ATM: (part 1 of 2)

• History
• Architecture
• Virtual Circuits
• Cells
• AAL
• Signaling and Maintenance

ATM History

• Asynchronous Transfer Mode (as opposed to SONET)
• Designed for Telephony: real-time plus data, short- or long-distance, high reliability
• 1990's technology
• 53-byte cells
• ATM Forum: telephone companies + networking companies
• Can I rent bandwidth (QoS) from my telephone company? On demand?
• Can I rent "dark fiber" from my telephone company and set up my intranet?

ATM Architecture

• Layer 3 (Network), but used as if it were layer 2 by IP
• Hosts and switches
• UNI (User-Network Interface) between hosts and switches
• NNI (Network-Network Interface) between switches (P-NNI within a private network)
• Negotiated Quality of Service (QoS)
• Virtual Circuit Switching: cell multiplexing with pre-allocation of resources
• In-order delivery of cells along a Virtual Circuit

Virtual Circuits

• like a telephone connection, but more efficient: statistical multiplexing (picture)
• A transmission link may carry a number of virtual paths (VPs), each identified by a VPI
• A virtual path may carry a number of virtual channels, each identified by a VCI
• The combination of (link-identifier, VPI, VCI) uniquely identifies a virtual circuit
• Every cell carries VPI and VCI; switches must be able to identify link from which cell was received
• Switch has routing table from incoming (link, VPI, VCI) to outgoing (link', VPI', VCI'): to route cells, changes VPI to VPI' and VCI to VCI', and places the cell on link'
• Can be multicast

ATM Cells

• 53 bytes = 424 bits
• cells sent at any time (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 (in-order delivery), payload CRC, addresses, or payload length

ATM AAL

• Adaptation Layer:
  • 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)
• Service Data Unit (SDU) is a packet
• Only done by end-systems, not switches

AAL 5: Segmentation and Reassembly

• Segmentation:
  • SDU from the higher layer
  • SDU size is multiple of 48 bytes
  • break it into 48-byte payloads
  • add 5-byte ATM headers
  • send
• Reassembly:
  • receive 53-byte cells in order
  • strip header (after checking CRC)
  • reassemble
  • when packet complete (as determined by bit in header), deliver to higher layer

AAL 5: Convergence Sublayer

• 4-byte header (unused)
• Payload (up to 2^16 bytes
• 0-47 byte pad to align to a 48-byte boundary
• 2-byte length field
• 4-byte CRC

AAL 1: CBR

• 47-byte payload per cell
• 1-byte header per cell carries 3-bit sequence number
• cells sent at constant (known, pre-allocated) rate

Signaling

• Permanent Virtual Circuits (PVCs)
• Switched Virtual Circuits (SVCs):
  • Host sends request (on reserved VPI/VCI) to switch
  • Switch finds route, sends request to next switch
  • Final switch sends request to destination
  • Destination replies, accepting/rejecting
  • Acceptance only if all switches and destination accept (may depend on resources requested)
  • In case of rejection, switch may try alternate route (crankback)
  • Switches must set up routing tables when processing successful reply

Maintenance

• VCI = 3 (hop-by-hop) or VCI = 4 (end-to-end)
• special cells for "ping" to check connectivity:
  • over a link
  • over a path