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Module 2: WAN Technology
Introducing ATM Technology / Concepts:
• Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the
ATM Forum and adopted by the ITU-T.
• Design Goals:
o Need for a transmission system to optimize the use of high-data-rate
transmission media, in particular optical fiber.
o Need for a system that can interface with existing systems, such as the various
packet networks, and provide wide area interconnectivity between them without
lowering their effectiveness or requiring their replacement.
o Need for a design that can be implemented inexpensively so that cost would not
be a barrier to adoption. If ATM is to become the backbone of international
communications, as intended, it must be available at low cost to every user who
wants it.
o The new system must be able to work with and support the existing
telecommunications hierarchies (local loops, local providers, long-distance
carriers, and so on).
o The new system must be connection-oriented to ensure accurate and predictable
delivery.
o Move as many of the functions to hardware as possible (for speed) and eliminate
as many software functions as possible (again for speed).
• Cell Networks:
o ATM is a cell network.
o A cell is a small data unit of fixed size that is the basic unit of data exchange in
a cell network. In this type of network, all data are loaded into identical cells
that can be transmitted with complete predictability and uniformity.
o Cells are multiplexed with other cells and routed through a cell network.
Because each cell is the same size and all are small, any problems associated
with multiplexing different-sized packets are avoided.
• Asynchronous TDM:
o ATM uses asynchronous time-division multiplexing, that is why it is called
Asynchronous Transfer Mode, to multiplex cells coming from different
channels. It uses fixed-size slots the size of a cell.
o ATM multiplexers fill a slot with a cell from any input channel that has a cell;
the slot is empty if none of the channels has a cell to send.
o Figure 1 shows how cells from three inputs are multiplexed. At the first tick of
the clock, channel 2 has no cell (empty input slot), so the multiplexer fills the
slot with a cell from the third channel.
o When all the cells from all the channels are multiplexed, the output slots are
empty.
Figure 1: ATM Multiplexing
ATM Architecture:
o ATM is a switched network.
o The user access devices, called the end points, are connected to the switches
inside the network. The switches are connected to each other using high-speed
communication channels. Figure 2 shows an example of an ATM network.
Figure 2: Architecture of an ATM network
o Virtual Connection:
▪ Connection between two end points is accomplished through
transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs).
▪ A transmission path (TP) is the physical connection (wire, cable,
satellite, and so on) between an end point and a switch or between two
switches.
▪ Think of two switches as two cities. A transmission path is the set of all
highways that directly connects the two cities.
▪ A transmission path is divided into several virtual paths. A virtual path
(VP) provides a connection or a set of connections between two
switches. Think of a virtual path as a highway that connects two cities.
Each highway is a virtual path; the set of all highways is the transmission
path.
▪ Cell networks are based on virtual circuits (VCs). All cells belonging
to a single message follow the same virtual circuit and remain in their
original order until they reach their destination. Think of a virtual circuit
as the lanes of a highway (virtual path) as shown in Figure 3.
Figure 3: Virtual circuits
▪ The figure also shows the relationship between a transmission path (a
physical connection), virtual paths (a combination of virtual circuits that
are bundled together because parts of their paths are the same), and
virtual circuits that logically connect two points together.
▪ In a virtual circuit network, to route data from one end point to another,
the virtual connections need to be identified. For this purpose, the
designers of ATM created a hierarchical identifier with two levels: a
virtual path identifier (VPI) and a virtual circuit identifier (VCI).
The VPI defines the specific VP and the VCI defines a particular VC
inside the VP. The VPI is the same for all virtual connections that are
bundled (logically) into one VP.
ATM Layers:
o The ATM standard defines three layers. They are, from top to bottom, the
application adaptation layer, the ATM layer, and the physical layer as shown in
Figure 4.
Figure 4: ATM Layers
o The physical and ATM layer are used in both switches inside the network and
end points (such as routers) that use the services of the ATM. The application
adaptation layer (AAL) is used only by the end points.
▪ Physical Layer:
• The physical layer defines the transmission medium, bit
transmission, encoding, and electrical to optical transformation.
• It provides convergence with physical transport protocols, such
as SONET as well as the mechanisms for transforming the flow
of cells into a flow of bits.
▪ ATM Layer:
• The ATM layer provides routing, traffic management,
switching, and multiplexing services.
• It processes outgoing traffic by accepting 48-byte segments from
the AAL sublayer.
• The addition of a 5-byte header transforms the segment into a
53-byte cell as shown in the Figure 5.
Figure 5: ATM Layer
• A cell is 53 bytes in length with 5 bytes allocated to header and
48 bytes carrying payload (user data may be less than 48 bytes).
• Header Format:
• ATM uses two formats for this header, one for user-to-network
interface (UNI) cells and another for network-to-network
interface (NNI) cells. Figure 6.
• shows these headers in the byte-by-byte format preferred by the
ITU-T (each row represents a byte).
• Most of the header is occupied by the VPI and VCI. Figure 6
shows the cell structure.
• The combination of VPI and VCI can be thought of as a label
that defines a virtual connection.
Figure 6: ATM Cell
• Generic flow control (GFC):
▪ The 4-bit GFC field provides flow control at the UNI level.
▪ The ITU-T has determined that this level of flow control is not
necessary at the NNI level.
▪ In the NNI header, therefore, these bits are added to the VPI. The
longer VPI allows more virtual paths to be defined at the NNI
level. The format for this additional VPI has not yet been
determined.
• Virtual path identifier (VPI):
▪ The VPI is an 8-bit field in a UNI cell and a 12-bit field in an NNI
cell (see above).
• Virtual circuit identifier (VCI):
▪ The VCI is a 16-bit field in both frames.
• Payload type (PT):
▪ In the 3-bit PT field, the first bit defines the payload as user data
or managerial information.
▪ The interpretation of the last 2 bits depends on the first bit.
• Cell loss priority (CLP):
▪ The -bit CLP field is provided for congestion control.
▪ A cell with its CLP bit sets to 1 must be retained as long as there
are cells with a CLP of 0.
• Header error correction (HEC):
▪ The HEC is a code computed for the first 4 bytes of the header.
▪ It is a CRC with the divisor x8
+ x2
+ x + 1 that is used to correct
single-bit errors and a large class of multiple-bit errors.
AAL Layer:
• The application adaptation layer (AAL) allows existing networks (such as
packet networks) to connect to ATM facilities.
• AAL protocols accept transmissions from upper-layer services (e.g., packet
data) and map them into fixed-sized ATM cells.
• The transmissions can be of any type (voice, data, audio, video) and can be of
variable or fixed rates.
• At the receiver, this process is reversed segments are reassembled into their
original formats and passed to the receiving service.
• 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. 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: AAL1, AAL2, AAL3/4, and AAL5.
• The common versions used today are AAL1 and AAL5. The first is used in
streaming audio and video communication; the second, in data
communications.
• AAL1:
o AAL1 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.
o Figure 7 shows how a bit stream of data is chopped into 47-byte chunks
and encapsulated in cells.
Figure 7: AAL1
o 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.
o 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.
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.
• AAL2:
o AAL2 was intended to support a variable-data-rate bit stream, but it has
been redesigned. It is now used for low-bit-rate traffic and short-frame
traffic such as audio (compressed or uncompressed), video, or fax.
o A good example of AAL2 use is in mobile telephony. AAL2 allows the
multiplexing of short frames into one cell.
o Figure 8 shows the process of encapsulating a short frame from the same
source (the same user of a mobile phone) or from several sources (several
users of mobile telephones) into one cell.
Figure 8: AAL2
o The CS layer overhead consists of five fields:
o Channel identifier (CID): The 8-bit CID field defines the channel (user)
of the short packet.
o Length indicator (LI): The 6-bit LI field indicates how much of the final
packet is data.
o Packet payload type (PPT): The PPT field defines the type of packet.
o User-to-user indicator (UUI): The UUI field can be used by end-to-end
users.
o Header error control (HEC): The last 5 bits is used to correct errors in the
header.
• AAL3/4:
o AAL3 was intended to support connection-oriented data services and AAL4
to support connectionless services.
o They have been combined into a single format called AAL3/4. Figure 9
shows the AAL3/4 sublayer.
MCQ Questions:
1) ATM and frame relay are ________
a. virtual circuit networks
b. datagram networks
c. virtual private networks
d. virtual public networks
e. ATM uses _______________
a) asynchronous frequency division multiplexing
f. asynchronous time division multiplexing
g. asynchronous space division multiplexing
h. asynchronous amplitude division multiplexing
Short Questions:
Long Questions:
Figure 9: AAL3/4
o 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 0.
o Begin tag (Btag): The value of this field is repeated in each cell to identify
all the cells belonging to the same packet. The value is the same as the
Etag.
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 1-byte AL field is included to make the rest of the
trailer 4 bytes long.
o Ending tag (Etag): The 1-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 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 ST of 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:
o AAL5, which is sometimes called the simple and efficient adaptation
layer (SEAL), 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.
o AAL5 is designed for connectionless packet protocols that use a datagram
approach to routing (such as the IP protocol in TCP/IP).
o AAL5 accepts an IP packet of no more than 65,535 bytes and adds an 8-
byte trailer as well as any padding required to ensure that the position of the
trailer falls where the receiving equipment expects it (at the last 8 bytes of
the last cell).
o As shown in Figure 10. Once the padding and trailer are in place, AAL5
passes the message in 48-byte segments to the ATM layer.
Figure 10: AAL5
o 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.
Need and Benefit of ATM:
• It is optimized to transport voice, data and video i.e. single network for everything. It
is used for mixed traffic, real-time and non-real time traffic types.
• It provides the dynamic bandwidth that is particularly suited for bursty traffic.
• Uniform packet size ensures that mixed traffic is handled efficiently.
• Small sized header reduces packet overload, thus ensuring effective bandwidth usage.
• It enables efficient use of network resources using bandwidth on demand concept.
• It uses simplified network infrastructure.
Drawbacks of ATM:
• Overhead of cell header (5 bytes per cell)
• Complex mechanisms are used to achieve QoS.
• Congestion may cause cell losses
• ATM switch is very expensive compare to LAN hardware. Moreover, ATM NIC is
more expensive compare to ethernet NIC.
• As ATM is connection-oriented technology, the time required for connection setup and
tear down is larger compare to time required to use it.
Faces of ATM:
• ATM as an Architecture and Technology
• ATM as an Interface and Protocol
• ATM as an Economical, Integrated Access
• ATM as a WAN Transport Service
BISDN Reference Model:
• The Broadband-ISDN (B-ISDN) protocol reference model consists of three planes: the
user plane, the control plane and the management plane.
• The layers are from down to top: Physical Layer, ATM Layer, ATM Adaptation Layer
(AAL) and the higher layer protocols.
• The B-ISDN protocol reference model is depicted in figure 11.
Figure 11: B-ISDN Reference Model
• User plane
o The user plane, with its layered structure, provides for user information flow
transfer, along with associated controls (e.g. flow control, and recovery from
errors, etc.).
• Control plane
o This plane has a layered structure and performs the call control and connection
control functions; it deals with the signalling necessary to set up, supervise and
release calls and connections.
o The distinction, if any, between local and global control plane functions in the
broadband environment is for further study.
• Management plane
o The management plane provides two types of functions, namely Layer
Management and plane management functions.
o Plane management functions
▪ The plane management performs management functions related to a
system as a whole and provides coordination between all the planes.
Plane management has no layered structure.
o Layer Management functions
▪ Layer Management performs management functions (e.g. meta-
signalling) relating to resources and parameters residing in its protocol
entities. Layer Management handles the operation and maintenance
(OAM) information flows specific to the layer concerned.
• Functions of the individual layers of the B-ISDN PRM
o The functions of each layer, the primitives exchanged between layers, and
primitives exchanged between the layers and the management plane are
described below.
o The information flows described do not imply a specific physical realisation.
Table 1 illustrates the layers of the PRM, and identifies the functions of the
Physical Layer, the ATM Layer, and the AAL.
Table 1: Functions of each layer
• Physical layer:
o The lowest layer in the ATM protocol. It describes the physical transmission
media. We can use shielded and shielded twisted pair, Coaxial cable and fibre
optic cable.
• ATM layer:
o It performs all function relating to the routing and multiplexing of cells over
VCs.
o It generates a header to the segment streams generated by the AAL.
o Similarly, on the receipt of a cell streams it removes the header from the cell
and pass the cell contents to the AA1 protocol. To perform all these functions,
the ATM layer maintains a table which contain a list of VCIs.
• ATM Adaptation Layer:
o Top layer in the ATM protocol model.
o It converts the submitted information into streams of 48-octet segments and
transports these in the payload field of multiple ATM cells.
o Similarly, a receipt of the stream of cells relating to the same cell, it converts
the 48-octet information field into required form for delivery to the particular
higher protocol layer.

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WAN Technology : ATM - Forouzan

  • 1. Module 2: WAN Technology Introducing ATM Technology / Concepts: • Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by the ITU-T. • Design Goals: o Need for a transmission system to optimize the use of high-data-rate transmission media, in particular optical fiber. o Need for a system that can interface with existing systems, such as the various packet networks, and provide wide area interconnectivity between them without lowering their effectiveness or requiring their replacement. o Need for a design that can be implemented inexpensively so that cost would not be a barrier to adoption. If ATM is to become the backbone of international communications, as intended, it must be available at low cost to every user who wants it. o The new system must be able to work with and support the existing telecommunications hierarchies (local loops, local providers, long-distance carriers, and so on). o The new system must be connection-oriented to ensure accurate and predictable delivery. o Move as many of the functions to hardware as possible (for speed) and eliminate as many software functions as possible (again for speed). • Cell Networks: o ATM is a cell network. o A cell is a small data unit of fixed size that is the basic unit of data exchange in a cell network. In this type of network, all data are loaded into identical cells that can be transmitted with complete predictability and uniformity. o Cells are multiplexed with other cells and routed through a cell network. Because each cell is the same size and all are small, any problems associated with multiplexing different-sized packets are avoided. • Asynchronous TDM: o ATM uses asynchronous time-division multiplexing, that is why it is called Asynchronous Transfer Mode, to multiplex cells coming from different channels. It uses fixed-size slots the size of a cell. o ATM multiplexers fill a slot with a cell from any input channel that has a cell; the slot is empty if none of the channels has a cell to send. o Figure 1 shows how cells from three inputs are multiplexed. At the first tick of the clock, channel 2 has no cell (empty input slot), so the multiplexer fills the slot with a cell from the third channel. o When all the cells from all the channels are multiplexed, the output slots are empty.
  • 2. Figure 1: ATM Multiplexing ATM Architecture: o ATM is a switched network. o The user access devices, called the end points, are connected to the switches inside the network. The switches are connected to each other using high-speed communication channels. Figure 2 shows an example of an ATM network. Figure 2: Architecture of an ATM network o Virtual Connection: ▪ Connection between two end points is accomplished through transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs). ▪ A transmission path (TP) is the physical connection (wire, cable, satellite, and so on) between an end point and a switch or between two switches. ▪ Think of two switches as two cities. A transmission path is the set of all highways that directly connects the two cities. ▪ A transmission path is divided into several virtual paths. A virtual path (VP) provides a connection or a set of connections between two switches. Think of a virtual path as a highway that connects two cities. Each highway is a virtual path; the set of all highways is the transmission path. ▪ Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual circuit and remain in their original order until they reach their destination. Think of a virtual circuit as the lanes of a highway (virtual path) as shown in Figure 3.
  • 3. Figure 3: Virtual circuits ▪ The figure also shows the relationship between a transmission path (a physical connection), virtual paths (a combination of virtual circuits that are bundled together because parts of their paths are the same), and virtual circuits that logically connect two points together. ▪ In a virtual circuit network, to route data from one end point to another, the virtual connections need to be identified. For this purpose, the designers of ATM created a hierarchical identifier with two levels: a virtual path identifier (VPI) and a virtual circuit identifier (VCI). The VPI defines the specific VP and the VCI defines a particular VC inside the VP. The VPI is the same for all virtual connections that are bundled (logically) into one VP. ATM Layers: o The ATM standard defines three layers. They are, from top to bottom, the application adaptation layer, the ATM layer, and the physical layer as shown in Figure 4. Figure 4: ATM Layers o The physical and ATM layer are used in both switches inside the network and end points (such as routers) that use the services of the ATM. The application adaptation layer (AAL) is used only by the end points. ▪ Physical Layer: • The physical layer defines the transmission medium, bit transmission, encoding, and electrical to optical transformation.
  • 4. • It provides convergence with physical transport protocols, such as SONET as well as the mechanisms for transforming the flow of cells into a flow of bits. ▪ ATM Layer: • The ATM layer provides routing, traffic management, switching, and multiplexing services. • It processes outgoing traffic by accepting 48-byte segments from the AAL sublayer. • The addition of a 5-byte header transforms the segment into a 53-byte cell as shown in the Figure 5. Figure 5: ATM Layer • A cell is 53 bytes in length with 5 bytes allocated to header and 48 bytes carrying payload (user data may be less than 48 bytes). • Header Format: • ATM uses two formats for this header, one for user-to-network interface (UNI) cells and another for network-to-network interface (NNI) cells. Figure 6. • shows these headers in the byte-by-byte format preferred by the ITU-T (each row represents a byte). • Most of the header is occupied by the VPI and VCI. Figure 6 shows the cell structure. • The combination of VPI and VCI can be thought of as a label that defines a virtual connection. Figure 6: ATM Cell
  • 5. • Generic flow control (GFC): ▪ The 4-bit GFC field provides flow control at the UNI level. ▪ The ITU-T has determined that this level of flow control is not necessary at the NNI level. ▪ In the NNI header, therefore, these bits are added to the VPI. The longer VPI allows more virtual paths to be defined at the NNI level. The format for this additional VPI has not yet been determined. • Virtual path identifier (VPI): ▪ The VPI is an 8-bit field in a UNI cell and a 12-bit field in an NNI cell (see above). • Virtual circuit identifier (VCI): ▪ The VCI is a 16-bit field in both frames. • Payload type (PT): ▪ In the 3-bit PT field, the first bit defines the payload as user data or managerial information. ▪ The interpretation of the last 2 bits depends on the first bit. • Cell loss priority (CLP): ▪ The -bit CLP field is provided for congestion control. ▪ A cell with its CLP bit sets to 1 must be retained as long as there are cells with a CLP of 0. • Header error correction (HEC): ▪ The HEC is a code computed for the first 4 bytes of the header. ▪ It is a CRC with the divisor x8 + x2 + x + 1 that is used to correct single-bit errors and a large class of multiple-bit errors. AAL Layer: • The application adaptation layer (AAL) allows existing networks (such as packet networks) to connect to ATM facilities. • AAL protocols accept transmissions from upper-layer services (e.g., packet data) and map them into fixed-sized ATM cells. • The transmissions can be of any type (voice, data, audio, video) and can be of variable or fixed rates. • At the receiver, this process is reversed segments are reassembled into their original formats and passed to the receiving service. • 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. Segmentation is at the source; reassembly, at the destination.
  • 6. • 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: AAL1, AAL2, AAL3/4, and AAL5. • The common versions used today are AAL1 and AAL5. The first is used in streaming audio and video communication; the second, in data communications. • AAL1: o AAL1 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. o Figure 7 shows how a bit stream of data is chopped into 47-byte chunks and encapsulated in cells. Figure 7: AAL1 o 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. o 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. 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. • AAL2: o AAL2 was intended to support a variable-data-rate bit stream, but it has been redesigned. It is now used for low-bit-rate traffic and short-frame traffic such as audio (compressed or uncompressed), video, or fax. o A good example of AAL2 use is in mobile telephony. AAL2 allows the multiplexing of short frames into one cell.
  • 7. o Figure 8 shows the process of encapsulating a short frame from the same source (the same user of a mobile phone) or from several sources (several users of mobile telephones) into one cell. Figure 8: AAL2 o The CS layer overhead consists of five fields: o Channel identifier (CID): The 8-bit CID field defines the channel (user) of the short packet. o Length indicator (LI): The 6-bit LI field indicates how much of the final packet is data. o Packet payload type (PPT): The PPT field defines the type of packet. o User-to-user indicator (UUI): The UUI field can be used by end-to-end users. o Header error control (HEC): The last 5 bits is used to correct errors in the header. • AAL3/4: o AAL3 was intended to support connection-oriented data services and AAL4 to support connectionless services. o They have been combined into a single format called AAL3/4. Figure 9 shows the AAL3/4 sublayer. MCQ Questions: 1) ATM and frame relay are ________ a. virtual circuit networks b. datagram networks c. virtual private networks d. virtual public networks e. ATM uses _______________ a) asynchronous frequency division multiplexing f. asynchronous time division multiplexing
  • 8. g. asynchronous space division multiplexing h. asynchronous amplitude division multiplexing Short Questions: Long Questions: Figure 9: AAL3/4 o 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 0. o Begin tag (Btag): The value of this field is repeated in each cell to identify all the cells belonging to the same packet. The value is the same as the Etag. 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 1-byte AL field is included to make the rest of the trailer 4 bytes long. o Ending tag (Etag): The 1-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 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 ST of 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.
  • 9. o CRC: The last 10 bits of the trailer is a CRC for the entire data unit. • AAL5: o AAL5, which is sometimes called the simple and efficient adaptation layer (SEAL), 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. o AAL5 is designed for connectionless packet protocols that use a datagram approach to routing (such as the IP protocol in TCP/IP). o AAL5 accepts an IP packet of no more than 65,535 bytes and adds an 8- byte trailer as well as any padding required to ensure that the position of the trailer falls where the receiving equipment expects it (at the last 8 bytes of the last cell). o As shown in Figure 10. Once the padding and trailer are in place, AAL5 passes the message in 48-byte segments to the ATM layer. Figure 10: AAL5 o 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. Need and Benefit of ATM: • It is optimized to transport voice, data and video i.e. single network for everything. It is used for mixed traffic, real-time and non-real time traffic types. • It provides the dynamic bandwidth that is particularly suited for bursty traffic. • Uniform packet size ensures that mixed traffic is handled efficiently. • Small sized header reduces packet overload, thus ensuring effective bandwidth usage. • It enables efficient use of network resources using bandwidth on demand concept. • It uses simplified network infrastructure.
  • 10. Drawbacks of ATM: • Overhead of cell header (5 bytes per cell) • Complex mechanisms are used to achieve QoS. • Congestion may cause cell losses • ATM switch is very expensive compare to LAN hardware. Moreover, ATM NIC is more expensive compare to ethernet NIC. • As ATM is connection-oriented technology, the time required for connection setup and tear down is larger compare to time required to use it. Faces of ATM: • ATM as an Architecture and Technology • ATM as an Interface and Protocol • ATM as an Economical, Integrated Access • ATM as a WAN Transport Service BISDN Reference Model: • The Broadband-ISDN (B-ISDN) protocol reference model consists of three planes: the user plane, the control plane and the management plane. • The layers are from down to top: Physical Layer, ATM Layer, ATM Adaptation Layer (AAL) and the higher layer protocols. • The B-ISDN protocol reference model is depicted in figure 11. Figure 11: B-ISDN Reference Model • User plane o The user plane, with its layered structure, provides for user information flow transfer, along with associated controls (e.g. flow control, and recovery from errors, etc.). • Control plane
  • 11. o This plane has a layered structure and performs the call control and connection control functions; it deals with the signalling necessary to set up, supervise and release calls and connections. o The distinction, if any, between local and global control plane functions in the broadband environment is for further study. • Management plane o The management plane provides two types of functions, namely Layer Management and plane management functions. o Plane management functions ▪ The plane management performs management functions related to a system as a whole and provides coordination between all the planes. Plane management has no layered structure. o Layer Management functions ▪ Layer Management performs management functions (e.g. meta- signalling) relating to resources and parameters residing in its protocol entities. Layer Management handles the operation and maintenance (OAM) information flows specific to the layer concerned. • Functions of the individual layers of the B-ISDN PRM o The functions of each layer, the primitives exchanged between layers, and primitives exchanged between the layers and the management plane are described below. o The information flows described do not imply a specific physical realisation. Table 1 illustrates the layers of the PRM, and identifies the functions of the Physical Layer, the ATM Layer, and the AAL. Table 1: Functions of each layer
  • 12. • Physical layer: o The lowest layer in the ATM protocol. It describes the physical transmission media. We can use shielded and shielded twisted pair, Coaxial cable and fibre optic cable. • ATM layer: o It performs all function relating to the routing and multiplexing of cells over VCs. o It generates a header to the segment streams generated by the AAL. o Similarly, on the receipt of a cell streams it removes the header from the cell and pass the cell contents to the AA1 protocol. To perform all these functions, the ATM layer maintains a table which contain a list of VCIs. • ATM Adaptation Layer: o Top layer in the ATM protocol model. o It converts the submitted information into streams of 48-octet segments and transports these in the payload field of multiple ATM cells. o Similarly, a receipt of the stream of cells relating to the same cell, it converts the 48-octet information field into required form for delivery to the particular higher protocol layer.