Biblio5

554ASYNCHRONOUS TRANSFER MODE

•Point-to-Point Signaling Virtual Channels. For point-to-point signaling, one virtual channel connection in each direction is allocated to each signaling entity. The same

/VCI value is used in both directions.

•General Broadcast Signaling Virtual Channel. The general broadcast signaling virtual channel (GBSVC) may be used for call offering in all cases. In cases where the “point” does not implement service profiles or where “the multipoints” do not support service profile identification, the GBSVC is used for call offering. The specific VCI value for general broadcast signaling is reserved per VP at the UNI. Only when meta-signaling is used in a VP is the GBSVC activated in the VP.

•Selective Broadcast Signaling Virtual Channels. Instead of the GBSVC, a virtual channel connection for selective broadcast signaling (SBS) can be used for call offering, in cases where a specific service profile is used. No other uses for SBSVCs are foreseen.

18.10 QUALITY OF SERVICE (QoS)

18.10.1 ATM Quality of Service Review

A basic performance measure for any digital data communication system is bit error rate (BER). Well-designed fiber-optic links will predominate now and into the foreseeable future. We may expect BERs from such links on the order of 1 × 10−12 and with end- to-end performance better than 5 × 10−10 (Ref. 14). Thus other performance issues may dominate the scene. These may be called ATM unique QoS items, namely:

•Cell transfer delay;

•Cell delay variation;

•Cell loss ratio;

•Mean cell transfer delay;

•Cell error ratio;

•Severely errored cell block ratio; and

•Cell misinsertion rate.

18.10.2 Selected QoS Parameter Descriptions

18.10.2.1 Cell Transfer Delay. In addition to the normal delay through network elements and transmission paths, extra delay is added to an ATM network at an ATM switch. The cause of the delay at this point is the statistical asynchronous multiplexing. Because of this, two cells can be directed toward the same output of an ATM switch or cross-connect, resulting in output contention.

The result is that one cell or more is held in a buffer until the next available opportunity to continue transmission. We can see that the second cell will suffer additional delay. The delay of a cell will depend upon the amount of traffic within a switch and thus the probability of contention.

The asynchronous path of each ATM cell also contributes to cell delay. Cells can be delayed one or many cell periods, depending on traffic intensity, switch sizing, and the transmission path taken through the network.

18.10.2.2 Cell Delay Variation (CDV). By definition, ATM traffic is asynchronous, magnifying transmission delay. Delay is also inconsistent across the network. It can be a function of time (i.e., a moment in time), network design/ switch design (such as buffer size), and traffic characteristics at that moment in time. The result is cell delay variation (CDV).

CDV can have several deleterious effects. The dispersion effect, or spreading out, of cell interarrival times can impact signaling functions or the reassembly of cell user data. Another effect is called clumping. This occurs when the interarrival times between transmitted cells shorten. One can imagine how this could affect the instantaneous network capacity and how it can impact other services using the network.

There are two performance parameters associated with cell delay variation: 1-point cell delay variation (1-point CDV) and 2-point cell delay variation (2-point CDV).

The 1-point CDV describes variability in the pattern of cell arrival events observed at a single boundary with reference to the negotiated peak rate 1/ T as defined in ITU-T Rec. I.371 (Ref. 13). The 2-point CDV describes variability in the pattern of cell arrival events as observed at the output of a connection portion (MP1).

18.10.2.3 Cell Loss Ratio. Cell loss may not be uncommon in an ATM network. There are two basic causes of cell loss: (1) error in cell header or (2) network congestion.

Cells with header errors are automatically discarded. This prevents misrouting of errored cells, as well as the possibility of privacy and security breaches.

Switch buffer overflow can also cause cell loss. It is in these buffers that cells are held in prioritized queues. If there is congestion, cells in a queue may be discarded selectively in accordance with their level of priority. Here enters the CLP (cell loss priority) bit, discussed in Section 18.4. Cells with this bit set to 1 are discarded in preference to other, more critical cells. In this way, buffer fill can be reduced to prevent overflow (Ref. 1).

Cell loss ratio is defined for an ATM connection as:

Lost cells/ Total transmitted cells.

Lost and transmitted cells counted in severely errored cell blocks should be excluded from the cell population in computing cell loss ratio (Ref. 3).

18.11 TRAFFIC CONTROL AND CONGESTION CONTROL

The following functions form a framework for managing and controlling traffic and congestion in ATM networks and are to be used in appropriate combinations from the point of view of ITU-T Rec. I.371 (Ref. 13):

1. Network Resource Management (NRM). Provision is used to allocate network resources in order to separate traffic flows in accordance with service characteristics.

2. Connection Admission Control (CAC). This is defined as a set of actions taken by the network during the call setup phase or during the call renegotiation phase in order to establish whether a VC or VP connection request can be accepted or rejected, or whether a request for reallocation can be accommodated. Routing is part of CAC actions.

556 ASYNCHRONOUS TRANSFER MODE

Figure 18.15 Reference configuration for traffic control and congestion control. (From ITU-T Rec. I.371, Figure 1/ I.371, p. 3 [Ref. 13].)

3. Feedback Controls. These are a set of actions taken by the network and by users to regulate the traffic submitted on ATM connections according to the state of network elements.

4. Usage/ Network Parameter Control (UPC/ NPC). This is a set of actions taken by the network to monitor and control traffic, in terms of traffic offered and validity of the ATM connection, at the user access and network access, respectively. Their main purpose is to protect network resources from malicious as well as unintentional misbehavior, which can affect the QoS of other already established connections, by detecting violations of negotiated parameters and taking appropriate actions.

5. Priority Control. The user may generate different priority traffic flows by using the CLP. A congested network element may selectively discard cells with low priority, if necessary, to protect as far as possible the network performance for cells with higher priority (Ref. 13).

Figure 18.15 is a reference configuration for traffic and congestion control on a B-ISDN/ ATM network.

18.12 TRANSPORTING ATM CELLS

18.12.1 In the DS3 Frame

One of the most popular high-speed digital transmission systems in North America is DS3 operating at a nominal transmission rate of 45 Mbps. It is also being widely implemented for transport of SMDS. The system used to map ATM cells into the DS3 format is the same that is used for SMDS.

To map ATM cells into a DS3 bit stream, the physical layer convergence protocol (PLCP) is employed. A DS3 PLCP frame is shown in Figure 18.16.

There are 12 cells in a PLCP frame. Each cell is preceded by a 2-octet framing pattern (A1, A2) to enable the receiver to synchronize to cells. After the framing pattern there is an indicator consisting of one of 12 fixed bit patterns used to identify the cell location

Figure 18.16 Format of DS3 PLCP frame. (From Ref. 1, courtesy of Hewlett-Packard.)

within the frame (POI). This is followed by an octet of overhead information used for path management. The entire frame is then padded with either 13 nibbles or 14 nibbles (1 nibble c 4 bits) of trailer to bring the transmission rate up to the exact DS3 bit rate. The DS3 frame, as we are aware, has a 125-ms duration.

DS3 has to contend with network slips (added/ dropped frames to accommodate synchronization alignment). Thus PLCP is padded with a variable number of stuff (justification) bits to accommodate possible timing slips. The C1 overhead octet indicates the length of padding. The BIP (bit-interleaved parity) checks the payload and overhead functions for errors and performance degradation. This performance information is transmitted in the overhead.

18.12.2 DS1 Mapping

One approach to mapping ATM cells into a DS1 frame is to use a similar procedure as used with the DS3 PLCP. In this case only 10 cells are bundled into a frame, and two of the Z overhead octets are removed. The padding of the frame is set at 6 octets. The entire frame takes 3 ms to transmit and spans many DS1 ESF (extended superframe) frames. This mapping is illustrated in Figure 18.17. The L2 PDU is terminology used with SMDS. It is the upper-level frame from which ATM cells derive through its segmentation.

One must also consider the arithmetic of the situation. Each DS1 time slot is 8 bits long or 1 octet in length. By definition, there are 24 octets in a DS1 frame. This, of course, leads to a second method of transporting ATM cells in DS1, by directly mapping ATM cells into DS1, octet-for-octet (time slot). This is done by groups of 53 octets (1 cell) and would, by necessity, cross DS1 frame boundaries to transport a complete cell.

Figure 18.19 Mapping ATM cells into STM-1 (155.520 Mbps rate), at the SDH-based UNI. (From ITU-T Rec. I.432, Figure 8/ I.432, p. 13 [Ref. 4].)

into the C-4, which, in turn, is mapped into the VC-4 container along with VC-4 path overhead. The ATM cell boundaries are aligned with STM octet boundaries. Since the C-4 capacity (2340 octets) is not an integer multiple of the cell length (53 octets), a cell may cross C-4 boundaries. The AU-4 pointer (octets H1 and H2 in the SOH) is used for finding the first octet in the VC-4.

18.12.4.2 At the STM-4 Rate (622.080 Mbps). As shown in Figure 18.20, the ATM cell stream is first mapped into the C-4-4c and then packed into the VC-4-4c container along with the VC-4-4c overhead. The ATM cell boundaries are aligned with the STM-4 octet boundaries. The C-4-4c capacity (9360 octets) is not an integer multiple of the cell length (53 octets); thus a cell may cross a C-4-4c boundary. The AU pointers are used for finding the first octet of the VC-4-4c.

560 ASYNCHRONOUS TRANSFER MODE

Figure 18.20 Mapping ATM cells into the STM-4 (655.080 Mbps rate) frame structure for the SDH-based UNI. (From ITU-T Rec. I. 432, Figure 10/ I.432, p. 15 [Ref. 4].)

18.12.5 Mapping ATM Cells into SONET

ATM cells are mapped directly into the SONET payload (49.54 Mbps). As with SDH, the payload in octets is not an integer multiple of cell length, and thus a cell may cross an STS cell boundary. This mapping concept is shown in Figure 18.21. The H4 pointer

Figure 18.21 Mapping ATM cells directly into a SONET STS-1 frame. (From Ref. 1, courtesy of HewlettPackard.)

can indicate where the cells begin inside an STS frame. Another approach is to identify cell headers, and thus the first cell in the frame.

REVIEW EXERCISES

562 ASYNCHRONOUS TRANSFER MODE

REFERENCES

1. Broadband Testing Technologies, Hewlett-Packard Co., Burlington, MA, Oct. 1993. 2. Broadband Aspects of ISDN, CCITT Rec. I.121, CCITT, Geneva, 1991.

3. ATM User–Network Interface Specification, Version 3.0, ATM Forum, PTR Prentice-Hall, Englewood Cliffs, NJ, 1993.

4. B-ISDN User-Network Interface—Physical Layer Specification, ITU-T Rec. I.432, ITU, Geneva, March 1993.

5. Broadband ISDN User–Network Interfaces—Rates and Formats Specifications, ANSI T1.624-1993, ANSI, New York, 1993.

6. B-ISDN User–Network Interface, CCITT Rec. I.413, ITU, Geneva, 1991.

7. Broadband ISDN–ATM Layer Functionality and Specification, ANSI T1.627-1993, ANSI, New York, 1993.

8. D. E. McDysan and D. L. Spohn, ATM Theory and Application, McGraw-Hill, New York, 1995.

9. B-ISDN ATM Layer Specification, ITU-T Rec. I.361, ITU, Geneva, March, 1993.

10. B-ISDN ATM Adaptation Layer (AAL) Functional Description, ITU-T Rec. I.363, ITU, Geneva, 1993.

11. Support of Broadband Connectionless Data Service on B-ISDN, ITU-T Rec. I.364, ITU, Geneva, March 1993.

12. B-ISDN General Network Aspects, ITU-T Rec. I.311, ITU, Geneva, March 1993.

13. Traffic Control and Congestion Control in B-ISDN, ITU-T Rec. I.371, ITU, Geneva, March 1993.

14. Bellcore Notes on the Network, SR-2275, Issue 3, Bellcore, Piscataway, NJ, Dec. 1997.