Download Document

Prof. Dr. Ing. PUSZTAI Kalman
ATM networks
1
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
2
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
3
ATM cell format
4
8
16
3
1
GFC
VPI
VCI
Type
CLP
8
HEC (CRC-8)
384 (48 bytes)
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
4
Segmentation And Reassembly (SAR)
AAL
AAL
…
…
ATM
ATM
• 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
5
AAL5 SAR protocol
< 64 KB
0– 47 bytes
16
16
32
Data
Pad
Reserved
Len
CRC-32
Padding
CS-PDU
trailer
User data
48 bytes
48 bytes
48 bytes
6
ATM header
Cell payload
VPIs and VCIs
Public netw ork
Netw ork A
Netw ork B
• 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
7
ATM in the LAN environment
• A bit of history: ATM vs Ethernet
• Major issue: how to implement broadcast and multicast in
an ATM LAN?
H4
H5
Ethernet links
ATM links
H6
H3
E2
E1
Ethernet sw itch
H7
ATM sw itch
ATM-attached
host
E3
H1
H2
8
ATL LAN Emulation (LANE)
• Objective: make ATM network look like shared-media
LAN (e.g., Ethernet) to higher-layer protocols
Higher-layer
protocols
(IP, ARP, . . .)
Signalling
+ LANE
Ethernet-like
interface
AAL5
ATM
PHY
Host
Higher-layer
protocols
(IP, ARP, . . .)
Signalling
+ LANE
AAL5
ATM
PHY
PHY
Switch
ATM
PHY
Host
9
LANE servers
LES
ATM netw ork
BUS
Point-to-point VC
Point-to-multipoint VC
H1
H2
• 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
10