Prof' Dr' Ing' PUSZTAI Kalman

1
Prof. Dr. Ing. PUSZTAI Kalman

• ATM networks

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ATM overview

• Promising technology in early 90s (why?)
• Connection-oriented (virtual circuits)
• Signalling protocol Q.2931
• Support for QoS (reserve bandwidth and buffer
  space for each VC at switches)
• Fixed-size packets (cells) 53 bytes

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Cells and cell size

• Pros and cons of variable-sized packets
• Bandwidth efficiency (no zero padding)
• Hardware complexity
• Benefits of fixed-size packets (cells)
• Switch processing is same for all cells
• Small cells reduce preemption delay
• Trade-offs in choosing cell size
• Bandwidth efficiency cell processing speed
• ATM 48B, 5B header, 53B total

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ATM cell format
4
16
3
1
8
384 (48 bytes)
8
GFC
HEC (CRC-8)
VPI
VCI
CLP
Type
Payload

• VPI VCI identify virtual-circuit
• Type bits
• Explicit Forward Congestion Indication
• User signalling bit (used by AAL5 shows last
  cell of a packet)
• Cell Loss Priority bit

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Segmentation And Reassembly (SAR)
AAL
AAL


A
TM
A
TM

• Also referred to as fragmentation and
  reassembly (in the IP context)
• SAR is performed by ATM Adaptation Layer (AAL)
• AAL attaches an additional header to the packet
  needed for reassembly at the receiver
• Five different AAL protocols were defined AAL5
  was the most popular in data networks

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AAL5 SAR protocol
Padding
CS-PDU
User data
trailer
48 bytes
48 bytes
48 bytes
ATM header
Cell payload
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VPIs and VCIs

• ATM connection identifiers 8b VPI 16b VCI
• Two-level hierarchy
• Virtual paths are used between major network
  nodes
• Virtual circuits are multiplexed in same virtual
  paths
• Objective backbone switches need smaller VC table

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ATM in the LAN environment

• A bit of history ATM vs Ethernet
• Major issue how to implement broadcast and
  multicast in an ATM LAN?

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ATL LAN Emulation (LANE)

• Objective make ATM network look like
  shared-media LAN (e.g., Ethernet) to higher-layer
  protocols

Higher-layer
Higher-layer
protocols
protocols
(IP, ARP,
.
.
.)
(IP, ARP,
.
.
.)
Ethernet-like
interface
Signalling
Signalling
LANE
LANE
AAL5
AAL5
ATM
ATM
ATM
PHY
PHY
PHY
PHY
Host
Switch
Host
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LANE servers

• LECS LAN Emulation Configuration Server
• Used to first connect an ATM host to a LANE
  network
• LES LAN Emulation Server
• Keeps track of ATM and MAC addresses for each
  host
• BUS Broadcast Unknown Server
• Maintains point-to-multipoint VC to all
  registered clients