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Chapter 18
Virtual Circuit
Switching:
Frame Relay
and
ATM
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©The McGraw-Hill Companies, Inc., 2004
Figure 18.1 Virtual circuit wide area network
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Main difference between a circuit-switched and a packet-switched
network is that in the latter the links are shared, channelized
between different communication paths.
Frame Relay and ATM [WAN technologies] use virtual circuit
switching.
Packet switching uses two different approaches: the datagram
approach [used in network layer] and the virtual circuit approach
[used in data link layer].
Global address:
 A source or destination needs to have a global address which is
unique address over the global space.
 However, Global addressing in virtual circuit networks is used
only to create a virtual circuit identifier.
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Figure 18.2 Virtual Circuit Identifier (VCI)
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This identifier is used for data transfer.
It is a small number.
It only has switch scope; it is used by a frame
between two switches. When a frame arrives at a
switch, it has one VCI; when it leaves, it has another.
VCI does not need to be a large number since each
switch can use its own unique set of VCIs.
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Figure 18.3 VCI phases
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To communicate, a source and destination need to go
through three phases:
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Setup phase: Source and destination use their global
addresses to help switches make table entries for the
connection.
Teardown phase: Source and destination inform the switches
to erase the corresponding entry.
Data Transfer phase: Occurs between these two phases.
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Figure 18.4
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Switch and table – Data Transfer Phase
To transfer a frame from a source to its destination,
all switches need to have a table entry for this virtual
circuit.
The switch table, in its simplest form, has 4 columns.
Switch holds four pieces of information for each
virtual circuit.
A frame arrives at port 1 with a VCI of 14. When the frame arrives, the
switch looks in its table to find port 1, and VCI 14. When it is found,
the switch knows to change the VCI to 22 and send out the frame from
port 3.
The procedure is same for all frames from the source to destination.
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Figure 18.5
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Source-to-destination data transfer
The figure below shows how a frame from source A reaches destination
B and how its VCI changes during the trip. Each switch changes the
VCI and routes the frame.
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Setup Phase
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Permanent Virtual Circuit [PVC]:
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Source and destination may choose to have a PVC.
In PVC, the connection setup is simple.
The corresponding table entry is recorded for all switches by the
administrator (of course remotely and electronically).
An outgoing VCI is given to the source, and an incoming VCI is given
to the destination.
The source always uses this VCI to send frame to that particular
destination; the destination knows that the frame is coming from that
particular source if the frame carries the corresponding incoming VCI.
If there is a need for duplex communication, two virtual circuits are
established.
PVC is like a leased telephone line.
Costlier: A connection is need all the time even if not in use. So, two
parties pay for the connection.
A connection is created from one source to one single destination. If
a source needs connections with several destinations, it needs a PVC
for each connection.
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SVC Setup Phase
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Switched Virtual Circuit [SVC]:
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SVC creates a temporary, short connection that exists only when data
are being transferred between source and destination.
SVC requires a connection setup phase.
To setup connections between A and B, we need setup request and
acknowledgement.
Setup Request:
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Source A sends a setup frame to switch I.
Switch I receives the setup request frame. It knows that a frame going
from A to B goes out through port 3. The switch creates an entry in its
table for this virtual circuit, but it is only able to fill three of the four
columns. The switch assigns the incoming port (1) and chooses and
available incoming VCI (14) and the outgoing port (3). It does not know
the outgoing VCI, which will be found during the acknowledgement step.
The switch then forwards the frame through port 3 to switch II.
Switch II receives the setup request frame and does as switch I.
Switch III receives the setup request frame and does as switch I and II.
Destination B receives the setup frame, and if it is ready to receive frames
from A, it assigns a VCI to the incoming frames that came from A, in this
case 77. This VCI lets the destination know that the frames comes from A,
and not from other sources.
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Figure 18.6 and 18.7
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Acknowledgement:
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SVC setup and SVC setup acknowledgment
Destination sends an acknowledgement to Switch III. Acknowledgement carries the
global source and destination addresses so the switch knows which entry in the
table is to be completed. The frame also carries VCI 77, chosen by the destination
as the incoming VCI for frames from A. Switch III uses this VCI to complete the
incoming VCI column for this entry. Note that 77 is incoming VCI for destination B,
but outgoing VCI for switch III.
Switch III sends an acknowledgement to switch II that contains its incoming VCI in
the table, chosen in setup phase. Switch II updates.
Switch II sends an acknowledgement to switch I and switch I sends an
acknowledgement to source A.
The source uses this as the outgoing VCI for the data frames to be sent to the
destination B.
Teardown phase:
After sending all frames to B, source A sends a special frame
called a teardown request. Destination B responds with a teardown confirmation frame.
All switches erase the corresponding entry from their tables.
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Figure 18.8
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Frame Relay is a virtual circuit wide area network that
was designed to respond to demands for a new type of
WAN.
Prior to Frame Relay, X.25 was used
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Frame Relay network
X.25 performs virtual circuit switching at network layer.
X.25 has a low 64-Kbps data rate.
X.25 has extensive flow and error control at both data link and
network layers. X.25 requires ACK for both data link layer
frames and network layer packets that are sent between nodes
and between source and destination.
X.25 has its own network layer, which differs from that of the
Internet. To use X.25 in Internet, Internet must deliver its
network-layer packet [called as datagram] to X.25 for
encapsulation in the X.25 packet, which doubles overhead.
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Instead of X.25, people lease T-1 or T-3 lines from public
service providers.
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Pay for the whole even if we use just 10%.
Assumes that the user has fixed data rate. With bursty rate and
the average is still within the limit, is not acceptable.
Bursty data requires ‘Bandwidth on demand’
Frame Relay
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Can run at higher speed [1.544Mbps, and recently 44.376 Mbps]
Operates in physical and data link layers. Thus, can run with the
network layer of the Internet.
Allows bursty data.
Allows a frame size of 9000 bytes, which can accommodate all
local area network frame sizes.
Less expensive.
Error detection at data link layer only. No flow or error control.
No retransmission policy. Flow and error control should be
handled at higher layers.
Provides PVC and SVC.
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Figure 18.9
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VCIs in Frame Relay are called (Data Link Connection
Identifier) DLCIs.
The table on the switches match an incoming port-DLCI
combination with an outgoing port-DLCI combination.
Physical Layer of Frame Relay
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No specific protocol at Physical layer. Supports any protocol
recognized by ANSI.
Data link layer of Frame Relay
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Frame Relay layers
Simplified version of HDLC at data link layer because HDLC
provides extensive error and flow control fields that are not needed in
Frame Relay.
Frame Relay has features for congestion control and quality
of service.
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Figure 18.10
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Frame Relay frame
Frame Relay frame format is similar to that of HDLC, except that control field
is missing because this field in HDLC is used for flow and error control.
Address field defines the DLCI as well as some bits used to control
congestion and traffic.
Address (DLCI) field: First 6 bits is used by part 1 of DLCI. Second 4 bits is
used by part 2 of DLCI. DLCI is of 10 bits.
Command/Response: Allows upper layers to identify a frame as command or
response. Not used by Frame Relay protocol.
Extended Address: Indicates whether the current byte is the final byte of
address. EA = 0 means another address byte is to follow.
FECN: Indicate congestion in the direction in which the frame is traveling.
Informs destination.
BECN: Indicate congestion in the direction opposite to which the frame is
traveling. Informs source.
Discard Eligibility: Indicate priority level of the frame. DE =1 means discard
this frame if there is congestion. Can be set by the sender or by any switch in
the network.
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Extended Address: To increase the range of DLCIs, Frame Relay
address has been extended from the original 2-byte address to 3- or
4-byte addrseses.
Frame Relay Assembler/Disassembler (FRAD): FRAD assembles and
disassembles frames coming from other protocols to allow them to
be carried by Frame Relay frames. FRAD can be implemented as a
separate device or part of a switch.
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Voice Over Frame Relay (VOFR):
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Local Management Information (LMI) is a protocol added to
Frame Relay to provide management features.
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Sends voice through the network.
Voice is digitized using PCM and then compressed and sent as data
frames over the network.
Quality is not as good as circuit-switched telephone network.
Keep-alive mechanism to check if data are flowing
Multicast mechanism to allow a local end system to send frames to
more than one remote end system
Mechanism to allow an end system to check the status of a switch.
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Figure 18.13
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Asynchronous Transfer Mode (ATM) is the cell relay
protocol.
Design Goals
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Multiplexing using different frame sizes
Work with high data rate by using the features [large
bandwidth, less susceptible to noise] of optical fiber.
Interact with existing systems and provide interconnections
without lowering their effectiveness or requiring their
replacement.
Cost effective.
Able to work with and support existing telecommunications
hierarchies (local loops, local providers, long-distance
carriers,…)
Connection-oriented to ensure accurate and predictable
delivery.
Move as many functions as possible to hardware and make the
software simple and faster.
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Problems in existing systems:
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As networks become complex and so headers need
more information in them.
Header size becomes larger. All network devices
should read and process a lot.
Variable sized frames.
Scenario
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One line uses large frames (data frame) and other uses small
frames (audio and video information).
If the large frame arrives a moment earlier, then the smaller
frames need to wait a long time in the multiplexer before being
transmitted. Thus data frames create unacceptable delays for
small frames.
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Figure 18.14
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Multiplexing using cells
A cell network uses the cell as the basic unit of data exchange. A cell is
defined as a small, fixed-sized
block of information.
As frames of different sizes and formats reach the cell network from a
tributary network, they are split into multiple small data units of equal
length and are loaded into cells. The cells are then multiplexed with
other cells and routed through the cell network.
High speed of links coupled with the small size of the cells means that,
despite interleaving, cells from each line arrive at their respective
destination in an approximation of continuous strem.
Cell network can handle real-time transmissions, such as phone call,
without the parties being aware of the segmentation or multiplexing at
all.
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Figure 18.15 Asynchronous Transfer Mode (ATM) multiplexing
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ATM uses asynchronous time-division multiplexing.
Multiplex cells coming from different channels.
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.
ATM is a cell-switched network.
The user access devices, called the endpoints, are connected
through a user-to-network interface (UNI) to the switches inside
the network. The switches are connected through network-tonetwork interfaces (NNIs).
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Figure 18.17 TP, VPs, and VCs
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Connection between two endpoints is accomplished
through TPs, VPs and VCs.
Transmission Path(TP): Physical connection (wire,
cable, satellite, and so on) between an endpoint and
a switch or between two switches. [All highways]
Transmission path is divided into several virtual paths
(VP). It provides a connection or a set of connections
between two switches. [Highway]
Virtual Circuits (VC): All cells belonging to a single
message follow the same virtual circuit and remain in
their original order until they reach their destination.
[Lanes]
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Figure 18.18
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Example of VPs and VCs
The first two VCs seem to share the same virtual
path from switch I to switch III, so it is reasonable to
bundle these two VCs together to form one VP.
The other two VCs share the same path from switch I
to switch IV, so it is also reasonable to combine them
to form one VP.
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Figure 18.19
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Connection identifiers
Virtual connection is defined by a pair of numbers:
the VPI and the VCI.
Virtual Path Identifier (VPI) defines the specific VP.
Virtual Circuit Identifier (VCI) defines the particular
VC inside the VP.
VPI is same for all virtual connections that are
bundled (logically) into one VP.
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Figure 18.20 Virtual connection identifiers in UNIs and NNIs
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Length of VPIs for UNIs is 8 bits and that for NNIs is 16 bits.
Length of VCIs [16 bits] is same for UNIs and NNIs.
Idea behind dividing a virtual connection identifier into two
parts is to allow hierarchical routing.
Most of the switches in a typical ATM network are routed
using VPIs.
The switches at the boundaries of the network, those that
interact directly with the endpoint devices, use both VPIs
and VCIs.
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Figure 18.21 An ATM cell
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Cell is only 53 bytes long with 5 bytes allocated to header and 48
bytes carrying payload (user data may less than 48 bytes). Header
contains the VPI and VCI.
ATM has two type of connections: PVC and SVC.
PVC:
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SVC:
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A permanent virtual circuit connection is established between two
endpoints by the network provider.
VPIs and VCIs are defined for the permanent connections, and the
values are entered for the tables of each switch.
Each time an endpoint wants to make a connection with another
endpoint, a new virtual circuit must be established.
Request for making virtual circuit is made using network layer address
and services of another protocol (such as IP).
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Figure 18.22
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ATM uses switches to route the cell from a source endpoint to the
destination endpoint.
A switch routers the cell using both the VPIs and the VCIs.
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Scenario:
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Routing with a switch
A cell with VPI of 153 & VCI of 67 arrives at switch interface (port)
1.
Switch checks its switching table, which stores six pieces of
information per row: arrival interface number, incoming VPI,
incoming VCI, corresponding outgoing interface number, the new
VPI, and the new VCI.
Switch finds the entry in the table and discovers that the
combination corresponds to output interface 3, VPI 140 and VCI
92. It changes the VPI and VCI and sends via interface 3.
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Figure 18.23 ATM layers
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Three layers: Application adaptation layer, ATM layer and
physical layer.
Physical layer
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ATM calls can be carried by any physical layer carrier.
SONET is preferred because:
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High data rate.
Using SONET, the boundaries of cells can be clearly defined. SONET
specifies the use of a pointer to define the beginning of a payload. If
the beginning of the first ATM cell is defined, the rest of cells in the
same payload can easily be identified because there are no gaps
between cells. Just count 53 bytes ahead to find the next cell.
Can run on wireless also. But, the problem of cell boundaries must
be solved. One solution for the receiver is to guess the end of cell
and apply the CRC to the 5-byte header. If there is no error, the
end of cell is found, to a high probability, correctly. Count 52 bytes
back to find the beginning of the cell.
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Figure 18.24 ATM layers in endpoint devices and switches
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ATM layer
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Provides routing, traffic management, switching and
multiplexing services.
Processes outgoing traffic by accepting 48-byte segments
from the AAL sublayers and transforming them into 53-byte
cells by the addition of a 5-byte header.
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Figure 18.26 ATM headers
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ATM uses two formats for this header, one for userto-network interface (UNI) cells and another for
network-to-network (NNI) cells.
GFC: 4-bit GFC field provides flow control at the UNI
level. It is not needed in NNI and so these fields are
added to VPI. Long VPI allows more virtual paths to
be defined at NNI level.
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VPI: 8-bit for UNI and 12-bit for NNI.
VCI: 16-bits for both UNI and NNI.
Payload Type (PT): 3 bits
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Cell loss Priority (CLP):
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Provided for congestion control.
A cell with CLP as 1 should be retained as long as there are
cells with CLP as 0.
Header Error Correction (HEC):
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First bit defines the payload as user data or management
information.
The interpretation of the last 2 bits depends on the first bit.
Code computed for the first 4 bytes of header.
It is a CRC with divisor x8+x2+x+1 that is used to correct
single-bit errors and a large class of multiple-bit errors.
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Figure 18.27 AAL1
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Application Adaptation Layer (AAL) should be to accept data
frames (data payload) and streams of bits (multimedia payload).
AAL has two sublayers: SAR & CS.
AAL sublayer SAR (Segmentation and reassembly) does
segmentation [divide the data frame or stream of bits into 48-byte
segments to be carried by a cell] at the source and 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 Convergence Sublayer (CS).
ATM defines four versions: AAL1, AAL2, AAL3/4, AAL5.
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AAL1
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Supports constant bit rate applications such as audio and video, Uses
existing digital telephone networks such as voice channels and T-lines.
Bit stream of data is divided into 47-byte chunks and encapsulated in
cells.
CS sublayer divides bit stream into 47-byte segments and passes them to
SAR sublayer. CS sublayer does not add a header.
SAR adds 1 byte of header and passes 48-byte segment to ATM layer.
Header has two fields:
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Sequence Number (SN): 4-bit field used to order the bits. First bit is
sometimes used for timing, which leaves 3 bits for sequencing (modulo 8)
Sequence Number Protection (SNP): 4-bit field protects the first field. First 3
bits automatically correct the SN field. Last bit is a parity bit that detects error
over all 8 bits.
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Figure 18.28 AAL2
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Originally designed for variable data rate but now used for low-bit-rate
traffic and short-frame traffic such as audio, video or fax. Ex. Mobile
telephony.
AAL2 allows the multiplexing of short frames into one cell.
CS overhead consists of five fields:
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CID: 8-bit defines the channel (user) of short packet; LI: 6-bit indicates how
much of the find packet is data; PPT: defines the type of packet; UUI: Used by
end-to-end users; HEC: last 5 bits used to correct errors in header.
SAR overhead is start field (SF) that defines the offset from the beginning
of the packet.
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Figure 18.29 AAL3/4
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AAL3 was intended to support connection-oriented data
services and AAL4 to support connectionless services.
But, they evolved to have the fundamental issues as
same. Thus combined as AAL3/4.
CS header and trailer has six fields:
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CPI: Defines how the subsequent fields are to be interpreted.
The value at present is 0.
Btag: This value is repeated in each cell to identify all the cells
beginning to the same packet. Value is same as Etag.
BAsize: 2-byte tells the receiver what size buffer is needed for
coming data.
AL: 1-byte included to make the rest of the trailer 4 bytes long.
ETag: 1-byte serves as ending flag.
L: 2-byte indicates the length of the data unit.
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SAR header and trailer consists of five fields:
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ST: 2-bit 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.
SN: sequence number.
MID: 10-bit field identifies cell coming from different data flows and
multiplexed on the same virtual connection.
LI: Defines how much of the packet is data, not padding.
CRC: last 10 bits of trailer is a CRC for the entire data unit.
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Figure 18.30 AAL5
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AAL3/4 provides comprehensive sequencing and error control
mechanisms that are not necessary for every application.
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.
ATM has provided a fifth AAL sublayer, called Simple and efficient
adaptation layer (SEAL).
Four trailer fields in CS layer are:
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UU: Used by end users.
CPI
L: 2-byte indicates the length of the original data.
CRC: last 4 bytes are for error control on the entire data unit.
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