channel; and the next sequence number to expect
on a frame that arrives on this channel. When the node has a frame to send, it uses
the lowest idle channel, and otherwise it behaves just like stop-and-wait.
In practice, the ARPANET supported 8 logical channels over each ground link and 16
over each satellite link. In the ground-link case, the header for each frame included
a 3-bit channel number and a 1-bit sequence number, for a total of 4 bits. This is
exactly the number of bits the sliding window protocol requires to support up to 8
outstanding frames on the link when RWS = SWS.
> Read full chapter
Digital Systems
Martin Plonus, in Electronics and Communications for Scientists and Engineers,
2001
Time-Division Multiplexing
Time-division multiplexing (TDM), on the other hand, is only used with digital
signals, which are a stream of pulses representing the 0 and 1's of a message. Since
modern digital equipment can process 0 and 1 's much faster than the 0 and 1 's that
come from a typical message, we can take several messages and interleave the 0 and
1's from the different messages and send the packet simultaneously over a single
channel. Thus TDM is a method for combining many low-speed digital signals into
a single high-speed pulse train. Multiplexing C channels, each sampled at S samples
per second and coded using n bits per sample, gives a rate R for the pulse train of
(9.38)
At the receiving end the messages are separated by a decoder. This is a very efficient
way to send many messages simultaneously over the same channel.
Figure 9.22a shows a multiplexer and demultiplexer, represented by a mechanically driven switch. N voice27 channels are placed sequentially on a high-capacity
channel and again separated at the receiving end by the demultiplexer. In TDM the
high-capacity channel is divided into N “logical” channels and data in each of the N.
incoming voice channels are placed in a designated “logical” channel. The procedure
is as follows: time on the high-capacity channel is divided into fixed length intervals
called frames. Time in each frame is further subdivided into N fixed-length intervals
usually referred to as slots: slot 1, slot 2,…, slot N. A slot is 1 bit wide.28 A “logical”
channel occupies a single slot in every frame. For example, the first “logical” channel
occupies slots 1,N+1, 2N + 1,…; the second occupies slots 2, N+ 2, 2N+ 2,…; the third
slots 3, N + 3, 2 N + 3,…; and so forth. A given “logical” channel therefore occupies
every Nth slot, giving us N “logical” channels in which to place the N incoming
messages. At the receiving end of the high-capacity channel the bit stream is readily
demultiplexed, with the demultiplexer detecting the framing pattern from which it
determines the beginning of each frame, and hence each slot. An integrated-circuit
codec (encoder/decoder) carries out antialiasing filtering, sampling, quantization,
and coding of the transmitted signal as well as decoding and signal recovery on
the receiving side. Figure 9.22b shows how TDM interleaves the three voice signals
represented by the dark, shaded, and clear pulses into a faster bit stream. The time
frames, denoted by the vertical lines, have three slots, one for each voice signal.
The first voice signal pulse occupies the first slot in each frame, the second signal
pulse the second slot, and the third the third slot. At the receiving end the three
pulse streams are separated by use of a reference timing signal (framing signal). The
framing signal identifies the pulse position of voice 1; the voice 2 and voice 3 pulse
positions are generated by an electronic counter that is synchronized to the framing
signal.
FIGURE 9.22. (a) Time-division multiplexing of N voice channels, (b) Three low-speed
digital voice signals are combined (by interleaving) into a higher-speed pulse train.
FIGURE 9.23. Time-division multiplexing of 24 telephone voice channels which
are sampled in sequence 8000 times a second, with the amplitude of each sample
represented in binary form.
> Read full chapter
Input Device Interfaces
Sanjeeb Mishra, ... Vijayakrishnan Rousseau, in System on Chip Interfaces for Low
Power Design, 2016
Operation
USB keyboards are classified as human interface devices (HIDs). They are expected to
follow the HID over USB protocol. Most operating systems have built-in support for
the protocol and USB keyboards that are HID compliant (which is the vast majority,
if not all of them) will work without having to install any device drivers on the host
machine.
In USB, there are one host and multiple devices connected via a tiered star topology.
There are logical entities on USB devices that are named endpoints. The host
communicates via pipes, which are logical channels, to the endpoints on the devices.
In USB terminology, the direction of an endpoint is based on the host. Thus, IN
always refers to transfers to the host from a device and OUT always refers to transfers
from the host to a device. USB devices can also support bidirectional transfers of
control data. Figure 11.1 shows a USB-based keyboard interface.
Figure 11.1. USB device (keyboard) connected to host.
Here, the IN endpoint is used to send keystrokes to the host, while the OUT endpoint
is used by the host to let the keyboard know which of its light-emitting diodes (LEDs)
must be turned on.
The USB interface overcomes all the disadvantages of the PS/2 interface:
1.
USB devices are designed to be hot swappable.
2.
The USB connector is quite robust, and there are no restrictions on the number
of times it can be plugged in and out.
A USB device may stop working, though it is quite rare. Since USB devices are
hot pluggable, removing it and plugging it back in usually causes the driver to
get reloaded and the device to start working.
In any case the kind of problems seen in PS/2 where a faulty device can bring
down the entire interface will not happen in USB.
3.
4.
> Read full chapter
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