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 (N*2 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 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’

10. VC Forwarding: Example
in-VCI
out
out-VCI … … … … in
in-VCI
out
out-VCI 1 7 4 1 … … … … … … … …
destination
source 3 5 4 11 … … … … 1 1 2 2 1 1 1 3 3 2 2 4 4 3 3 11 5 4 4 1 1 7 2 2 3 3 4 4 in
in-VCI
out
out-VCI … … … … 2 11 3 7 … … … …

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. Protocol Data Unit (PDU)
AAL Structure
Protocol Data Unit (PDU)
CPCS
header
PDU
CPCS
trailer
ATM cell
header
AAL
header
Payload data <= 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 (cont’d) 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 (cont’d) 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 IP over ATM
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)