oach to a number of layers. Another trend is to reduce the number of layers to a minimum. 7.4.4 TCP/IP AND ATM PACKETIZATION EFFECTS Delays and Delay Variations in Packet Systems Packetized systems are queuing systems. Traditionally all packets were treated equally: first in, first out. Having all packets share the same queue has much the same effect as having all highway traffic share a single lane: congestion and delays at the intersections, depending on the distribution of packet arrivals. The burstiness (peak rate to average rate) of many interactive applications should be borne in mind. It might be 200 times as much as voice, with a burstiness set to 1 for a reserved path. Bandwidth sharing led to a situation where bandwidth started to be related for the first time to QoS, and delay handling became an important issue. IP networks have basically Understanding Changing Telecommunications - Building a Successful Telecom Business very little sensitivity to delay as data packets do not need synchronous reception and eventually can be retransmitted on error detection, some time after the original dispatch. The packetized voice applications (VoIP, VoFR, VoATM) use short packet/cell sizes (typically below 100 bytes) to minimize delay and jitter. In a packet-handling device at least three processes can create queues: the arrival pro- cess; the serving (switching) process; and especially the output serialization process (see Figure 7.8). Serialization is a kind of multiplexing. When long data packets are occupying the output, excessive delay may arise for waiting packets used for real-time traffic. The packetization delay for an ATM cell is 6 ms, when used for a single voice channel and with a payload of 48 bytes. This is the time for a 64 kbit/s or 8 kbytes/s stream to fill the payload space. In IP data networks the best-effort data traffic has an average packet size of around 1.5 kbytes and a possible maximum of 64 kbytes. Now let us consider the case when this traffic competes for network resources with packetized 