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Table of Contents

Connection Management

Connection Management

This chapter is provided for users who wish to have an in-depth knowledge of the Cisco IPX narrowband connection management functions. It describes packet queuing and the various queue types. It also discusses circuit routing and rerouting, delay for various types of connections, and circuit bandwidth requirements and utilization.

The chapter contains the following:

Packet Queuing

As previously discussed, packets may be created in an Cisco IPX narrowband switch by any of the following cards: NPC, CDP, SDP, LDP, or FRP. Each of these cards creates one or more different types of packets, each of which is handled separately for purposes of packet queuing in the NTC cards.

Each NTC contains a routing table in RAM to determine which packets it should take from the system bus MUXBUS for transmission on its trunk. It checks the address on each MUXBUS packet against this table and, if a match is found, it reads the packet into one of its queues. The separate queues allow the NTC to set transmission priority for different packets depending on the type of information they carry.

Packets are removed from the system bus MUXBUS in the node and queued for transmission by a trunk card. Different types and models of trunk cards support different packet types. For example, the NTC Model B support only high priority, non-timestamped, timestamped, and voice packets. The NTC Model C support all six packet types.

In the NTC Model C or later, trunk cards, the queue service algorithm:

This is accomplished using the Credit Manager as described in the section on frame relay. In these cards, high priority packets are not subject to the credit manager scheme. The other five packet types, however, are issued credits one by one at a rate equal to the configured load of that type of packets on the trunk. Furthermore, each packet type may not receive a credit if it has not used its previous credit.

As an example, assume that a trunk has the following load:

Then, as frames go by, each of these packet types is eligible to accrue credits as indicated in Table 16-1.


Table 16-1: Credit Accumulation
Frame 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Voice Credits

X

X

X

X

Timestamped Credits

X

X

Non-timestamped Credits

X

X

X

X

X

X

X

At every opportunity to send a packet, the NTC Model C runs the following queue service algorithm to determine which packet to send.


Step 1   If there is a high priority packet, send it.

Step 2   If there is no high priority packet, then examine each other queue in order of highest to lowest configured bandwidth. If a queue has a packet and a credit, send the packet.

Step 3   If no queue has a credit, then use the following priority to send a packet.

Step 4   If there is no packet to send, send a 4-byte idle packet.

This scheme allows every queue to use at least some minimum configured bandwidth. Any packets that exceed the configured bandwidth are handled in the order described, which gives a slight edge to non-timestamped, then timestamped data


.

Delay in a Cell Network

The overall delay of information through the network includes:

Delay in Packet Frame Relay Networks

Bursty data packets are built in the FRP card as the data is received from the port. Therefore, the packetization delay is inversely proportional to the speed of the port. Essentially, the time to fill a packet is the time it takes to assemble 160 bits at the bit rate of the port. This delay is only relevant if the connection is not throttled in the FRP due to the credit manager scheme implemented there to prevent network congestion.

When designing an Cisco IPX narrowband frame relay network, the goals are to minimize delay, congestion, data loss and cost and to maximize bandwidth utilization. Minimizing delay and maximizing bandwidth utilization are usually conflicting goals.

Minimizing delay in the FRP card minimizes congestion and data loss at the source (or destination) point. However, care must be taken not to shift these problems to the network trunks. Note that the frame relay parameters that reduce delay always increase bandwidth utilization and vice versa. For instance, increasing MIR to reduce delay also increases the trunk bandwidth allocated. Or, reducing %utilization to reduce the trunk bandwidth allocated to frame relay can cause congestion in the NTC or AIT trunk cards, resulting in greater delay.

The following are a couple of general suggestions that can be applied when setting up an Cisco IPX narrowband frame relay network.

Delay at the Source

Delay in the most common devices sending frames to the FRP (e.g. bridges, routers, etc.) follow the standard store-and-forward model for simple queues. This delay is changed by changing the port speed parameter (configured clock in cnfrport command). For a given amount of traffic, increasing port speed reduces delay in the access device.

However, when modifying port speed, delay in the access device must be considered in conjunction with delay in the FRP. Increasing port speed and holding MIN constant will increase delay in the FRP for that connection. While this produces a small overall reduction in delay, it also moves delay to the FRP where there is more control over it.

Delay in the FRP can be controlled by modifying MIR, Cmax, and VC Q depth. For a given amount of traffic, the greater value for MIR, the less the delay in the source FRP. Likewise, if MIR is less than the setting for port speed, the larger Cmax is, the smaller the delay. Increasing Cmax has the same effect on delay as increasing MIR. However, large Cmax can cause occasional congestion on the trunks. The value for VC Q depth sets the maximum allowable delay in the source FRP. But reducing this may result in discarded frames, which is generally unacceptable.

Delay in the Network

There are two primary sources of frame relay connection delay in the Cisco IPX narrowband switch network:

Intermediate node delay consists of processing, queuing, and transmit delay and is generally one to two milliseconds per hop even in a heavily loaded network. Even on a 10-hop connection, this delay would be under 20 milliseconds. This is assuming that care has been taken to prevent data loss that can significantly increase overall delay.

Propagation delay is generally small except in international networks. At roughly one millisecond per 100 miles, propagation delay on a 2600 mile connection (e.g. San Francisco to Boston) would be about 26 milliseconds.

If several of the connections on a trunk have large values for Cmax (on the order of 100 or more), then the possibility of short-term congestion arises. If all connections burst at once, the bursty data queue in the NTC or AIT will get very long. However, this condition should normally be of a short duration.

The connection utilization parameter, %utl, controls bandwidth allocation for frame relay connections on the trunks. Oversubscription, where the bandwidth allocated is significantly less than the connection MIR values, can allow trunks to become overloaded. This can result in congestion and data loss over network trunks resulting in significant increases in end-user delays.

Delay at the Sink

The sink FRP is the card that sends frames to the destination access device. Generally, delay in the sink FRP follows the same model as delay in the access device. However, if the sink access device is attached to a LAN, then increasing port speed can significantly reduce delay in the sink FRP without significantly increasing delay in the access device.

Another method for decreasing delay at the terminating FRP for selected connections is to assign a high priority to these connections. Frames for high priority connections are assembled in a separate output port queue from low priority connections. All frames in the high priority queue are transmitted before any frames are transmitted from the low priority queue.

Care should be taken when reducing the Port Queue Depth parameter in the cnfrport command as this could result in unnecessarily dropping frames if this queue should overflow. Where the sink FRP receives traffic from only one source and has a port speed that is greater or equal to the MIR, the queuing delay does not follow the normal model and is very small.

Synchronous Data Connection Delay

For all time-stamped and non-timestamped data packets, the number of information bytes in a packet varies from 4 to 21 bytes depending on the type and speed of the connection. The packetization delay for the two types of data packets can be calculated by using Table 16-2 or Table 16-3 or looked up in the tables at the end of this chapter.

There is a delay from the first information byte clocked into the packet buffer to the last. The lower the bit rate of the channel, the longer the packetizing delay would become. To keep this time low, packets are formed from as few as 4 bytes of information for low-speed channels. This, and the time necessary for the card's firmware to add address, priority, DFM and timestamp information to the buffer, constitutes packetization delay. The packet is then placed on the system bus.

In the SDP, non-timestamped packets are received for a particular channel, the header is discarded and the information placed in a flexible buffer. When the connection is first set up, the buffer is half-filled. This allows variations in transmission delay to be accommodated until the buffer overflows or underflows. It also allows for short-term variations in the clocks at the transmitting and receiving interfaces.

Timestamped packets are buffered in the receiving SDP until the timestamp has reached the maximum age set in the Configure System Parameter (cnfsysparm) command, then clocked out. Therefore all timestamped connections have a one-way delay approximately equal to the "maximum timestamped packet age" set in the Configure System Parameter command plus packetization and transmission delays.

EIA lead information (non-interleaved) and clock-speed information (isochronous connections) is sent in supervisory packets, SDP to SDP. These packets appear to the network like the data packets of the same connection. Therefore, their delay through the network should be the same as the data stream. However, because they are sampled asynchronously and packetized and depacketized through different paths of the SDP, their changes are time-shifted with respect to the data.

Ways to Reduce Data Connection Delays

Normally, data circuit delay is not a problem. For some user data devices transmitting over large networks, the data delay may appear to cause some minor problems. The following are several suggestions for reducing the network delay.


Table 16-2: Calculation of Non-Timestamped Data Packet Overall Delay
Source Delay (ms.)

Transmitting SDP packetizing

1-3

Transmission delay (terrestrial, per mile)

0.01

Transmission delay (satellite, per hop)

300

Miscellaneous dejitter delays (per hop)

0.25

Receiving SDP null timing buffer (per hop) 1

2.5-5.0

Receiving SDP processing delays

4.0

Receiving SDP isochronous buffer delay 2

10.0

Minimum delay (one-hop, colocated nodes)

7.75

1Includes trunk queuing delays
2Isochronous connections only


Table 16-3: Calculation of Timestamped Data Packet Overall Delay
Source Delay (ms.)

Transmitting SDP packetizing

3-33

Transmission delay (terrestrial, per mile)

0.01

Transmission delay (satellite, per hop)

300

Miscellaneous dejitter delays (per hop)

0.25

Receiving SDP null-timing buffer

40

Receiving SDP processing delay

4.0

Receiving SDP isochronous buffer delay 1

10.0

Minimum delay (one hop, colocated nodes)

47.25

1Isochronous connections only
2.Ages timestamp to 40 (default).

Voice Connection Delay

Non-VAD voice packets

For a voice channel without VAD ("p", "d" or "a"), the packetization delay is constant. It is the time for 21 bytes or 42 bytes to be processed by the CDP or 2.625 msecs for a "p" connection and
5.25 msecs for an "a32" connection.

VAD voice packets

For a voice channel with VAD ("v" or "c"), there is a VAD software parameter, sample input delay (SID) that defines the size of a serial register in the CDP. This adjusts "front end clipping" but increases the end-to-end delay of the connection by the amount of buffer delay.

Transmission delay across a trunk is generally a function of the distance travelled. For a terrestrial trunk, signals travel an average of about 100 miles per millisecond, or 0.01 msec/mile. For a satellite trunk, the propagation delay of the signal to the satellite and back adds approximately 300 msec per satellite hop. Table 16-4 can be used to calculate delay for the four types of voice connections.


Table 16-4: Sources of Delay in Voice Connections
Delay Source t & p v a16 a24 a32 c16 c24 c32

Circuit T1 transmitter dejitter

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

Transmitting CDP sample input delay

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

Transmitting CDP packetization

2.6

2.6

10.5

7.0

5.25

10.5

7.0

5.25

Transmitting TXR queuing (per hop)

< 2.5

< 2.5

< 2.5

< 2.5

< 2.5

< 2.5

< 2.5

< 2.5

Transmission delay (terrestrial, per mile)

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

Transmission delay (satellite, per hop)

300

300

300

300

300

300

300

300

Miscellaneous dejitter delays (per hop)

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

Table 16-5 illustrates some typical expected delays for various number of terrestrial hops. The calculations do not include any PBX or channel bank delay.


Table 16-5: Typical Voice Connection Delays
Expected Delay (ms.) t or p v a c

1-hop, colocated nodes1

9

30

14

35

2-hops, colocated nodes1

12

33

17

38

3-hops, colocated nodes1

15

36

20

41

4-hops, colocated nodes1

17

38

23

43

5-hops, colocated nodes1

20

41

25

45

1For nodes that are not colocated nodes, add 0.01 ms./mile.

Maximum Hops—Voice Connections

With the NTC, as the number of hops for a connection increases, the possible fluctuation in delay increases also. This is because each NTC in the path may add delay up to the maximum allowed by the queuing parameters, depending on the other traffic passing over that trunk. The CDP has a large buffer so the buffer size does not limit the maximum number of hops for a voice connection.

Routing and Rerouting

This section discusses how the Cisco IPX narrowband switch determines the route each circuit takes when added to the network and the algorithms and considerations involved when the Cisco IPX narrowband switch must automatically reroute circuits because of a failure(s) detected in the network.

Load Model

The Cisco IPX narrowband switch maintains a load model of the network and uses it to make decisions for routing and failing to route connections. The inputs to this model are the number and type of all connections routed on each trunk, and the configured utilization figures for VAD and DFM connections (measured as a percent of nominal connection bandwidth).

These utilization figures are set by the network administrator. Defaults are 40 percent for VAD and 100 percent for DFM and frame relay. It is these figures that are used to determine how many connections may be routed over a trunk, and when no more bandwidth is available. This is without reference to the "real world" performance of VAD and DFM and frame relay.

The challenge of network optimization is to make these configured utilizations reflect reality, in order to gain the maximum possible E1 or T1 pair gain, and to predict and influence the performance of the network in extreme cases of trunk failures or unfavorable statistics.

Load Model and Routing for Frame Relay

Frame relay connections differ from others because they have a greater range of instantaneous packet rates. A connection with a minimum rate of 512 Kbps may generate no packets for a long time, then suddenly generate 10 or 20 packets in a row (depending on the value of Cmax) at the frame relay port speed.

Since the connection has accepted data and processed it through the FRP card very quickly, and since the delay across the connection depends directly on the queuing delay of the last packet in the frame, it is important to ensure there are no unnecessary bottlenecks in the network trunking.

When the first frame relay connection is routed over the trunk, the load model in software allocates the entire bursty data peak bandwidth. This is important for networks mixing frame relay with other traffic, as it ensures that when a frame relay burst reaches the Cisco IPX narrowband trunk card, the bandwidth available is at least the bursty data peak.

As more frame relay connections are routed over the same trunk, the statistical addition of the different sources allows them to share bandwidth more efficiently. Because of this, the user can allocate only a portion of the trunk bandwidth required for each new frame relay connection added (the default is 121%, which equates to 100% usage for user data and the remainder for the overhead of encapsulating the frame relay data into FastPackets). This oversubscription of bandwidth is can also be extended to the Cisco IPX narrowband switch MUXBUS bandwidth reserved for each FRP. This factor can be decreased for slots where there are many PVCs transmitting at lower rates (e.g. 56 Kbps and less).

The routing algorithm (using frame relay optimization) allocates routes for new connections to minimize extra bandwidth allocation, and so tends to route frame relay connections over the same trunks that the earlier connection took. This results in good bandwidth efficiency.

A priority (high/low) can be assigned each ForeSight frame relay connection as it is added to the network. High priority connections are routed through a separate transmit queue in the FRP receiving the packets. The frames in the high priority queue are output before frames in the low priority queue. This reduces the queuing delay for these frames.

ForeSight is a closed-loop system that dynamically allocates trunk bandwidth based on the connection parameters set. If there is any excess bandwidth available after all the committed information rates have been satisfied, it will portion out the excess bandwidth based on each connection's CIR.

Routing Algorithm

Each node has, in a database, a representation of the network topology. This includes all trunks and their status, and all connections, their type and route. From this, the node calculates the load (packets/sec or cells/sec) on all trunks in the network.

When a connection is routed, the owner node determines the bandwidth requirements from lookup tables and the destination node and channel from the network database. If the connection has a preferred route (direct routing), it will attempt to comply if at all possible. The routing can also be specified by the operator to be restricted to a terrestrial route only or if a satellite route is acceptable.

The search for a circuit route is begun by first examining all trunks to adjacent nodes (in order of trunk number). If the route has enough bandwidth for the circuit (or bundle), and the terminating node is found to be the other end of the connection, the route has been found and the search is terminated. Otherwise, the search is continued.

When all single-hop routes have been examined but found lacking, the search is extended to nodes at a distance of two hops. The search radius is enlarged from the master node. Eventually, the search is successful, or completes without success, or the search times out without success. If a preferred route is specified and is unavailable for a connection that has directed routing, the connection will be marked immediately as failed.

When a search is successful, the route information (trunks and nodes on the path) is broadcast to all nodes on the chosen route so they can update their network topology models in their database.

The network does not continually look for new routes unless there are connections failed for lack of a route. If this is the case, the addition of trunks or deletion of connections is necessary. The network does not rearrange connections that are already routed to accommodate a connection that is not routed, even though the new connection may have a high priority Class of Service.

Likewise, if the statistical reserve on trunks is decreased, the network takes no action except to route any failed connections that can now be routed. However, if statistical reserve is increased, all connections in the network will be failed and rerouted as some previously used routes may no longer be available.

The Cisco IPX narrowband switch attempts to balance loads between trunks. This allows the adaptive voice feature to give better results, but affects all connections. The reroute algorithm finds all routes with the shortest hop count. It chooses the route where the current load on the most heavily loaded line of the route is a minimum. In order to force even balancing, the size of a routing bundle is restricted.

When a connection is first added to the network, software identifies the first route available in the usual way, finding the fewest hops given restrictions of trunk type (satellite/terrestrial) and current loading (there must be bandwidth available). It then finds all other routes of the same number of hops and chooses the route with the lowest loading factor.

Causes of Rerouting

The Cisco IPX narrowband switch does not move connections from existing routes unless one of the following conditions exists:

It is important to realize that the algorithm does not move working connections between trunks to balance load: the balancing occurs when a connection without a route is allocated one. A working connection is rerouted when its preferred route (when different from the current route) becomes available.

Reroute Priority and Order

For every connection, there is a master node (the owner). This node, where the connection was added is responsible for finding a route and rerouting the connection in the event of a failure. Master nodes act independently. If a trunk fails in a network, all nodes owning connections routed over that line recognize the failure since the information is broadcast to all nodes in the network. As each node recognizes the failure it attempts to reroute its connections without reference to the others.

For this reason, it is recommended that ownership of connections be concentrated in a small number of nodes. There will be fewer collisions in rerouting, and, since the class-of-service priority is followed within each node but not coordinated between nodes, performance will be more predictable and closer to that desired.

Within each node, the order of precedence for routing connections is determined by:

When a node has to find routes for a number of connections at the same time, it uses the rules above to determine the order in which it considers them. They are hierarchical. Bundle size will only be considered if there are a number of bundles of connections of the same type and COS. If a route cannot be found for a particular connection, the owning node will leave it failed and go on to the next. This is why the "largest first" rule is important. The network cannot reroute some connections to make room for others. Rerouting only occurs as the result of the failure of routed connections.

When a group of connections is failed, a timer is started at the node owning the connections. COS 0 connections may be rerouted immediately, and there is a 250 millisecond delay before each subsequent COS may be rerouted. This is to allow COS to have a network-wide effect. Therefore, COS 8 connections will be rerouted after a pause of 2 seconds although there may still be COS 0 connections awaiting rerouting. The low COS gives a "head start" rather than absolute priority.

After the COS timer, priority is given to connections with the highest bandwidth (packets/second) of the group awaiting rerouting. This is because, as available bandwidth diminishes, it is more difficult to find routes for the higher bandwidth connections. The data block for each connection contains the packet/second requirement, so prioritizing is easy. The general rerouting priority order is given in Table 16-6.

When several similar connections have the same source and destination node, they can be routed as a bundle. This saves time, as only one route is found for several connections. Bundle size is the least important rerouting priority.


Table 16-6: Priority for Rerouting
Priority Connection Type

1

high speed data connections (>64 Kbps)

2

"p" or "d" connections

3

"a" connections

4

"v" connections

5

"c" connections

6

low speed data connections (< 9.6 Kbps)

Courtesy Downing

Courtesy downing is the process of monitoring voice connections and downing those connections that are configured for downing when they go inactive. This frees up network bandwidth for other uses, generally for bandwidth reservation.

Only voice type connections can be monitored for inactivity, and then only when the on-hook status is configured by the user. All other connection types and voice types with no defined on-hook status are treated as active and cannot be courtesy downed.

System Message Traffic Routing

System messages are carried between node controller cards (NPC and BCC) in high priority packets called CC packets. The route used by any pair of controller cards to communicate is determined automatically by the network and is fixed as long as there are no changes to the network topology that affect the choice.

The criteria used to select a route between two controller cards are as follows.

    1. The network selects the route with the fewest trunks that restrict controller traffic. A user may want to restrict a trunk that uses almost all of its bandwidth for customer traffic from carrying inter-node traffic to free up the bandwidth. This is done with the Restrict CC Traffic parameter in the Configure Trunk (cnftrk) command.

    2. The network then considers the route with the fewest satellite trunks. A satellite trunk is entered in the Link type option of the Configure Trunk (cnftrk) command. The network has no way of determining whether a trunk actually uses a satellite.

    3. The network then selects the route with the biggest "choke point." The network determines for every route, a "choke point", which is the trunk that has the least total bandwidth capacity. The network then selects the route with the least restrictive choke point.

    4. The network then selects the route with the least total number of hops.

    5. If there are still choices available, internode communication will travel over the lowest numbered trunk (of the choices being considered) on the node that has the lowest number of the two nodes.

Every packet or cell that is sent between node controllers is acknowledged by the recipient. The maximum time that a controller will wait for an acknowledgment is 1.7 seconds. If no acknowledgment is received in time, the node will retransmit the packet/cell and wait another
1.7 seconds.

The maximum number of attempts, 5 or 7, depending on whether there are satellite trunks in the communication path between the nodes or not. If acknowledgment is received after the maximum allowed attempts, the far node is declared unreachable. This represents a communication break condition.

Bandwidth Allocation

One of the benefits of the Cisco IPX narrowband network is the compression of voice (VAD) and data (DFM) connections to allow cost savings through pair-gain. However, these features both depend on statistical properties of the data offered to the system. Therefore, their exact level of effectiveness is not easily predicted. VAD may result in a 0 percent to 70 percent bandwidth savings, for instance, whereas the effectiveness of ADPCM, (50 percent savings for 32 Kbps ADPCM), is predictable and unchanging.

Since the total traffic capacity of an Cisco IPX narrowband network is somewhat difficult to predict, Cisco has developed a Network Modeling Tool (NMT). This tool allows users to analyze their proposed networks to determine if there will be sufficient capacity available. For further information on the NMT, refer to the Network Modeling Tool User's Guide.

Network Trunk Bandwidth

The system calculates the available bandwidth of each network trunk as follows:

T3 Framed:
E3 Framed:
T1 Framed:
E1 Framed:
E1 Unframed:
Subrate:

Depends on the number of DS0's available in the subrate trunk. See Table 16-7.


Table 16-7: Subrate Packet Line Bandwidth
DS0s BW DS0s BW DS0s BW DS0s BW

1

n/a

9

3000

17

5666

25

8333

2

n/a

10

3333

18

6000

26

8666

3

n/a

11

3666

19

6333

27

9000

4

1333

12

4000

20

6666

28

9333

5

1666

13

4333

21

7000

29

9666

6

2000

14

4666

22

7333

30

10000

7

2333

15

5000

23

7666

31

10333

8

2666

16

5333

24

8000

32

10666


Note   It is recommended that a subrate trunk be configured with at least four DS0s to provide sufficient statistical reserve for inter-node communications traffic.

A packet slice on the TDM bus is 1000 packets/sec, therefore an E1 trunk requires 11 packet slices of TDM bandwidth for a total of 11,000 packets/sec per E1 trunk. The T1 trunk requires 8 packet slices for a total of 8,000 packets/sec. per T1 trunk.

The total bandwidth available on the Cisco IPX narrowband switch backplane MUXBUS, excluding NPC-reserved bandwidth, is 30.72 Mbps. This corresponds to 30.72 Mbps / 192 = 160,000 packets/sec. Therefore, the maximum number of E1 trunks in a node is 160,000 / 11,000 = 14. The maximum number of T1 trunks per node is 160,000 / 8,000 = 20. However, software limits this to 16 trunks.

Each 64 Kbps time slot, or DS0, provides 1/3 X 1000 or approximately 333 packets per second of available bandwidth on a trunk. Table 16-7 shows the packet bandwidth available on a subrate trunk as a function of the number of DS0s regardless of the trunk type.

Voice Compression Bandwidth Requirements

The bandwidth required on a trunk to carry the information on a DS0 circuit depends on which one of the five compression types is selected for the circuit as indicated in Table 16-8. The equivalent bit rate after compression is also listed in this table.

Compression is an effective means of reducing the network bandwidth requirements but does degrade the quality of the voice circuit. Note, however, that any circuit that may at times have a fast modem or FAX connection will automatically revert to a "p" connection during the transmission with attendant increase in bandwidth required.


Table 16-8: Cisco IPX Narrowband Switch Voice Grades
Type Equivalent Bit Rate Required BW

p

64 Kbps

381 pkts/sec.

t

64 Kbps

381 pkts/sec.

v

32 Kbps

191 pkts/sec.

a32

32 Kbps

191 pkts/sec.

a24

24 Kbps

143 pkts/sec.

a16

16 Kbps

95 pkts/sec.

a16(z)

16 Kbps

95 pkts/sec.

c32 1

16 Kbps

95 pkts/sec.

c24 1

12 Kbps

72 pkts/sec.

c16 1

8 Kbps

48 pkts/sec.

c16(z) 1

8 Kbps

48 pkts/sec.

1Assumes 50% VAD.

VAD and DFM Effects

Voice activity detection takes place in the CDP card before speech is transmitted over a trunk. If speech is present, packets are sent. If speech is not present, no packets are sent and the trunk bandwidth may be used by other connections.

Similarly, DFM allows packets whose contents can be predicted by the receiving card, those containing repetitive patterns, to be suppressed. It should be noted that a DFM packet uses one more byte for control information (sequence byte) than the packet for a corresponding non-DFM connection. If the data cannot be compressed to less than 90 percent utilization by DFM, bandwidth savings will be made by disabling DFM for that connection.

Before the development at Cisco of statistical tools, VAD was assumed to save 60 percent of nominal bandwidth. Experience has shown this to be a good estimate in most cases. But if a connection is not off-hook all the time (less than 36 ccs/hr) this estimate may be too high. Likewise, if there is high background noise on the circuit, this estimate may be too low.

With new statistical tools provided by Cisco WAN Manager NMS, this utilization can now be measured on an active network. Voice and data compression can be treated in similar ways. For effective traffic studies, it is necessary to configure utilization figures for voice in the same way as data and this section treats both forms of compression similarly.

Data Channel Packet Generation Rates

The synchronous data channels use widely varying amounts of trunk bandwidth depending on whether they use timestamped data packets or not and how the control lead information is carried. Refer to Table 16-9 through Table 16-13 for bandwidth requirements or calculate as follows.

Exceptions are the low-speed connections listed in Table 16-11, Table 16-12, and Table 16-13, where partially-filled packets are used to reduce packetization delay. Divide the bit rate of the connection by the number of user bits per packet and the result is the number of packets/second.

For DFM connections, the actual packet generation rate will depend upon the actual utilization. The load model uses the user-configured utilization to calculate the expected number of packets/second. Add between 0 and 20 packets/second for EIA updates (an isochronous clock implies 20 packets/second in the direction the clock is propagated).

Traffic Statistics

The Cisco IGX multiband switch provides a number of statistical tools to assist in traffic studies. The object of such a study is to collect enough information so that an accurate figure for configured utilization may be chosen for each connection. The display of Cisco IGX multiband switch statistics requires a Cisco WAN Manager workstation connection to the Cisco IGX multiband switch network. The Cisco WAN Manager collects all of the operating statistics for a network and stores it in its database (usually on its own hard disk). Refer to the Cisco WAN Manager Operations publication for details of statistics displays and examples.


Table 16-9: Data Load Table with Standard EIA and No DFM
Bit Rate 7/8 Coding 8/8 Coding
Kbps Pkt/sec Byte/pkt Delay, ms Pkt/sec Byte/pkt Delay, ms

1.2

43

4

24

38

4

27

1.8

65

4

16

57

4

18

2.4

35

10

29

30

10

33

3.2

46

10

22

40

10

25

3.6

52

10

20

45

10

22

4.8

35

20

29

30

20

33

6.4

46

20

22

40

20

25

7.2

52

20

20

45

20

22

8

58

20

18

50

20

20

9.6

69

20

15

60

20

17

12

86

20

12

75

20

14

12.8

92

20

11

80

20

13

14.4

103

20

10

90

20

11

16

115

20

9

100

20

10

16.8

120

20

9

105

20

10

19.2

138

20

8

120

20

9

24

172

20

6

150

20

7

28.8

206

20

5

180

20

6

32

229

20

5

200

20

5

38.4

275

20

4

240

20

5

48

343 1

20 1

3 1

300

20

4

56

381

21

3

350

20

3

57.6

392

21

3

360 1

20 1

3 1

64

436

21

3

381

21

3

72

490

21

3

429

21

3

76.8

523

21

2

458

21

3

84

572

21

2

500

21

2

96

654

21

2

572

21

2

112

762

21

2

667

21

2

115.2

784

21

2

686

21

2

128

871

21

2

762

21

2

144

980

21

2

858

21

2

168

1143

21

1

1000

21

1

192

1307

21

1

1143

21

1

224

1524

21

1

1334

21

1

230.4

1568

21

1

1372

21

1

256

1742

21

1

1524

21

1

288

1960

21

1

1715

21

1

336

2286

21

1

2000

21

1

384

2613

21

1

2286

21

1

448

3048

21

1

2667

21

1

512

3483

21

1

3048

21

1

672

4572

21

1

4000

21

1

768

5225

21

1

4572

21

1

772

5252

21

1

4596

21

1

896

6096

21

1

5334

21

1

1024

6966

21

1

6096

21

1

1152

7837

21

1

6858

21

1

1344

9144

21

1

8000

21

1

1Connections below this rate generate time-stamped data packets. Connections above this rate generate non-time-stamped data packets.


Table 16-10: Data Load Table with Standard EIA and DFM
Bit Rate 7/8 Coding 8/8 Coding
Kbps Pkt/sec Byte/pkt Delay, ms Pkt/sec Byte/pkt Delay, ms

1.2

58

3

18

50

3

20

1.8

86

3

12

75

3

14

2.4

39

9

27

34

9

30

3.2

51

9

20

45

9

23

3.6

58

9

18

50

9

20

4.8

37

19

28

32

19

32

6.4

49

19

21

43

19

24

7.2

55

19

19

48

19

22

8

61

19

17

53

19

19

9.6

73

19

14

64

19

16

12

91

19

12

79

19

13

12.8

97

19

11

85

19

12

14.4

109

19

10

95

19

11

16

121

19

9

106

19

10

16.8

127

19

8

111

19

10

19.2

145

19

7

127

19

8

24

181

19

6

158

19

7

28.8

217

19

5

190

19

6

32

241

19

5

211

19

5

38.4

289

19

4

253

19

4

48

361

19

4

316

19

4

56

422

19

3

369

19

3

57.6

434

19

3

379

19

3

64

482

19

3

422

19

3

72

542

19

2

474

19

3

76.8

578

19

2

506

19

2

84

632

19

2

553

19

2

96

722

19

2

632

19

2

112

843

19

2

737

19

2

115.2

867

19

2

758

19

2

128

963

19

2

842

19

2

*All of the connections below 56 Kbps generate time-stamped data packets.


Table 16-11: Data Load Table with Partially-Filled Packet and No DFM
Bit Rate 7/8 Coding 8/8 Coding
Kbps Pkt/sec Byte/pkt Delay, ms Pkt/sec Byte/pkt Delay, ms

2.4/4

86

4

12

75

4

14

3.2/4

115

4

9

100

4

10

3.6/4

129

4

8

113

4

9

4.8/10

69

10

15

60

10

17

4.8/4

172

4

6

150

4

7

6.4/10

92

10

11

80

10

13

6.4/4

229

4

5

200

4

5

7.2/10

103

10

10

90

10

12

7.2/4

258

4

4

225

4

5

8/10

115

10

9

100

10

10

9.6/10

138

10

8

120

10

9

12/10

172

10

6

150

10

7

12.8/10

183

10

6

160

10

7

14.4/10

206

10

5

180

10

6

* All of the above connections generate time-stamped data packets.


Table 16-12: Data Load Table with Partially-Filled Packet and DFM
Bit Rate 7/8 Coding 8/8 Coding
Kbps Pkt/sec Byte/pkt Delay, ms Pkt/sec Byte/pkt Delay, ms

2.4/4

115

3

9

100

3

10

3.2/4

153

3

7

134

3

8

3.6/4

172

3

6

150

3

7

4.8/10

77

9

14

67

9

15

4.8/4

229

3

4

200

3

5

6.4/10

102

9

10

89

9

12

6.4/4

305

3

4

267

3

4

7.2/10

115

9

9

100

9

10

7.2/4

343

3

3

300

3

4

8/10

127

9

9

112

9

9

9.6/10

153

9

7

134

9

8

12/10

191

9

6

167

9

6

12.8/10

204

9

5

178

9

6

14.4/10

229

9

5

200

9

5

*All of the above connections generate time-stamped data packets.


Table 16-13: Data Load Table with Fast EIA
Bit Rate 7/8 Coding 8/8 Coding
Kbps Pkt/sec Byte/pkt Delay, ms Pkt/sec Byte/pkt Delay, ms

1.2f

35

5

29

30

5

33

1.8f

52

5

20

45

5

22

2.4f

35

10

29

30

10

33

3.2f

46

10

22

40

10

25

3.6f

52

10

20

45

10

22

4.8f

69

10

15

60

10

17

6.4f

92

10

11

80

10

13

7.2f

103

10

10

90

10

11

8f

115

10

9

100

10

10

9.6f

138

10

8

120

10

9

12f

172

10

6

150

10

7

12.8f

183

10

6

160

10

7

14.4f

206

10

5

180

10

6

16f

229

10

5

200

10

5

16.8f

240

10

5

210

10

5

19.2f

275

10

4

240

10

5

24f

343 *

10 *

3 *

300

10

4

28.8f

412

10

3

360 *

10 *

3 *

32f

458

10

3

400

10

3

38.4f

549

10

2

480

10

3

48f

686

10

2

600

10

2

56f

800

10

2

700

10

2

57.6f

823

10

2

720

10

2

64f

915

10

2

800

10

2

72f

1029

10

1

900

10

2

76.8f

1098

10

1

960

10

2

84f

1200

10

1

1050

10

1

96f

1372

10

1

1200

10

1

112f

1600

10

1

1400

10

1

115.2f

1646

10

1

1440

10

1

128f

1829

10

1

1600

10

1

144f

2058

10

1

1800

10

1

168f

2400

10

1

2100

10

1

192f

2743

10

1

2400

10

1

224f

3200

10

1

2800

10

1

230.4f

3292

10

1

2880

10

1

256f

3658

10

1

3200

10

1

288f

4115

10

1

3600

10

1

336f

4800

10

1

4200

10

1

384f

5486

10

1

4800

10

1

448f

6400

10

1

5600

10

1

512f

7315

10

1

6400

10

1

*Connections below this rate generate time-stamped data packets. Connections above this rate generate non-time-stamped data packets.


Table 16-14: Data Load Table—Fast EIA with Partially-Filled Packet
Bit Rate 7/8 Coding 8/8 Coding
Kbps Pkt/sec Byte/pkt Delay, ms Pkt/sec Byte/pkt Delay, ms

1.2f/2

86

2

12

75

2

14

1.8f/2

129

2

8

113

2

9

2.4f/5

69

5

15

60

5

17

2.4f/2

172

2

6

150

2

7

3.2f/5

92

5

11

80

5

13

3.2f/2

229

2

5

200

2

5

3.6f/5

103

5

10

90

5

12

3.6f/2

258

2

4

225

2

5

4.8f/5

138

5

8

120

5

9

6.4f/5

183

5

6

160

5

7

7.2f/5

206

5

5

180

5

6

*All of the above connections generate time-stamped data packets.

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Posted: Tue Jan 16 11:03:22 PST 2001
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