First, ATM must accept any type of payload, both data frames and streams of bits. A
data frame can come from an upper-layer protocol that creates a clearly defined frame
to be sent to a carrier network such as ATM. A good exampre is the Internet. ATM must
also carry multimedia payload. It can accept continuous bit streams and break them
into chunks to be encapsulated into a cell at the ATM layer. AAL uses two sublayers to
accomplish these tasks.
The CS sublayer divides the bit stream into 47-byte segments and passes them to
the SAR sublayer below. Note that the CS sublayer does not add a header.
The SAR sublayer adds 1 byte of header and passes the 48-byte segment to the
ATM layer. The header has two fields:
o Sequence number (SN). This 4-bit field defines a sequence number to order the
bits. The first bit is sometimes used for timing, which leaves 3 bits for sequencing
(modulo 8).
o Sequence number protection (SNP). The second 4-bit field protects the first
field. The first 3 bits automatically correct the SN field. The last bit is a parity bit
that detects error over all 8 bits.
QOS in ATM
Network-Related Attributes:
CLR The cell loss ratio (CLR) defines the fraction of cells lost (or delivered so late
that they are considered lost) during transmission. For example, if the sender sends
100 cells and one of them is lost.
CTD The cell transfer delay (CTD) is the average time needed for a cell to travel
from source to destination. The maximum CTD and the minimum CTD are also considered
attributes.
CDV The cell delay variation (CDV) is the difference between the CTD maximum
and the CTD minimum.
CER The cell error ratio (CER) defines the fraction of the cells delivered in error.
B-ISDN
The B-ISDN reference protocol model consists of three planes:
data frame can come from an upper-layer protocol that creates a clearly defined frame
to be sent to a carrier network such as ATM. A good exampre is the Internet. ATM must
also carry multimedia payload. It can accept continuous bit streams and break them
into chunks to be encapsulated into a cell at the ATM layer. AAL uses two sublayers to
accomplish these tasks.
Whether the data are a data frame or a stream of bits, the payload must be segmented
into 48-byte segments to be carried by a cell. At the destination, these segments
need to be reassembled to recreate the original payload. The AAL defines a sublayer,
called a segmentation and reassembly (SAR) sublayer, to do so. Segmentation is at
the source; reassembly, at the destination.
into 48-byte segments to be carried by a cell. At the destination, these segments
need to be reassembled to recreate the original payload. The AAL defines a sublayer,
called a segmentation and reassembly (SAR) sublayer, to do so. Segmentation is at
the source; reassembly, at the destination.
Before data are segmented by SAR, they must be prepared to guarantee the integrity
of the data. This is done by a sublayer called the convergence sublayer (CS).
ATM defines four versions of the AAL: AALl, AAL2, AAL3/4, and AAL5.
Although we discuss all these versions, we need to inform the reader that the common
versions today are AALI and AAL5. The first is used in streaming audio and video
communication; the second, in data communications.
of the data. This is done by a sublayer called the convergence sublayer (CS).
ATM defines four versions of the AAL: AALl, AAL2, AAL3/4, and AAL5.
Although we discuss all these versions, we need to inform the reader that the common
versions today are AALI and AAL5. The first is used in streaming audio and video
communication; the second, in data communications.
AALI: AALI supports applications that transfer information at constant bit rates,
such as video and voice. It allows ATM to connect existing digital telephone networks
such as voice channels and T lines. Figure 18.20 shows how a bit stream of data is
chopped into 47-byte chunks and encapsulated in cells.
such as voice channels and T lines. Figure 18.20 shows how a bit stream of data is
chopped into 47-byte chunks and encapsulated in cells.
The CS sublayer divides the bit stream into 47-byte segments and passes them to
the SAR sublayer below. Note that the CS sublayer does not add a header.
The SAR sublayer adds 1 byte of header and passes the 48-byte segment to the
ATM layer. The header has two fields:
o Sequence number (SN). This 4-bit field defines a sequence number to order the
bits. The first bit is sometimes used for timing, which leaves 3 bits for sequencing
(modulo 8).
o Sequence number protection (SNP). The second 4-bit field protects the first
field. The first 3 bits automatically correct the SN field. The last bit is a parity bit
that detects error over all 8 bits.
AAL3/4: Initially, AAL3 was intended to support connection-oriented data services and
AAL4 to support connectionless services. As they evolved, however, it became evident
that the fundamental issues of the two protocols were the same. They have therefore been
combined into a single format calledAAL3/4.
AAL4 to support connectionless services. As they evolved, however, it became evident
that the fundamental issues of the two protocols were the same. They have therefore been
combined into a single format calledAAL3/4.
The CS layer header and trailer consist of six fields:
o Common part identifier (CPI). The CPI defines how the subsequent fields are to
be interpreted. The value at present is O.
o Begin tag (Btag). The value of this field is repeated in each ceU to identify all the
cells belonging to the same packet. The value is the same as the Etag (see below).
o Buffer allocation size (BAsize). The 2-byte BA field tells the receiver what size
buffer is needed for the coming data.
o Alignment (AL). The I-byte AL field is included to make the rest of the trailer
4 bytes long.
o Ending tag (Etag). The I-byte ET field serves as an ending flag. Its value is the
same as that of the beginning tag.
o Length (L). The 2-byte L field indicates the length of the data unit.
o Common part identifier (CPI). The CPI defines how the subsequent fields are to
be interpreted. The value at present is O.
o Begin tag (Btag). The value of this field is repeated in each ceU to identify all the
cells belonging to the same packet. The value is the same as the Etag (see below).
o Buffer allocation size (BAsize). The 2-byte BA field tells the receiver what size
buffer is needed for the coming data.
o Alignment (AL). The I-byte AL field is included to make the rest of the trailer
4 bytes long.
o Ending tag (Etag). The I-byte ET field serves as an ending flag. Its value is the
same as that of the beginning tag.
o Length (L). The 2-byte L field indicates the length of the data unit.
The SAR header and trailer consist of five fields:
o Segment type (ST). The 2-bit ST identifier specifies the position of the segment
in the message: beginning (00), middle (01), or end (10). A single-segment message
has an STof 11.
o Sequence number (SN). This field is the same as defined previously.
o Multiplexing identifier (MID). The 10-bit MID field identifies cells coming from
different data flows and multiplexed on the same virtual connection.
o Length indicator (LI). This field defines how much of the packet is data, not
padding.
o CRC. The last 10 bits of the trailer is a CRC for the entire data unit.
o Segment type (ST). The 2-bit ST identifier specifies the position of the segment
in the message: beginning (00), middle (01), or end (10). A single-segment message
has an STof 11.
o Sequence number (SN). This field is the same as defined previously.
o Multiplexing identifier (MID). The 10-bit MID field identifies cells coming from
different data flows and multiplexed on the same virtual connection.
o Length indicator (LI). This field defines how much of the packet is data, not
padding.
o CRC. The last 10 bits of the trailer is a CRC for the entire data unit.
AAL5: AAL3/4 provides comprehensive sequencing and error control mechanisms
that are not necessary for every application. For these applications, the designers of
ATM have provided a fifth AAL sublayer, called the simple and efficient adaptation
layer (SEAL). AAL5 assumes that all cells belonging to a single message travel
sequentially and that control functions are included in the upper layers of the sending
application. Figure shows the AAL5 sublayer.
that are not necessary for every application. For these applications, the designers of
ATM have provided a fifth AAL sublayer, called the simple and efficient adaptation
layer (SEAL). AAL5 assumes that all cells belonging to a single message travel
sequentially and that control functions are included in the upper layers of the sending
application. Figure shows the AAL5 sublayer.
The four trailer fields in the CS layer are:
o User-to-user (UU). This field is used by end users, as described previously.
o Common part identifier (CPI). This field is the same as defined previously.
o Length (L). The 2-byte L field indicates the length of the original data.
o CRC. The last 4 bytes is for error control on the entire data unit.
o User-to-user (UU). This field is used by end users, as described previously.
o Common part identifier (CPI). This field is the same as defined previously.
o Length (L). The 2-byte L field indicates the length of the original data.
o CRC. The last 4 bytes is for error control on the entire data unit.
QOS in ATM
Classes:
CBR The constant-bit-rate (CBR) class is designed for customers who need realtime
audio or video services. The service is similar to that provided by a dedicated line
such as a T line.
VBR The variable-bit-rate (VBR) class is divided into two subclasses: real-time
(VBR-RT) and non-real-time (VBR-NRT). VBR-RT is designed for those users who
need real-time services (such as voice and video transmission) and use compression
techniques to create a variable bit rate. VBR-NRT is designed for those users who do
not need real-time services but use compression techniques to create a variable bit rate.
ABR The available-bit-rate (ABR) class delivers cells at a minimum rate. If more
network capacity is available, this minimum rate can be exceeded. ABR is particularly
suitable for applications that are bursty.
UBR The unspecified-bit-rate (UBR) class is a best-effort delivery service that does
not guarantee anything.
audio or video services. The service is similar to that provided by a dedicated line
such as a T line.
VBR The variable-bit-rate (VBR) class is divided into two subclasses: real-time
(VBR-RT) and non-real-time (VBR-NRT). VBR-RT is designed for those users who
need real-time services (such as voice and video transmission) and use compression
techniques to create a variable bit rate. VBR-NRT is designed for those users who do
not need real-time services but use compression techniques to create a variable bit rate.
ABR The available-bit-rate (ABR) class delivers cells at a minimum rate. If more
network capacity is available, this minimum rate can be exceeded. ABR is particularly
suitable for applications that are bursty.
UBR The unspecified-bit-rate (UBR) class is a best-effort delivery service that does
not guarantee anything.
User-Related Attributes:
SCR The sustained cell rate (SCR) is the average cell rate over a long time interval.
The actual cell rate may be lower or higher than this value, but the average should be
equal to or less than the SCR.
PCR The peak cell rate (PCR) defines the sender's maximum cell rate. The user's
cell rate can sometimes reach this peak, as long as the SCR is maintained.
MCR The minimum cell rate (MCR) defines the minimum cell rate acceptable to the
sender. For example, if the MCR is 50,000, the network must guarantee that the sender
can send at least 50,000 cells per second.
CVDT The cell variation delay tolerance (CVDT) is a measure of the variation in
cell transmission times. For example, if the CVDT is 5 ns, this means that the difference
between the minimum and the maximum delays in delivering the cells should not
exceed 5 ns.
SCR The sustained cell rate (SCR) is the average cell rate over a long time interval.
The actual cell rate may be lower or higher than this value, but the average should be
equal to or less than the SCR.
PCR The peak cell rate (PCR) defines the sender's maximum cell rate. The user's
cell rate can sometimes reach this peak, as long as the SCR is maintained.
MCR The minimum cell rate (MCR) defines the minimum cell rate acceptable to the
sender. For example, if the MCR is 50,000, the network must guarantee that the sender
can send at least 50,000 cells per second.
CVDT The cell variation delay tolerance (CVDT) is a measure of the variation in
cell transmission times. For example, if the CVDT is 5 ns, this means that the difference
between the minimum and the maximum delays in delivering the cells should not
exceed 5 ns.
Network-Related Attributes:
CLR The cell loss ratio (CLR) defines the fraction of cells lost (or delivered so late
that they are considered lost) during transmission. For example, if the sender sends
100 cells and one of them is lost.
CTD The cell transfer delay (CTD) is the average time needed for a cell to travel
from source to destination. The maximum CTD and the minimum CTD are also considered
attributes.
CDV The cell delay variation (CDV) is the difference between the CTD maximum
and the CTD minimum.
CER The cell error ratio (CER) defines the fraction of the cells delivered in error.
Function of ATM layer:
-The ATM Adaptation Layer (AAL) interfaces the higher
layer protocols to the ATM Layer.
The ATM adaption layer is divided into two sublayers:
Convergence Sublayer (CS) :
-It wraps the user-service data units in a header and
trailer
-The information in the header and trailer depends on
the class of information to be transported.
Segmentation and reassembly sublayer :
- It recieves the CS prorocol data unit and divides it
up into peices which it can place
in an ATM cell to transport to ATM Layer..
-It adds to each peice a header which contains
information used to reassemble the peices
at the destination.
B-ISDN
The B-ISDN reference protocol model consists of three planes:
- Management Plane
- User Plane
- Control Plane
Management Plane
Two types of functions exist in this plane
Two types of functions exist in this plane
Layer management:
All the management functions related to the resources and parameters
residing in its protocol entities such as signaling are performed by
layer management.
Plane management: All the management functions that relate to the whole system are located in the plane
management.
management.
USER PLANE
The function of the user plane is to transfer the user information from point A to point B in the network.
All associated mechanisms, such as flow control congestion control, or recovery from errors are included.
CONTROL PLANE
This plane is responsible for call control and connection control functions related to setting up and tearing down a connection.
Physical Layer Functions
Divided into two sublayers
Divided into two sublayers
Physical medium: It is the lowest layer of the B-ISDN protocol, and it includes the functions that are only
physical-medium-dependent. It itself provides line coding and if necessary, electrical to optical conversion.
physical-medium-dependent. It itself provides line coding and if necessary, electrical to optical conversion.
Transmission convergence:
The main functions of this sublayer are cell rate decoupling, HEC
(Header Error Control) header sequence generation, cell delineation,
transmission frame adaptation, transmission frame generation.
X.25 Protocol
X.25 Protocol
The X.25 is an International
Telecommunication Union-Tell-Communication Standarization (ITU-T). The X.25
protocol allows computers on different public networks (such as CompuServe,
Tymnet, or a TCP/IP network) to communicate through an intermediary computer at
the network layer level. The X.25 protocols works at the Layers 1 to 3 of the OSI
model. Basically these network devices fall into 3 general categories. Those
are
- Data Terminal Equipment (DTE)
- Data Circuit Terminating Equipment (DCE) and
- Packet Switching Exchange (PSE)
DTE are usually terminals, PC or network hosts. DCE devices are communication devices, such as modem and packet switches and PSE are generally located in the carrier’s facilities.
X.25 defines
the procedures for the exchange of data between user devices and a packet of
node. It is interface between Data Terminal Equipment (DTE) and Data Circuit
Terminating Equipment (DCE) for terminals operating in the packet mode on
Public Data Networks. The x.25 standard specifies calls for three layers for
functionality. Those are,
- Physical Layer
- Data Link Layer
- Network Layer
These three
layers correspond to the lowest three layers of the OSI model.
The Physical Layer deals with the physical
layer interface between computer and the link that station to the packet
switching node. The X.25 makes use of the physical layer specification in a
standard known as X.21 but in many cases, other standard such as EIA-23 are
substituted. The physical level of X.25 does not perform significant control
functions. It is more of a passive conduit, with control provided by the data
link and network layers.
The Data link Layer provides for the
reliable transfer of data across the physical link transmitting the data as
sequence of frames. The data link layer standard is referred to as Link Access
Protocol Balanced (LAPB). LAPB is the subset of HDLC. The X.25 packets are
carried within the LAPB frame as the information field.
Network layer provides an external virtual circuit
service. User data are passed down to X.25 level 3, which appends control
information as a header, creating a packet. X.25 protocol uses this control
information for its operation.
The entire
X.25 packet is then passed down to the LAPB entity, which appends control
information at the front and back of the packet, forming an LAPB frame. X.25 operates
on the premise of the virtual circuit services. X.25 provies for two types of
virtual circuit. Those are,
- Virtual Call (VC)
- Permanent Virtual Circuit (PVC)
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