Asynchronous Transfer Mode

ATM AAL-5

ATM adaptation layers provide additional services that the lower layers do not provide
for example, sending a frame that is larger than 48 bits: AAL 3/4, AAL 5
AAL 5 is the normal way to send data frames
each frame can have 1 to 2¹⁶-1 data bytes
the data bytes are padded with 0-47 bytes, then the (unpadded) length, using 16 bits, and a 32-bit CRC
the padding must make the entire frame, including length and CRC, be a multiple of 48 bytes
the user signaling bit marks the last cell only
this bit is used to determine when to check the length and CRC, which are always at a fixed offset within the last cell
AAL-5 is suitable for software OR hardware implementation

modern version of Integrated Services Data Network (ISDN)
packet oriented and modeled on ATM, but much simpler than SONET, ATM
virtual circuits, including PVCs (permanent virtual circuits) and SVCs (switched virtual circuits)
range of speeds, from 64Kb/s to 44.736 Mb/s

Frame format:
- 8-bit "flags" field (always 7E)
- 16-bit "address" field (what we normally think of as a header)
- variable sized data
- 16-bit frame check sequence (FCS, a 16-bit CRC)
- 8-bit "flags" field (always 7E)
the "address" (header) field has the following fields:
- 10-bit DLCI (Data Link Connection Identifier), similar to the combined VPI+VCI and stored as a 6-bit and a 4-bit part
  Note that, as for VCI and VPI,
  devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection. (from http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm#wp1020669)
- bits indicate forward and backward congestion, a discard eligibility bit, address extension bit, and a command/response bit which "is not currently defined" (same reference).
QoS, congestion management, signaling similar to ATM
easy, relatively cheap way to bridge distant LANs or connect to an ISP

High-level Data Link Control
frame relay (and other protocol) use HDLC to actually carry data, so the frame relay header is a subset of the possible HDLC header formats
HDLC header can contain no address (for point-to-point), or an 8- or 16-bit address
HDLC trailer has CRC
different frame types, including information frames, for data, and supervisory frames for control
connections are set up by unnumbered frames
a reasonably good description: http://www2.rad.com/networks/1994/hdlc/hdlc.htm
to keep the flag from occurring in the data, HDLC uses bit stuffing, so flags and frames can be identified unambiguously

Synchronous Optical NETwork
synchronous standard for public carrier networks on optic fibers (ATM is Asynchronous Transfer Mode)
one frame every 125 u s, 8000 frames per second
basic speed is 51.84 Mb/s, selected multiples are defined
therefore, basic frames are 6480 bits, 810 bytes
two-dimensional frame representation: 9 rows of 90 bytes each
first 3 bytes of each row are the SONET header, for a total of 27 bytes per frame.

section: the connection between repeaters (9 bytes per frame overhead)
line: the connection between multiplexers (18 bytes per frame)
path: the end-to-end connection (9 bytes per payload overhead)
so the frame header is 27 bytes, but there are an additional 9 bytes of payload (one byte per row) that are used to carry path header information.
these headers are for management, signaling, framing, CRCs, and supervisory voice channels
Raj Jain of Washington University in St. Louis has posted a comprehensive tutorial on SONET, providing many details, e.g. of the header.

synchronous payload may not be ready to send when multiplexer is ready to send frame
could delay until the next frame, but would be better to start sending immediately
the synchronous payload floats in the SONET frame, usually spanning successive frames
line overhead includes a pointer to the start of the SPE
if payload arrives faster than we send, we may need to send an additional byte (sent as part of the line overhead)

SONET: real-time data (mostly voice) over the public carriers
Frame Relay: non-real-time data over the public carriers
ATM: real-time and non-real-time data over the public carriers, also for LAN use
IP: non-real-time data over anything (including avian carriers!)
complexity:
- connection-oriented allows more complexity, since price is only paid at connection establishment time
- connectionless allows faster recovery in the event of failure, but has more overhead and so forces greater simplicity