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EE 122: Lecture 19 (Asynchronous Transfer Mode - ATM)

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Constant bit rate (CBR) Variable bit rate real-time (VBR-rt) ... CBR very similar to VBR-rt, but the source is expected to send at a specified ... – PowerPoint PPT presentation

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Title: EE 122: Lecture 19 (Asynchronous Transfer Mode - ATM)


1
EE 122 Lecture 19(Asynchronous Transfer Mode -
ATM)
  • Ion Stoica
  • Nov 13, 2001

( based on some on-line slides of J. Kurose K.
Rose)
2
Goals
  • Provide better services than best-effort
  • Able to carry both data and voice (telephony)
    traffic
  • Can be implemented at very high speeds

3
ATM Protocol Stack
  • 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
4
Design Decisions
  • Fixed size packets cells
  • Based on virtual circuit

5
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)

6
ATM Cell Header
  • 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

7
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

8
ATM VCs
  • 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 (N2 connections needed)
  • VC introduces call setup latency, processing
    overhead for short lived connections

9
Virtual Circuit (VC) Forwarding
  • Each router maintains a routing table
  • A routing entry (input port, input VCI, output
    port, output VCI)
  • VCI Virtual Circuit Identifier
  • Upon a cell arrival at interface i
  • Input port uses i and the packets 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

10
VC Forwarding Example
out
in-VCI
out-VCI
in




out
in-VCI
out-VCI
in
1
4
1
7








destination
source
11
4
3
5




1
1
2
2
1
1
3
3
2
2
4
4
3
3
4
4
1
1
2
2
3
3
4
4
out
in-VCI
out-VCI
in




7
3
2
11




11
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 VCI

Backbone(Public) Network
Virtual path
Network A
Network B
12
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
Network B
13
ATM Adaptation Layer
  • 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
  • Usually CS adds
  • Common Part Convergence Sub-layer (CPCS) header
    and trailer
  • Checksum

14
AAL Structure
Protocol Data Unit (PDU)
CPCS header
PDU
CPCS trailer
ATM cell header
AAL header
Payload data lt 48
ATM cell trailer
  • Example of AAL header information
  • Type (e.g., first cell, last cell in PDU)
  • Sequence of the cell within PDU

15
ATM Quality of Service (QoS)
  • 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)

16
ATM QoS (contd)
  • VBR-rt similar to Guaranteed Service in Intserv
  • Traffic is specified by a token bucket, and the
    end-to-end delay is specified
  • 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
  • VBR-nrt similar to the Controlled load Service
    in Intserv
  • Traffic specified by token-bucket, but no hard
    delay guarantees

17
ATM QoS (contd)
  • 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
  • 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

18
AAL Types
  • 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)

19
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
20
IP-Over-ATM
  • Issues
  • IP datagrams into ATM AAL5 PDUs
  • From IP addresses to ATM addresses

ATM network
Ethernet LANs
21
Datagram Transmission in IP-over-ATM Network
  • 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
  • ATM network moves cell along VC to destination
  • At destination
  • AAL5 reassembles cells into original datagram
  • if CRC OK, datgram is passed to IP

22
Summary
  • 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
  • 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
  • Today ATM is used largely by
  • Voice carrier
  • In the Internet backbones (see IP-over-ATM)
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