Download ATM

NETE0510
ATM
Dr. Supakorn Kungpisdan
[email protected]
NETE0510: Communication Media and Data
Communications
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Outline
 Overview
 Cells
 Segmentation and Reassembly
 Virtual Paths
 Physical Layers for ATM
 ATM in the LAN
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ATM Overview
 Asynchronous Transfer Mode
 ATM is a competing technology with Ethernet switching,
but the areas of application for these two technologies
only partially overlap
 ATM is a connection-oriented, packet-switched
technology
 Use virtual circuits
 Connection phase is called “signaling”
 The main signaling protocol is known as Q.2931
 Q.2931
 Discover a suitable route across an ATM network
 Responsible for allocating resources at the switches among the
circuit
 This is to ensure the circuit a particular quality of service (QoS),
one of the greatest ATM strengths
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ATM Overview (cont’d)
 ATM has specific format of identifying addresses: E.164
and NSAP (network service access point), different from
MAC address in LANs
 Packets that are switched in ATM has fixed length: 53
bytes
 5 bytes of header followed by 48 bytes of payload
 To distinguish between fixed-length packets and
variable-length packets, they are given a special name
“cells”
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Outline
 Overview
 Cells
 Segmentation and Reassembly
 Virtual Paths
 Physical Layers for ATM
 ATM in the LAN
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Cells
 Variable-length packets are normally constrained
to fall within some bounds
The lower bound is set by the minimum amount of
information that needs to be contained in the packet
 Typically a header with no optional extensions
The upper bound may be set by a variety of factors
 Maximum FDDI packet size
• How long each station is allowed to transmit without passing on
the token, and thus determine how long a station might have to
wait for the token to reach it
 Cell are both fixed-length and small in size
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Cell Size
 Cell has fixed length in order to facilitate the
implementation of hardware switches
 When ATM was being created in the mid- and
late 1980s, 10-Mbps was the cutting-edge
technology in terms of speed.
 To go much faster, most people thought in terms
of hardware
 Fixed-length packets turn out to be very helpful
thing if want to build fast, highly scalable
switches
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Cell Size (cont’d)
Two main reasons:
Easier to build hardware to do simple jobs, and
the job of processing packets is simple when
you already know how long each one will be
If all packets have the same length, then you
can have lots of switching elements all doing
much the same thing in parallel, each of them
taking the same time to do its job
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Cell Size (cont’d)
 The second reason improves scalability of
switch designs
Ease the task of building such hardware and that there
was a lot of knowledge available about how to build cell
switches in hardware at the time the ATM standards
were being defined
 Fixed-length cell is also good for queuing
Queues build up in a switch when traffic from several
inputs may be heading for a single output
Once you extract a packet from a queue and start
transmitting it, you need to continue until the whole
packet is transmitted
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Cell Size (cont’d)
 The longest time that a queue output can be tied up is
equal to the time it takes to transmit a maximum-sized
packet
 Tqueue = Ttrans-max-size-packet
 Fixed-length cell means that a queue output is never tied
up for more than the time it takes to transmit one cell,
which is almost certainly shorter than the maximumsized packet on a variable-length packet network
 Tqueue-cell ≤ Ttrans-max-size-packet
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Cell Size (cont’d)
 A network with variable-length packets,
 Maximum packet size is 4kB
 Link speed is 100 Mbps
 Thus the time to transmit a max-size packet is 4096 x 8 /100M =
327.68 µs
 A high-priority packet that arrives just after the switch starts to
transmit a 4-kB packet will have to wait in a queue 327.68 µs
 In contrast, if the switch were forwarding 53-byte cells, the longest
wait would be 53 x 8 /100M = 4.24 µs
 Also queues of cells also tend to be a little shorter than queues of
packets
 When a packet begins to arrive in an empty queue, switch has to wait
for the whole packet to arrive before it can start transmitting the packet
on an outgoing link
 The link sits idle when the packet arrives
 Large-size packet VS small-size cell
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How large is The Cell Size?
 If the size is too short, the amount of header relative to the amount
of data gets larger
 Percentage of link bandwidth used to carry data goes down
 If you build up a device that processes cells at some maximum
number of cells per second, then as cells get shorter, the total data
rate drops in direct proportion to cell size
 E.g. network adapter that reassembles cells into larger units before
handling them up to the host
 The performance of such device depends on cell size
 If the cell is too big, there is a problem of waste bandwidth caused
by the need to pad transmitted data to fill a complete cell
 If the payload size is 48 bytes and you want to transmit 1 byte of data 
need to pad 47 bytes
 This lowers link utilization
 53-byte size were chosen, 5 bytes of header and 48 bytes of
payload
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Cell Format
 Two different cell formats
 UNI (user-network interface): between a telephone company and
its user
 NNI (network-network interface): between a pair of telephone
companies
 The only significant difference in cell formats is that the NNI format
replaces the GFC (Generic Flow Control) fielded with 4 extra bits of
VPI
ATM Cell Format at the UNI
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ATM Cell Format (cont’d)
 GFC (Generic Flow Control)
 4 bits, not widely used, were intended to have local
significance at a site
 To provide a means to arbitrate access to the link if the local
site used some shared medium to connect to ATM
 VPI (Virtual Path Identifier) and VCI (Virtual Circuit
Identifier)
 8-bit VPI and 16-bit VCI
 Both are used to identify a virtual connection
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ATM Cell Format (cont’d)
 Type
3 bits  8 possible values
4 of them, where the 1st bit is set, relate to
management functions
If the 1st bit is not set, the cell contains user data
The 2nd bit is the “explicit forward congestion
indication” (EFCI) bit
 Used for congestion control in conjunction with the available
bit rate (ABR) service  set by the congested switch to tell
an end node that it is congested
The 3rd bit is the “user signaling” bit
 Used in conjunction with ATM adaptation Layer 5 (AAL5) to
delineate frames. Indicate the last cell of the frame
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ATM Cell Format (cont’d)
 CLP (cell loss priority)
 A user or network element may set this bit to indicate cells that
should be dropped preferentially in the event of overload
 E.g. a video coding application could set this bit for cells that
would not dramatically degrade the quality of the video if
dropped
 A network element might set this bit for cells that have been
transmitted by a user in excess of the amount that was
negotiated
 CRC
 Known as header error check (HEC), use CRC-8
 Provide error detection and single-bit error correction capability
on the cell header only
 Important because error in the VCI will cause the cell to be
misdelivered
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Outline
 Overview
 Cells
 Segmentation and Reassembly
 Virtual Paths
 Physical Layers for ATM
 ATM in the LAN
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Segmentation and Reassembly
 The packet handed down from high-level protocol are
often larger than 48 bytes  need fragmentation and
reassembly
 In ATM, often called Segmentation And Reassembly (SAR)
 To deal with this, a protocol was added in between ATM
and the variable-length packet protocols that might use
ATP, such as IP
 This layer is called ATM Adaptation Layer (AAL)
 AAL header contains the information needed by the
destination to reassemble the individual cells back into
the original message
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AAL and ATM
Segmentation and reassembly in ATM
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AALs
 ATM supports all sorts of services, including voice,
video, and data
 Need different AALs
 4 adaptation layers was originally defined:
 1 and 2 designed to support applications, like voice, that require
guaranteed bit rates
 3 and 4 support packet data running over ATM
 AAL3 used by connection-oriented packet services (e.g. X.25)
 AAL4 used by connectionless services (e.g. IP)
 Later 3 and 4 were combined into AAL3/4
 Some perceived shortcomings in AAL3/4 caused AAL5
 Now there are 4 types of AALs: 1, 2, 3/4, and 5
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ATM Adapter Layer 3/4
 Main function of AAL3/4 is to provide enough information
to allow variable-length packets to be transported
across the ATM network as a series of fixed-length
cells
 A packet is this layer is called “protocol data unit
(PDU)”
 The task of segmentation/reassembly involves two
different packet formats:
 Convergence sublayer PDU (CS-PDU): defines a way of
encapsulating variable-length PDUs prior to segmenting them
into cells
 The PDU passed down to the AAL is encapsulated by adding
a header and a trailer, and the resultant CS-PDU is
segmented into ATM cells
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AAL3/4 CS-PDU Packet Format
ATM AAL3/4 packet format
 CPI indicates which version of the CS-PDU format is in use
 Only the value 0 is currently defined
 Beginning tag (Btag) is supposed to match the end tag (Etag) for
a given PDU
 Protect against the situation in which the loss of the last cell of one PDU
and the first cell of another causes two PDUs to be inadvertently joined
into a single PDU and passed up to the next layer in the protocol stack
 Buffer allocation size (BASize) is not necessarily the length of the
PDU
 Supposed to be a hint to the reassembly process as to how much buffer
space to allocate for the reassembly
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AAL3/4 Packet Format (cont’d)
 Before adding the CS-PDU trailer, the user data is padded to one
byte less than a multiple of 4 bytes, by adding up to 3 bytes of
padding
 Ensure that the trailer is aligned on a 32-bit boundary, making for
more efficient processing
 The CS-PDU trailer contains the Etag and the real length of the
PDU (Len)
 AAL3/4 specifies a header and trailer that are carried in each cell
ATM cell format for AAL3/4
 The CS-PDU is segmented into 44-byte chunks plus 4 bytes of its
header and trailer  bring up to 48 bytes
 Then carried as the payload of an ATM cell
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AAL3/4 Packet Format (cont’d)
 Type
 The first two bits of the AAL3/4 header contain Type field
indicating is this is the first cell of a CS-PDU, the last cell of a
CS-PDU, a cell in the middle of a CS-PDU, or a single-cell PDU
(in which case it is both first and last)
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AAL3/4 Packet Format (cont’d)
 SEQ (sequence number)
 Detect cell loss or misordering so that reassembly can be
aborted
 MID (multiplexing identifier)
 Multiplex several PDUs onto a single connection
 Length
 Show the number of bytes of PDU that are contained in the
cell
 Must equal 44 for BOM and COM cells
 CRC
 Detect errors anywhere in the 48-byte cell payload
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Encapsulation and Segmentation for
AAL3/4
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ATM Adaptation Layer 5
 AAL3/4 seems to take a lot of fields and thus a lot of
overhead to perform conceptually simple function of SAR
 Could just have 1 bit in the ATM header (as opposed to
the AAL header) to delineate the end of a frame, them
SAR could be accomplished without using any of the 48byte ATM payload for SAR information
 This led to AAL5
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AAL5 (cont’d)
 Replace 2-bit Type filed of AAL3/4 with 1 bit of
framing information in the ATM cell header
Set this bit to identify the last cell of a PDU
The next cell is assumed to be the first cell of the next
PDU
Subsequent cells are assumed to be COM cells until
another cell is received with this bit set.
 Other features of AAL3/4 are provided in AAL5
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AAL5 CS-PDU
 AAL5 CS-PDU consists of data portion and 8-byte trailer
 To make sure that the trailer always falls at the tail end of
an ATM cell, there may be up to 47 bytes of padding
between the data and trailer (min data allowed is 1 byte)
 The first 2 bytes of trailer is Reserve and must be 00
 Len is the number of bytes carried in the PDU, not
including the trailer or any padding before trailer
ATM AAL5 packet format
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Encapsulation and Segmentation for
AAL5
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Encapsulation and Segmentation for
AAL5 (cont’d)
 AAL5 provides almost the same functionality as AAL3/4
without using an extra 4 bytes out of every cell
 The main feature missing from AAL% is the ability to
provide an additional layer of multiplexing onto one
virtual circuit using the MID
 AAL5 is preferred in the IETF form transmitting IP
datagrams over ATM
 More efficient use of bandwidth and simple design are
the main features that make it more appealing than
AAL3/4
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Outline
 Overview
 Cells
 Segmentation and Reassembly
 Virtual Paths
 Physical Layers for ATM
 ATM in the LAN
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Virtual Paths
 ATM uses 24-bit identifier for virtual circuits
 8-bit virtual path identifier (VPI)
 16-bit virtual circuit identifier (VCI)
 This creates two levels of virtual connections
 A corporation has two sites connecting to a public ATM
network and each site the corporation has a network of
ATM switches
 We can establish a virtual path between two sites using
only VPI field
 The switch in the public network would use the VPI as the
only field on which to make forwarding decisions
 It is a virtual circuit network with 8-bit circuit identifier
 The 16-bit VCI has no interest to these public switches
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Virtual Paths (cont’d)
 Within corporate sites, the full 24-bit space is used for
switching
 Traffic flowing between two sites is routed to a switch
that has an connection to the public network, and its top
(most significant bits) 8 bits VPI are mapped onto the
appropriate value to get the data to the other site
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Virtual Paths (cont’d)
 Advantages
Support many virtual connections across the public
network
The switches in the public network behave as if there is
only one connection
There needs to be much less connection-state
information stored on the switches, avoiding the need
for big, expensive tables of per-VCI information
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Outline
 Overview
 Cells
 Segmentation and Reassembly
 Virtual Paths
 Physical Layers for ATM
 ATM in the LAN
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Physical Layers for ATM
 From early in the process of standardizing ATM, it was
assumed that ATM would run on top of a SONET
physical layer
 Standard ways of carrying ATM cells inside a SONET
frame have been defined. Can buy ATM-over-SONET
products
 However, they are entirely separable.
 Can lease a SONET link from a telephone company to carry
variable-length packets
 Can send ATM cells over many other physical layers instead of
SONET
 Notable early physical layers for ATM was TAXI, the physical
layer used in FDDI
 Wireless physical layers for ATM are also being defined
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How to find boundaries of the ATM cells
 With SONET, find ATM cell boundaries:
 One of the overhead bytes in the SONET frame can
be used as a pointer into the SONET payload to start
of an ATM cell
 Having found the start of one cell, it is known that the
next cell starts 53 bytes further on in the SONET
payload
 In theory, need to read this pointer only once, but in
practice, may read it every time the SONET overhead
goes by to detect errors or resynchronize if needed
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Outline
 Overview
 Cells
 Segmentation and Reassembly
 Virtual Paths
 Physical Layers for ATM
 ATM in the LAN
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ATM in the LAN
 ATM can be used in LANs  a replacement for
Ethernet and 802.5
 Its popularity can be attributed to two main
factors:
ATM is a switched technology, whereas Ethernet and
802.5 were originally envisioned as shared-media
technologies
ATM was designed to operate on links with speeds of
155 Mbps and above, compared to the original 10 Mbps
of Ethernet and 4 or 16 Mbps of token rings
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ATM in the LAN (cont’d)
 Switched networks have a big performance advantage
over shared-media networks
 A single shared-media network has a fixed total bandwidth
that must be shared among all hosts, whereas each host gets
its own dedicated link to the switch in a switched network
 However, bridge that connects a number of sharedmedia network together is also a switch, giving dedicated
access
 Also when ATM were launched, high-speed Ethernet
became available. It speed began to approach that of
ATM
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ATM in the LAN (cont’d)
 One advantage of ATM over Ethernet that remains is the
lack of distance limitation for ATM links. Also high-speed
ATM links (e.g. 622 Mbps) became available.
 This made ATM fairly popular for the high-performance
“backbone” of larger LANs
 One common configuration is to connect hosts to
Ethernet switches, which in turn could be interconnected
by ATM switches
 High-performance servers might also be connected
directly to the ATM switch
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ATM in the LAN
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ATM in the LAN
 One significant problem with running ATM in a LAN is
that it doesn’t look like a “traditional” LAN
 Because most LANs are shared-media networks (i.e.
every node on the LAN is connected to the same link), it
is easy to implement broadcast and multicast
 Many protocols that people depend on in their LANs, e.g.
ARP, depend in turn on the ability of the LAN to support
broadcast and multicast
 However, ATM is not a shared-media network.
 How do we can broadcast to all nodes on an ATM LAN if
don’t know all their addresses and set up VCs to all of
them?
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ATM in the LAN (cont’d)
 Two possible solutions
 Redesign the protocols that make assumption about
LANs that are not in face true for ATM
 ATMARP does not depend on broadcast
 Make ATM behave more like a shared-media LAN 
supporting broadcast and multicast without losing the
performance advantages of a switched network
 LAN Emulation or LANE: aim to add functionality to ATM
LANs so that anything that runs over a shared-media LAN
can operate over an ATM LAN
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ATM in the LAN (cont’d)
 One aspect of LANE that can be confusing is the variety
of different addresses and identifiers that are used
 ATM devices must have ATM address used when
signaling to establish a VC
 Different from IEEE 802 MAC address used in Ethernet and
token rings
 LANE does not actually change functionality of ATM
switches, but add functionality to the network through the
additional of a number of servers.
 Devices connecting to the ATM network – hosts, bridges, routers
– are referred to as LAN emulation clients (LECs).
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ATM in the LAN (cont’d)
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LAN Emulation
 The servers that are required to build an emulated LAN
are:
 The LAN emulation configuration server (LECS)
 The LAN emulation server (LES)
 The broadcast and unknown server (BUS)
 These servers can be physically located in one or more
devices
 LECS and LES primarily perform configuration functions
 BUS has a central role in making data transfer in an ATM
network resemble that of a shared-media LAN
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LAN Emulation (cont’d)
LES
H1
BUS
H2
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Broadcasting in LAN Emulation
1. LECS enables a newly attached or rebooted LAN emulation client to
get some essential information
2. First, the client must find the LECS, which may use a well-known,
predefined VC that is always set up. Alternatively, the client must
have prior knowledge of the ATM address of the LECS so it can set
up a VC to it
3. Once connected, the client provides the LECS with its ATM address
4. LECS responds by telling the client
 what type of LAN is being emulated (Ethernet or token ring)
 What the maximum packet size is
 The ATM address of the LES. One LES might support many separate
emulated LANs
5. The client signals for a connection to the LES whose ATM address is
just learned.
6. Once connected to the LES, the client registers its MAC and ATM
addresses with the LES. The LES provides the client with the ATM
address of the BUS
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Broadcasting in LAN Emulation (cont’d)
7. The BUS maintains a single point-to-multipoint VC that connects it to
all registered clients. BUS and multipoint VC are crucial to LANE:

Enable the broadcast capability of traditional LAN to be emulated
in a VC environment
8. Once LEC has the ATM address of the BUS, it signals for a
connection to the BUS
9. The BUS adds the LEC to the point-to-multipoint VC


Now LEC is ready to participate in data transfer
BUS is the place to send any packet that needs to be
broadcast to all clients on the LAN, not efficient to deliver
unicast packets
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Unicasting in LANE
 Assume that a host has a packet to send to a particular MAC
address
 In traditional LAN, simply send packets over on the wire. It will be
picked up by intended recipient
 But in LANE, packet needs to be delivered over a VC. The sending
host also does not know the ATM address of the recipient, which is
required to set up a VC
 The host performs the following steps:
1. It sends the packet to the BUS, which it knows can deliver the
packet to the destination using its point-to-multipoint VC
2. It sends an “address resolution” request to the LES, of the
form “what ATM address correspond to this MAC address?”
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Unicasting in LANE (cont’d)



Since all clients should have registered their MAC
and ATM addresses with the LES, the LES should
be able to answer the query and provide an ATM
address to the client
The client now can signal for a VC to the recipient,
which may use to forward subsequent frames to the
destination
The reason for using BUS to send the first packet is
to minimize delay, since it may take some time to get
a response from the LES and establish a VC
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PVC VS SVC
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SVC Call Setup and Teardown
 SVCs use Q.2931 user-to-network interface (UNI)
signaling to establish and release switched connections.
Before you create SVCs, you configure a PVC signaling
channel that the router uses to pass signaling messages
during call setup and teardown.
 To set up an ATM SVC connection:
1. Source initiates a call setup request message and forwards
the request to the network to establish a connection.
2. The call setup request message includes information, such as
traffic category and service quality information, needed to
define and support the connection.
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SVC Call Setup and Teardown (cont’d)
3. The network routes the call setup request through the network to
the destination. While routing the connection, the network
makes sure that the path has the resources to support the
connection information specified in the call setup request.
4. The destination receives the setup request message and either
accepts or rejects the call by sending a connect message back
to the initiator. The network can also verify the address prefix of
the initiator so that the destination can accept or reject the call
on that basis as well.

Once established, the connection is available for use
until either the source or destination issues a call
release message and the end user who receives the
message acknowledges the message.
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ATM Quality of Service
 Real-time application: application that is sensitive to the
timeliness of data
 Voice and video applications, industrial control e.g. controlling a
robot
 Real-time applications need assurance from the network
that data is likely to arrive on time
 Non-real-time application can use an end-to-end
retransmission strategy to make sure that data arrives
correctly. Such strategy cannot provide timeliness
 Best effort model is not sufficient for real-time applications
 Some applications need higher assurances that can ask
the network for it
 Different applications require different levels of services
 Quality of Service (QoS)
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ATM Quality of Service (cont’d)
 QoS capabilities that are provided in ATM networks are similar to
those provided in an IP network.
 Five ATM service classes:
 Constant bit rate (CBR)
 Variable bit rate – real-time (VBR-rt)
 Variable bit rate – non-real-time (VBR-nrt)
 Available bit rate (ABR)
 Unspecified bit rate (UBR)
 In ATM, QoS is defined at the time a virtual circuit is set up. This
is done be including information in the signaling messages that
are sent at VC setup time
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ATM Quality of Service (cont’d)
 AAL1 is used for constant bit rate (CBR) services and
circuit emulation.
 AAL2 through AAL4 are used for variable bit rate (VBR)
services, and AAL5 for data.
 Which AAL is in use for a given cell is not encoded in the
cell. Instead, it is negotiated by or configured at the
endpoints on a per-virtual-connection basis.
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Variable Bit Rate – real-time (VBR-rt)
 Designed for intolerant applications
 Transmission rate varies, but guarantee minimum
transmission rate
 Guarantee maximum total delay with some specified
value
 Early arrived packets can always be handled by
buffering
 Popular for voice traffic
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Constant Bit Rate (CBR)
 Provide constant, predefined data rate at all times
 Not too different than VBR-rt except that sources of CBR
traffic are expected to send at a constant rate
 Very important to telephone companies, since the
majority of the services they offer today , voice and
leased lines, provide a pipe of fixed bandwidth to the end
user.
 CBR is a relatively easy service to specify and
implement so that many early ATM switches could
support CPR but not VBR
 The early availability of CBR in ATM products certainly
helped the acceptance of ATM in the marketplace
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Variable Bit Rate – non-real-time
(VBR-nrt)




Meet the needs of tolerant, adaptive applications
transmission rate varies, delay tolerable
Can adjust transmission rate as network delay varies
The aim is to emulate a lightly loaded network for those
applications that request the service, even though the
network as a whole may in fact be heavily loaded
 Use queuing mechanism to isolate the controlled loaded
traffic from the other traffic and some form of admission
control to limit the total amount of controlled load traffic
on a link such that the load is kept reasonably low.
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Unspecified Bit Rate (UBR)
 ATM’s best effort service
 no transmission rate guaranteed, cells will be transmitted
when capacity is available.
 Lowest priority  first to be discarded when network is
busy
 Non guaranteed capacity
 Because ATM always requires a signaling phase before
data is sent, it is possible to convey information about
the source at VC setup time. UBR allows the source to
specify a maximum rate at which it will send, which may
be less than the line rate
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Available Bit Rate (ABR)
 Define a set of congestion-control mechanism
 An ATM virtual circuit clearly has two ends, source and
destination. VCs are usually bidirectional, so a node that
is the source in one direction is generally the destination
in the other
 The ABR mechanisms operate over a VC by exchanging
special ATM cell called resource management (RM)
cells between the source and destination of the VC
 The goal of RM cell is to get information about the state
of congestion in the network back to the source so that it
can send traffic at an appropriate rate
 RM cell is said to be an explicit congestion feedback mechanism
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Available Bit Rate (ABR) (cont’d)
 Initially, the source sends the cell to the destination and
includes in it the rate at which it would like to send data
cells.
 Switches along the path look at the requested rate and
decide if sufficient resources are available to handle that
rate, based on the amount of traffic being carried on
other circuits.
 If enough resources are available, the RM cell is passed
on unmodified; otherwise, the requested rate is
decreased before the cell is passed along.
 At destination, the RM cell is turned around and sent
back to the source, which thereby learns what rate it can
send at
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Available Bit Rate (ABR) (cont’d)
 The intention of ABR is to allow a source to increase or
decrease its allotted rate as conditions dictate.
 As a consequence, RM cells are sent periodically and
may contain either higher or lower requested rates.
 The rate at which a source is allowed to send decays
with time if not used. This is intended to discourage a
source from requesting capacity “just in case”.
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Virtual Source and Virtual Destination
 End points of RM cells do not have to be the endpoints
of the VC
 ABR extends the notion of source and destination,
introducing the concepts of virtual source (VS) and
virtual destination (VD)
 “Virtual” in the sense that they are not the true endpoints of the
VC
 Enable control loop around which RM cells flow to be made
shorter than the VC itself
 Making the control loop shorter, the response time of the
system can be reduced
 It may reduce buffer requirements of the switched by
reducing the time between when they sense congestion
and when the (virtual) source slows down
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Virtual Source and Virtual Destination
(cont’d)
 The real source sends RM cells to the switch providing VS/VD
capability, and that switch, acting as VD, turns around the RM cells
 It will include in this cell the rate at which it is willing to accept traffic
on this VC.
 The switch (as VS) originates RM cells toward the real destination,
which will in turn be sent back to the VS telling it at what rate it can
send traffic on this VC
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Questions?
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