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ATM, SONET, and SDH
- Asynchronous Transfer Mode (ATM) and Synchronous Optical Network (SONET) work in
conjunction to provide high-speed voice and data services. SONET provides L1 functions –
defines how to transmit high-speed data over optical links, while ATM provides L2 functions
over SONET – framing, addressing, and error detection.
- Synchronous Digital Hierarchy (SDH) represents the same standards as SONET outside US.
- Below list the speeds and bit rates for all SONET and SDH link types:
SONET Optical Carrier Level SONET STS Level SDH STM Level Speed / Bit Rate
OC-1 STS-1 STM-0 52Mbps
OC-3 STS-3 STM-1 155Mbps
OC-12 STS-12 STM-4 622Mbps
OC-24 STS-24 STM-8 1.2Gbps
OC-48 STS-48 STM-16 2.4Gbps
OC-96 STS-96 STM-32 5Gbps
OC-192 STS-192 STM-64 10Gbps
OC-768 STS-768 STM-256 40Gbps
Synchronous Transport Signal (STS) and Synchronous Transport Module (STM) are
referred to as the frame format for SONET and SDH respectively.
- ATM does not forward frames, it forwards cells. IP packets and Ethernet frames can vary in size,
but ATM cells are fixed 53-byte in length – 5 bytes header and 48 bytes payload. The ATM
header contains 2 fields – Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI)
that function like Frame Relay DLCI for identifying virtual circuits. ATM switches forward
ATM cells based on the VPI/VCI pair.
Cell
Header
48-byes
Payload
Frame
Header
Cell
Header
Packet
48-byes
Payload
Cell
Header
48-byes
Payload
RT1 ATM Network
RT2
Figure 21-2: ATM Segmentation and Reassembly
- The Segmentation and Reassembly (SAR) of Ethernet frames and ATM cells are performed by
the routers that are connected to the ATM network.
- ATM has similar functions to Frame Relay, and hence is also being considered as a type of
packet-switching service. However, it is more often referred to as a cell-switching service.
- Frame Relay, ATM, and MPLS are connection-oriented packet-switched networks; while
SONET/SDH is a connection-oriented circuit-switched network. They are considered
connection-oriented as virtual channels or virtual circuits must be established across the network
prior to data transmission. Note: Refer to Page 238 for explanation of connectionless networks.
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com