Transcript lecture19

EE 122: Lecture 19
(Asynchronous Transfer
Mode - ATM)
Ion Stoica
Nov 13, 2001
(* based on some on-line slides of J. Kurose & K. Rose)
Goals
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Provide better services than best-effort
Able to carry both data and voice (telephony)
traffic
Can be implemented at very high speeds
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2
ATM Protocol Stack

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Physical layer
ATM layer: performs routing (similar in function to
IP + data link layer)
ATM Adaptation Layer (AAL) – performs
segmentation and reassembly, multiplexing
(similar in function to the transport layer)
Source
Destination
AAL
AAL
ATM
ATM
ATM
Phys
Phys
Phys
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3
Design Decisions


Fixed size packets – cells
Based on virtual circuit
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4
Why Fixed Size?

Easier to implement high speed switches
- Easier to do processing when the cell length is known in advance
- Easier to implement parallel and pipeline solutions when the
processing of all cells take the same time

A cell is in general much smaller than the maximum
packet size
- A high priority cell needs to wait less before being transmitted (a
low priority cell will take less time to be transmitted than a packet
of maximum size)
- When the is empty, the first data bits are in general transmitted
faster

Cell size: 48 byte payload + 5 byte header
- A compromise between US (64 byte payload) and Europe (32 byte
payload)
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5
ATM Cell Header
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VCI: virtual channel ID
- will change from link to link through network
PT: Payload type (e.g. control cell versus data cell)
CLP: Cell Loss Priority bit
- CLP = 1 implies low priority cell, can be discarded if
congestion
HEC: Header Error Checksum
- cyclic redundancy check
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6
ATM Cell Header


First 4 bits in VCI – GFC (Generic Flow Control)
Local significance at the end-host
- Arbitrate the access to the link if a shared medium is
used to connect to ATM

GFC bits can be overwritten by the newtork
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7
ATM VCs
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Advantages of ATM VC approach:
- QoS performance guarantee for connection
mapped to VC (bandwidth, delay, delay jitter)
Drawbacks of ATM VC approach:
- Inefficient support of datagram traffic
- One VC between each source/dest pair) does
not scale (N*2 connections needed)
- VC introduces call setup latency, processing
overhead for short lived connections
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8
Virtual Circuit (VC) Forwarding
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Each router maintains a routing table
- A routing entry: (input port, input VCI, output port, output VCI);
• VCI – Virtual Circuit Identifier
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Upon a cell arrival at interface i
- Input port uses i and the packet’s VCI v to find the routing entry (i,
v, i’, v’)
- Replaces v with v’ in the packet header
- Forwards packet to output port I’
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9
VC Forwarding: Example
in in-VCI out out-VCI
…
…
… …
in in-VCI out out-VCI
…
…
… …
source
3
…
5
5
…
4
…
1
2
3
4
1
2
3
4
1
…
7
…
4
…
1
…
destination
11
…
1
2
3
4
1
2
3
4
11
1
2
3
4
1
2
3
4
1
7
in in-VCI out out-VCI
…
…
… …
2
…
11
…
3
…
7
…
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10
Virtual Path Identifier (VPI)
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ATM splits the VCI in two
- 16 bits Virtual Path Identifier (VPI)
- 8-12 bits VCI
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Use to implement hierarchical routing
All VCI between two subnets share the same VPI
- Backbone switches switch based on VPI
- Switches in edge networks switch based on the entire
VCI
Backbone(Public) Network
Virtual path
Network A
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Network B
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Virtual Path Identifier (VPI)

ATM splits the VCI in two
- 16 bits Virtual Path Identifier (VPI)
- 8-12 bits VCI


Use to implement hierarchical routing
All VCI between two subnets share the same VPI
- Backbone switches switch based on VPI
- Switches in edge networks switch based on the entire
VPI
Backbone(Public) Network
Virtual path
Network A
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Network B
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ATM Adaptation Layer
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Goal: allow existing protocols and applications to
run on top of ATM
AAL is implemented only at endpoints
AAL has two sub-layers
- Convergence Sub-layer (CS)
- Segmentation and Reassembly (SAR) sub-layer
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Usually CS adds
- Common Part Convergence Sub-layer (CPCS) header
and trailer
- Checksum
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13
AAL Structure
Protocol Data Unit (PDU)
CPCS
header
PDU
CPCS
trailer
ATM cell AAL
ATM cell
Payload
data
<=
48
header header
trailer
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Example of AAL header information
- Type (e.g., first cell, last cell in PDU)
- Sequence # of the cell within PDU
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14
ATM Quality of Service (QoS)
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Constant bit rate (CBR)
Variable bit rate – real-time (VBR-rt)
Variable bit rate –non-real-time (VBE-nrt)
Available bit rate (ABR)
Unspecified bit rate (UBR)
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ATM QoS (cont’d)
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VBR-rt – similar to Guaranteed Service in Intserv
- Traffic is specified by a token bucket, and the end-toend delay is specified
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CBR – very similar to VBR-rt, but the source is
expected to send at a specified rate (e.g.,
telephony traffic)
- Specified by a token-bucket with very small bucket,
e.g., 1 cell
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VBR-nrt – similar to the Controlled –load Service
in Intserv
- Traffic specified by token-bucket, but no hard delay
guarantees
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16
ATM QoS (cont’d)
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UBR - similar to best-effort service, but
- There can still be an admission control:
- However, UBR allows the source to specify a maximum rate,
which can be used in the admission control
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ABR – implements congestion control: use explicit
notification
- Source sends periodic Resource Management (RM) cells
- Each switch put in the RM cell the available bit rate of the output
link – after the RM cell traverses all switches it will have the
minimum available rate amongst all routers along the path
• Routers can use fair queueing to compute the available bit rate
- Receiver sends back the RM cell to the sender
- Sender adjust its rate accordingly
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17
AAL Types
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AAL 1: support CBR
AAL 2: support VBR
AAL 3/4: support variable-length packets
AAL 5: more efficient support for data packets
(used to implement to support data packets and
UBR service)
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18
IP-Over-ATM
Classic IP only
 3 “networks” (e.g., LAN
segments)
 MAC (802.3) and IP
addresses
IP over ATM
 Replace “network” (e.g., LAN
segment) with ATM network
 ATM addresses, IP addresses
ATM
network
Ethernet
LANs
Ethernet
LANs
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19
IP-Over-ATM
Issues:
 IP datagrams into
ATM AAL5 PDUs
 From IP addresses
to ATM addresses
ATM
network
Ethernet
LANs
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20
Datagram Transmission in IP-overATM Network
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At source:
- IP layer finds mapping between IP, ATM dest address
(using ARP)
- passes datagram to AAL5
- AAL5 encapsulates data, segments to cells, passes to
ATM layer
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ATM network: moves cell along VC to destination
At destination:
- AAL5 reassembles cells into original datagram
- if CRC OK, datgram is passed to IP
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21
Summary
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
ATM was an effort to build a new network to support data
and voice applications, and to provide QoS
Two main design decisions:
- Use fixed-size, packets (cells)
- Use virtual circuit switching
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ATM did not replace IP
- To many legacy applications
- Hard to change these applications to take advantage of ATM QoS
- Not appropriate to Web traffic
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Today ATM is used largely by
- Voice carrier
- In the Internet backbones (see IP-over-ATM)
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