ATM : Asynchronous Transfer Mode
ATM basics
concept: simple and fast
data rate: 155 Mbps and 622 Mbps
packet-switched
fixed size 53-byte packet called cell
small promotes multiplexing with good response to all
fixed size facilitates switching (many functions in HW: faster)
connection-oriented service (virtual circuit)
sequence guaranteed
cell delivery not guaranteed!
Single-bit error correction instead of retransmission (OK for fiber)
Usable for either LAN or WAN
Defines OSI layers 1-4 (physical - transport) only
ATM is underlying technology for B-ISDN
Broadband-ISDN for very high speed services
Proposed B-ISDN services include:
- video on demand
- CD-quality audio
- multi-media e-mail
- LAN interconnections
- high speed data transmission
Layered network architecture
a. physical layer
- PMD sublayer
- P
hysical Medium Dependent
- volts, bit timing, etc.
- OSI layer 1
- TC sublayer
- T
ransmission Convergence
- checksums, framing
- OSI layer 2
b. ATM layer
- cell layout, header fields
- virtual circuit management (routing, switching)
- OSI layer 2/3
c. ATM adaptation layer (AAL)
- interface to applications (to be carrier for many different applications)
- At least 4 application types have been defined:
- AAL1 : constant bit rates (such as video)
- AAL2 : variable bit rates
- AAL3/4 : connection-oriented/connectionless
- AAL5 : sequencing and error control
- SAR sublayer
- S
egment And Reassembly
- provides variable-length packets (convert variable / fixed)
- CS sublayer
- C
onvergence Sublayer
- provide various packet services (packetizing, adding headers)
ATM links
category 5 twisted pair good for 100 meters
fiber good for several kilometers
all links are point-to-point
- host to switch
- switch to switch
unidirectional, cable pairs
ATM Switches
a. generic behavior:
- cells arrive asynchronously on input lines
- arrive at each switching cycle
- cells accepted into "switching fabric"
- cells may require several switching cycles to switch
(switch looks in routing table to match cell’s virtual circuit number)
b. constraints:
- discard cell only in emergency
- never reorder cells from given virtual circuit
c. major problem to solve:
- multiple cells contend for same output line
- use knockout switch or Batcher-banyan switch
d. knockout switch
- basically crossbar
- each output line has queue
- fix maximum queue length N
- if >N cells contend, discard based on tournament
- discarded cells are "knocked out"
e. Batcher-Banyan switch
- use multistage switching to simplify crosspoints
Cell format
53 bytes fixed total length : 48 data, 5 header
Two header formats: UNI, NNI
- User-Network Interface (between host and net)
- Network-Network Interface (between switches)
Header format
- First 12 bits differ by format.
- UNI header: 4 bit GFC (Generic Flow Control), 8 bit VPI
- NNI header: 12 bit VPI (Virtual Path Id)
- 16 bit VCI
- 3 bit PT
- 1 bit CLP
- Cell Loss Priority
- if 1, switch cannot discard cell if cells with CLP==0 remain)
- 8 bit HEC
- Header Error Control (CRC applied to first 4 bytes of header)
- corrects single bit errors (most common on fiber) and some multibit
Framing (TC sublayer)
No framing bits! No synchronization! No ACKs!
Maybe underlying xmit technology (e.g. SONET, FDDI) will contain whole cells
Failing that, how to detect cell boundary?
- Hold data in shift register
- Guess where checksum is
- See if checksum matches "header"
Problem is false positives
Solution is to hold several cells length and check each.
Routing
host to switch (UNI) routing uses VPI and VCI (on both ends)
switch to switch (NNI) routing may use VPI only!
- no standard, switch designers determine
- no fixed routing algorithm
- advantage of using VPI only: small tables
- If VCI not used in routing, all channels from same path go same route
VPI value is relative to a switch!
- can can change at each hop!
- Each routing table has two VPI: incoming-VPI and outgoing-VPI
- Switch matches cell VPI to incoming-VPI for routing table lookup
- Switch changes cell VPI to corresponding outgoing-VPI
- Allows VPI to be short (need IDs only for neighbor switches, not global)
ATM is virtual circuit, so route setup once per connect
Related Home Pages:
notes | CSC 465 | Peter Sanderson | Computer Science | SMSU
Last reviewed: 4 May 1998
Peter Sanderson ( pete@csc.smsu.edu )