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Figure 12.140 illustrates the relationship between virtual channel, virtual path, and transmission path.
Figure 12.140 Virtual channel, virtual path, and transmission path.
Transmission Path VP
VP VP
VP VP
VP VC
VC VC
VC VC
VC
Two types of switching are possible: VP switching and VC switching. A VP switch connects a set of incoming VP terminations to a set of outgoing VP
terminations. The mapped VPs will have different VPIs, but the VCI will remain unchanged. A VC switch, on the other hand, connects a set of incoming VC
terminations to a set of outgoing VC termination. VCIs are not preserved. Because VCs are embedded in VPs, VC switching also involves VP switching.
12.2.2. ATM Cells
An ATM cell consists of 53 consecutive octets, of which 48 octents are used to represent user information and the remaining 5 octets form a header for the private
use of the ATM protocols see Figure 12.141.
Figure 12.141 ATM cell structure. Field
Description
GFC Generic Flow Control field for end-to-end flow control
VPI Virtual Path Identifier for routing
Header VCI
Virtual Channel Identifier for routing 5 octets
PT Payload Type for the information field
CLP Cell Loss Priority for discarding of cells in congestions
HEC Header Error Control code
Body 48 octets
Information User information or inband signaling information
The GFC field is intended for the user-network interface only. It does not
appear in the header format of cells used for the internal operation of the network. Its 4 bits are instead used by the VPI. It can be used by applications to provide end-
to-end flow control. The VPI and VCI fields collectively support the routing of a cell by the network. The PT field denotes the type of information stored in the
information field.
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The CLP field prioritizes the order in which cells may be discarded. The HEC
field is an 8-bit CRC calculated over the remaining 32 bits in the header. It is used for error detection and single-bit error correction.
12.3. Physical Layer
This section describes the transmission convergence functions of the physical layer. The physical layer may be either cell-based or SDH-based SDH stands for
Synchronous Digital Hierarchy and will be explained in Section 12.7. The generation, recovery, and adaptation of transmission frames are done differently for
these two interfaces, and are therefore described separately below. The remaining functions are identical in both cases.
12.3.1. SDH-Based Interface
Figure 12.142 shows the transmission frame format of the 155.52 Mbps SDH- based interface. It consists of a table of 9 rows by 270 columns of octets total of
2430 octets. The frame is transmitted row-by-row at a frequency of 8 kHz, thus yielding a data rate of 2430 × 8kHz × 8 bits = 155.52 Mbps. The technical term for
the frame format is Synchronous Transport Module 1 STM-1.
Figure 12.142 Frame format for 155.52 Mbps SDH-based interface STM-1.
9 Octets 261 Octets
Section AU-4 Pointer
Administrative Unit 4 AU-4 Overhead
The first 9 columns of octets in the frame consist of transmission overheads called the Section Overhead SOH. This includes framing bytes, parity bytes,
additional narrowband channels for other uses, and the AU-4 pointer which points to a Virtual Container 4 VC-4 block. The VC-4 block starts somewhere in the AU-4
which comprises the remaining 261 columns of the frame.
The format of a VC-4 block is shown in figure 12.143. It consists of 9 rows by 261 columns of octets. Of these, the first column is called the Path Overhead POH
and the remaining 260 columns consist of ATM cells. Because the total size of the latter is not an integral multiple of 53 octets, ATM cells may cross VC-4 boundaries.
