ATM

ATM Background

1990s, Asynchronous Transfer Mode (as opposed to SONET, Synchronous Optical Network)
designed for telephony: real-time plus data, short- or long-distance, high reliability
53-byte cells
ATM Forum: telcos + networking companies
UNI (User-Network Interface) between hosts and switches
NNI (Network-Network Interface) between switches (P-NNI within a private network)
in-order delivery, low loss

53 bytes = 424 bits
cells sent when ready (Asynchronous Transfer Mode)
5 byte header + 48 byte payload
Header (40 bits):
- GFC (4 bits; UNI only),
- VPI (8 bits/UNI, 12 bits/NNI),
- VCI (16),
- Payload Type (3),
- Cell Loss Priority (1),
- Header Error Correction (CRC, 1)
No sequence numbers (since delivery is guaranteed to be in-order), no payload CRC, no addresses, no payload length

ATM Adaptation Layer interprets the contents of the ATM cells:
- AAL 1: constant-bit-rate (CBR) traffic
- AAL 3/4: data (full service)
- AAL 5: data (simpler)
AAL Structure:
1. Segmentation and Reassembly (cell-level services)
2. Convergence (message-level services)
a Service Data Unit (SDU) is a packet
AAL is only done by end-systems, not switches

Segmentation:
- SDU from the higher layer
- add a 4-byte header (which is not used), 0-47 byte pad (to make multiple of 48 bytes after adding length and CRC), 2-byte length and 4-byte CRC
- break this into 48-byte payloads
- add 5-byte ATM headers
- send
Reassembly:
- receive 53-byte cells in order
- check HEC and strip header
- reassemble
- when SDU complete (specified by a bit in the payload type in the header), check length/CRC, if they match, deliver packet

like a telephone connection, but more efficient: statistical multiplexing
a transmission link may carry a number of virtual paths (VPs), each identified by a different VPI
a virtual path may carry a number of virtual channels, each identified by a different VCI
the combination of (link, VPI, VCI) uniquely identifies a virtual channel
virtual paths and virtual channels are virtual circuit: data is sent end-to-end, but there is no actual physical circuit, as there might be in an old-fashioned telephone switch

cell incoming on a link l of a given switch has specific VPI and VCI values in the header
the ATM switch has table which maps incoming triples (l, VPI, VCI) to outgoing triples (l', VPI', VCI')
if an incoming cell matches an incoming triple, the cell is sent on the outgoing link, and the header of the outgoing cell has the specified outgoing VPI, VCI
to route cells, change VPI to VPI' and VCI to VCI', and places the cell on l' (to route onto a virtual path, leave VCI unchanged while changing VPI to VPI')
multicast is easy -- just list multiple output link/VPI/VCI for an incoming link/VPI/VCI
in-class exercise: given this table, forward cell from link 5, VPI=33, VCI=7

Quality of Service: what the network provides to the connection
Traffic Descriptor: what the host provides to the network (traffic contract)
Service classes:
- Constant bit-rate (CBR)
- Variable bit-rate (VBR): Real-Time (RT), Non-Real-Time(NRT)
- Available bit-rate (ABR)
- Unspecified bit-rate (UBR)
QoS parameters include maximum Cell Delay Variation (CDV) and Cell Loss Ratio (CLR)

Dual Leaky Bucket, in parallel (any overflow is tagged by Cell Loss Priority)
Each Leaky Bucket has:
- maximum sustained rate R (cells/s)
- capacity C (cells)
Peak rate leaky bucket: PCR (Peak Cell Rate), CDVT (Cell Delay Variation Tolerance)
Mean rate leaky bucket: SCR (Sustained Cell Rate), BT (Burst Tolerance)

20-byte ATM addresses (telephone number superset, hierarchical)
once connected, transfer (including multicast) is easy
how to find an ATM address given a higher-level address?
- broadcast every message to everyone in the Virtual LAN (VPN) -- this is LAN emulation, LANE
- configure end systems with the ATM address of an address server. The server records my addresses when I connect, and provides translations of others' addresses. Subsequent transfers occur over a regular ATM connection -- CLassical IP over ATM, CLIP