Flow and Rate Control

The primary method of flow control in the Paris network is a "delivery rate control" system rather than a flow control mechanism. The concept is to control the rate at which packets are allowed to enter the network rather than

controlling individual flows. All flow and rate control in a Paris network takes place in the EPP.

Chapter 8. High Speed Packet Networking 163

The scheme used is called "leaky bucket" rate control. In order for a packet to pass the leaky bucket the counter must be non-zero.

I

I

- if maximum value reached - do not change

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Packets Arrive

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Decrement Counter when passing barrier - if counter equals zero packet must wait

Packets Ent.er Network

Figure 64. Leaky Bucket Rate Control

The "leaky bucket" is a counter which has a defined maximum value. This counter is incremented (by one) n times per second. When a packet arrives it may pass the leaky bucket if (and only if) the counter is non-zero. When the packet passes the barrier to enter the network, the counter is decremented.

This scheme has the effect of limiting the packet rate to a defined average, but

"allowing short (definable size) bursts of packets to enter the network at

maximum rate. If the node tries to send packets at a high rate for a long period of time, the rate will be equal to "n" per second. If however, there has been no traffic for a while, then the node may send at full rate until the counter reaches zero.

Paris in fact uses two leaky buckets in series with the second one using a

maximum bucket size of 1 but a faster clock rate. The total effect is to limit input to a defined average rate but with short bursts allowed at a higher rate (but not the full speed of the transmission medium). The scheme is a bit conservative but allocates capacity fairly. Paris has an adaptive modification based on

network congestion. It can alter the maximum rates at which th~ buckets "leak".

The network provides feedback via a congestion stamp on the reverse path.

This feedback is used to alter the rate at which the counters are incremented and thus the rate at which packets are able to enter the network.

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11

' End to End Flow Control ' -Figure 65. Paiis Flow and Rate Control

In addition to the delivery rate control, which restricts the rate at which data may enter the network, there are end-to-end protocols which allow for error recovery (

and for flow control for each individual connection. ~

These controls are different depending on the type of connection. For data a protocol is used that allows for error recovery by retransmission of error (or lost)

8.3.6 Interfaces

packets and also provides flow control to allow for speed matching between the end users. Voice traffic does not have any retransmission for recovery from errors but it does need a method of playout that allows for the possibility of irregular network delays and for lost packets etc.

8.3.6.1 Physical Interfaces

The Paris system will operate over almost any clear channel physical interface of appropriate speed. Typical connection could be through G.703 electrical interface to an optical transmitter at (say) 150 megabits. Also possible would be connection to SonetlSDH at a number of different rates such as STS-3c.

8.3.6.2 User Data Interfaces

End user equipment is connected to the network via an adaptation (or interfacing function). As with ATM (see section 7.1.8, "ATM Adaptation (Interfacing) Layer (AAL)" on page 137), an adaptation layer is needed to interface and adapt the network to different kinds of user traffic. A number of different interfacing layers (or modes of operation of a common protocol) would be needed depending on the type of traffic to be handled.

Paris requires much less adaptation function than does ATM. This is because:

• Paris sends full frames (there is no need to break logical user blocks of data up into small units for transmission).

• Paris performs error detection on the data part of the frame as well as the routing header. There is no need for any additional function to provide error detection on the data.

In addition to the adaptation function there is also an interfacing function needed which converts the network interface protocol to the internal network protocol.

This is not quite the same thing as adaptation.

For example, a Frame Relay service could be built using a Paris backbone network very easily. To use a Paris network for TCP/IP traffic requires that IP routers be built at the Paris network entry points. These routers would have to accomodate the difference between the connection oriented nature of the Paris network and the connectionless nature of IP (this adaptation is a standard function of IP).

Handling SNA is a different matter and is discussed in section 5.8.1, "SNA in a High Speed Network" on page 97.

8.3.6.3 Transporting Voice

The principles of handling voice in a high speed packet network are discussed in section 5.2, "Transporting Voice in a Packet Network" on page 79. These principles hold for voice transport in a Paris network. The major difference is that a packet size larger than 48 bytes may be used. This means that echo cancellation is necessary.

This is discussed fully in A Blind Voice Packet Synchronisation Strategy (see bibliography).

Chapter 8. High Speed Packet Networking 165