Cell Switching: ATM

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• Cell Switching ATM

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Last time Spanning Tree Algorithm

• A message is considered better than the recorded
  info if
• it ids a root with smaller id or
• it ids a root with equal id but shorter distance
  or
• the root id and distance are equal, but the
  sending bridge has smaller id.
• Before updating the info with the received
  message, the bridge adds 1 to the hop count.
• When a bridge discovers it is not the root, it
  stops sending out messages of its own (only
  forwards those from other bridges after adding 1
  to the hop count).

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Spanning Tree Algorithm
When a bridge discovers its not the designated
bridge for that port, it stops sending
configuration messages over that
port. Eventually, the system stabilizes only the
root sends out configuration messages and the
other bridges only forward them around.
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Broadcast and Multicast

• Broadcast messages should still reach all the
  nodes in the extended LAN.
• Multicast messages should reach all the nodes in
  a multicast group independently of the LAN to
  which the nodes belong.

Question How can one implement these kinds of
messages on the extended LAN?
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Cell Switching ATM
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Asynchronous Transfer Mode

• Connection-oriented, packet switched or rather,
  cell switched.

header
payload
ATM cell
fixed-length 53 bytes (48 bytes payload)
Why use fixed-length packets

• Processing packets of fixed length is simpler.
• Possible to divide the task of switching into a
  number of elements that work in parallel
  (scalability).

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Queueing behavior
switch (latency)

inputs
outputs

• Get packet from queue, start transmitting (no
  preemption).
• If another packet for the same destination
  arrives, it must wait.
• Question What is the longest time that the
  output gets tied up to a single packet?
• Question Does the use of fixed-length cells
  reduce queue length?

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Queueing behavior
switch
packet
100 Mbps
4 KB (max length)
Question What is the time to transmit a max
length packet?
Question What is the time to transmit an ATM
cell?
Question What is the longest time that a packet
or cell has to wait until it is transmitted?
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Queueing behavior
switch
100 Mbps
packet A
4MB
packet B
4MB
Say that when a packet starts to arrives, the
switch waits for the complete packet to be
captured before it is forwarded. Question What
happens to the utilization of the outgoing link?
switch
100 Mbps

A

B
Question Does the utilization of the outgoing
link change with small, fixed-length cells?
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Cell format
(UNI)
GFC generic flow control VPI virtual path
identifier VCI virtual circuit identifier Type
used for management (congestion, signaling,
framing) CLP cell loss priority (what gets
tossed in case of overload) HEC header error
check
Question Why is it important to check the header
for errors?
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Segmentation and Reassembly
IP
IP
outgoing packet from upper layer
incoming packet to upper layer
ATM Adaptation Layer
cells
cells
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ATM Adaptation Layer 3/4
packet

cells
AAL 3 connection-oriented packet services
(X.25) AAL 4 connectionless packet services
(IP) Both ended up merged into one layer AAL 3/4
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Encapsulation and Segmentation for AAL 3/4
PDU protocol data unit think of it as a kind of
packet specific to a certain protocol.
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AAL 3/4 Cell Types
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ATM Adaptation Layer 5
Created to simplify what AAL 3/4 had made
complicated.
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ATM Adaptation Layer 5
Created to simplify what AAL 3/4 had made
complicated.
AAL 3/4 cell
Change the ATM cell header to drop the 2-bit Type
and use instead 1 bit to help with framing. If
that bit is set, the cell is the last chunk of
information in a PDU. The next cell corresponds
to the next PDU subsequent cells are assumed to
be COM until another cell is received with user
signaling bit set.
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Virtual Path
VPI (8 bits) Establishes a path between two
sites. VCI (16 bits) Establishes the connection
between the endpoints in each of the two
sites. Effectively what you have is a two-level
hierarchy of VC connections. Switches in the
public network do not have to deal with
individual virtual circuits between two sites.
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The ATM Reference Model

• The ATM reference model.

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The ATM Reference Model (2)

• The ATM layers and sublayers and their functions.

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Physical Layer of ATM

• ATM could run on top of any physical media in
  theory
• Most popular medium is the fiber optics network
  (e.g. SONET (Synchronous Optical Network)), the
  current most popular long distance network medium

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SONET

• Time-division multiplexing (TDMA)
• 810-byte frame is transmitted every 125 u seconds
  (making it 8000 frames/sec, compatible with PCM
  channel in digital phone systems)
• The basic rate is thus 51.84 Mbps (81088000
  51.84 Mbps). This is called STS-1 for Synchronous
  Transport Signal 1.
• All SONET trunks are a multiple of STS-1

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Other Common Bandwidth
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ATM and LAN

• LAN is designed to have easy broadcast and
  multicast capability
• ATM is designed as a switch, not easy to
  broadcast or multicast
• To make the two inter-operable, two approaches
• Re-design protocols that rely on broadcast (e.g.
  ARP has its sibling ATMARP)
• Have ATM emulate LAN