Slot Collisions

• Slot Commissions
• Slot Collisions Games
• Slot Collisions Meaning

In this, when the collision occurs, we probe for i2th slot in ith iteration, and this probing is performed until an empty slot is found. The cache performance in quadratic probing is lower than the linear probing. Quadratic probing also reduces the problem of clustering. Slot collisions themselves aren't indicative of a problem, but if there is a period of high write response time on a set of FA's coupled with high slot collisions on those same FA's, it could point towards (but not definitively confirm) a filesystem/partition alignment problem. I am getting the following log messages in Nexus 7k. 2016 Sep 26 20:20:52 EE-PLY-I-SLRNXB-TIER2%MTM-SLOT13-4-BUCKETZEROCOLLISION: Mac 001f.da0b.a0d9 vlan 108 got replaced from FE inst 9 due to collision 2016 Sep 27 02:26:13 EE-PLY-I-SLRNXB-TIER2%MTM-SLOT13-4-BUCKETZEROCOLLISION: Mac 0012.43dc.9f05 vlan 62 got replaced from FE inst 9 due to collision Version in 6.2 and none of these vlans. After a collision is sensed by the channel, time is divided up into discrete slots. For example, if first collision identified then each station waits for either 0 or 1 slot time. Similarly, if third collision occurs then random interval will be 0 to 7 and for ith collision random number interval will be 0 to 2 i -1.

Just by adding a physics body to the balls and bouncers we already have some collision detection because the objects bounce off each other. But it's not being detected by us, which means we can't do anything about it.

In this game, we want the player to win or lose depending on how many green or red slots they hit, so we need to make a few changes:

1. Add rectangle physics to our slots.
2. Name the slots so we know which is which, then name the balls too.
3. Make our scene the contact delegate of the physics world – this means, 'tell us when contact occurs between two bodies.'
4. Create a method that handles contacts and does something appropriate.

The first step is easy enough: add these two lines just before you call addChild() for slotBase:

The slot base needs to be non-dynamic because we don't want it to move out of the way when a player ball hits.

The second step is also easy, but bears some explanation. As with UIKit, it's easy enough to store a variable pointing at specific nodes in your scene for when you want to make something happen, and there are lots of times when that's the right solution.

But for general use, Apple recommends assigning names to your nodes, then checking the name to see what node it is. We need to have three names in our code: good slots, bad slots and balls. This is really easy to do – just modify your makeSlot(at:) method so the SKSpriteNode creation looks like this:

Then add this to the code where you create the balls:

We don't need to name the bouncers, because we don't actually care when their collisions happen.

Now comes the tricky part, which is setting up our scene to be the contact delegate of the physics world. The initial change is easy: we just need to conform to the SKPhysicsContactDelegate protocol then assign the physics world's contactDelegate property to be our scene. But by default, you still won't get notified when things collide.

What we need to do is change the contactTestBitMask property of our physics objects, which sets the contact notifications we want to receive. This needs to introduce a whole new concept – bitmasks – and really it doesn't matter at this point, so we're going to take a shortcut for now, then return to it in a later project.

Let's set up all the contact delegates and bitmasks now. First, make your class conform to the SKPhysicsContactDelegate protocol by modifying its definition to this:

Then assign the current scene to be the physics world's contact delegate by putting this line of code in didMove(to:), just below where we set the scene's physics body:

Now for our shortcut: we're going to tell all the ball nodes to set their contactTestBitMask property to be equal to their collisionBitMask. Two bitmasks, with confusingly similar names but quite different jobs.

The collisionBitMask bitmask means 'which nodes should I bump into?' By default, it's set to everything, which is why our ball are already hitting each other and the bouncers. The contactTestBitMask bitmask means 'which collisions do you want to know about?' and by default it's set to nothing. So by setting contactTestBitMask to the value of collisionBitMask we're saying, 'tell me about every collision.'

This isn't particularly efficient in complicated games, but it will make no difference at all in this current project. And, like I said, we'll return to this in a later project to explain more. Until then, add this line just before you set each ball's restitution property:

That’s the only change required for us to detect collisions, so now it's time to write the code that does the hard work.

But first, a little explanation: when contact between two physics bodies occurs, we don't know what order it will come in. That is, did the ball hit the slot, did the slot hit the ball, or did both happen? I know this sounds like pointless philosophy, but it's important because we need to know which one is the ball!

Before looking at the actual contact method, I want to look at two other methods first, because this is our ultimate goal. The first one, collisionBetween() will be called when a ball collides with something else. The second one, destroy() is going to be called when we're finished with the ball and want to get rid of it.

Put these new methods into to your code:

The removeFromParent() method removes a node from your node tree. Or, in plain English, it removes the node from your game.

You might look at that and think it's utterly redundant, because no matter what happens it's effectively the same as writing this:

But trust me on this: we're going to make these methods do more shortly, so get it right now and it will save refactoring later.

With those two in place, our contact checking method almost writes itself. We'll get told which two bodies collided, and the contact method needs to determine which one is the ball so that it can call collisionBetween() with the correct parameters. This is as simple as checking the names of both properties to see which is the ball, so here's the new method to do contact checking:

If you're particularly observant, you may have noticed that we don't have a special case in there for when both bodies are balls – i.e., if one ball collides with another. This is because our collisionBetween() method will ignore that particular case, because it triggers code only if the other node is named 'good' or 'bad'.

Run the game now and you'll start to see things coming together: you can drop balls on the bouncers and they will bounce, but if they touch one of the good or bad slots the balls will be destroyed. It works, but it's boring. Players want to score points so they feel like they achieved something, even if that 'something' is just nudging up a number on a CPU.

Before I move on, I want to return to my philosophical question from earlier: “did the ball hit the slot, did the slot hit the ball, or did both happen?” That last case won’t happen all the time, but it will happen sometimes, and it’s important to take it into account.

If SpriteKit reports a collision twice – i.e. “ball hit slot and slot hit ball” – then we have a problem. Look at this line of code:

And now this line of code:

The first time that code runs, we force unwrap both nodes and remove the ball – so far so good. The second time that code runs (for the other half of the same collision), our problem strikes: we try to force unwrap something we already removed, and our game will crash.

To solve this, we’re going to rewrite the didBegin() method to be clearer and safer: we’ll use guard to ensure both bodyA and bodyB have nodes attached. If either of them don’t then this is a ghost collision and we can bail out immediately.

It takes a little more explanation and a little more code, but the result is safer – and that’s always worth striving for!

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Overview:

Interface and cable issues can be due to collisions, errors, duplex mismatch or speed mismatch

Study Notes:

Collisions

• In full-duplex Ethernet, collision detection is disabled
• A collision is the mechanism used by Ethernet to control access and allocate shared bandwidth among stations that want to transmit at the same time on a shared medium.
• Because the medium is shared, a mechanism must exist where two stations can detect that they want to transmit at the same time. This mechanism is collision detection.
• Ethernet uses CSMA/CD (Carrier Sense Multiple Access/Collision Detect) as its collision detection method. Here is a simplified example of Ethernet operation:
  1. Station A wishes to send a frame. First, it checks if the medium is available (Carrier Sense). If it isn't, it waits until the current sender on the medium has finished.
  2. Suppose Station A believes the medium is available and attempts to send a frame. Because the medium is shared (Multiple Access), other senders might also attempt to send at the same time. At this point, Station B tries to send a frame at the same time as Station A.
  3. Shortly after, Station A and Station B realize that there is another device attempting to send a frame (Collision Detect). Each station waits for a random amount of time before sending again. The time after the collision is divided into time slots; Station A and Station B each pick a random slot for attempting a retransmission.
  4. Should Station A and Station B attempt to retransmit in the same slot, they extend the number of slots. Each station then picks a new slot, thereby decreasing the probability of retransmitting in the same slot.
• Collisions are a way to distribute the traffic load over time by arbitrating access to the shared medium. Collisions are not bad; they are essential to correct Ethernet operation.
• The deferred counter counts the number of times the interface has tried to send a frame, but found the carrier busy at the first attempt (Carrier Sense). This does not constitute a problem, and is part of normal Ethernet operation.
• An increasing collision rate (number of packets output divided by the number of collisions) does not indicate a problem: it is merely an indication of a higher offered load to the network. An example of this could be because another station was added to the network.
• There is no set limit for 'how many collisions are bad' or a maximum collision rate.
• The collisions counter does not provide a very useful statistic to analyze network performance or problems.
• The station that reports a late collision merely indicates the problem; it is generally not the cause of the problem. Possible causes are usually incorrect cabling or a non-compliant number of hubs in the network. Bad network interface cards (NICs) can also cause late collisions.
• Excessive collisions indicate a problem. Common causes are devices connected as full-duplex on a shared Ethernet, broken NICs, or simply too many stations on the shared medium. The excessive collisions can be resolved by hardcoding speed and duplex.

Use this command to view collisions:

• CRC - a high number of CRCs is usually the result of collisions or a station transmitting bad data.
• frame - shows the number of packets received incorrectly having a CRC error and a noninteger number of octets. On a LAN, this is usually the result of collisions or a malfunctioning Ethernet device.
• collisions - gives the number of messages retransmitted due to an Ethernet collision. This is usually the result of an overextended LAN (Ethernet or transceiver cable too long, more than two repeaters between stations, or too many cascaded multiport transceivers). A packet that collides is counted only once in output packets.

Slot Commissions

Errors

Use this command to view errors:

Slot Collisions Games

• Ethernet is up, down or administratively down
• packets input gives the total number of error-free packets received by the system
• bytes input gives the total number of bytes, including data and MAC encapsulation, in the error-free packets received by the system
• input error includes runts, giants, no buffer, CRC, frame, overrun, and ignored counts. Other input-related errors can also cause the input error count to be increased, and some datagrams may have more than one error; therefore, this sum may not balance with the sum of enumerated input error counts.
• frame shows the number of packets received incorrectly having a CRC error and a noninteger number of octets. On a LAN, this is usually the result of collisions or a malfunctioning Ethernet device
• input packets with dribble condition detected gives the dribble bit error, which indicates that a frame is slightly too long. This frame error counter is incremented just for informational purposes; the router accepts the frame
• output errors gives the sum of all errors that prevented the final transmission of datagrams out of the interface being examined. Note that this may not balance with the sum of the enumerated output errors because some datagrams may have more than one error, and others may have errors that do not fall into any of the specifically tabulated categories
• restarts gives the number of times a Type 2 Ethernet controller was restarted because of errors.

Duplex and Speed

• duplex and speed should match on both ends or else you will have problems
• traffic can still pass with mismatched duplex and speed, but you will experience retransmissions and reduced throughput
• to verify duplex and speed run the command

Slot Collisions Meaning

• If you want to hard code the speed and duplex on a switch that runs Cisco IOS Software (turn off auto-negotiation), issue the speed and duplex commands underneath the specific interface.
• Duplex is subservient to speed in the sense that if speed is set to auto, then the duplex cannot be manually set.
• You might see cyclic redundancy check (CRC) error messages when both the speed and duplex settings are hardcoded on the two devices. This might be because any one of the devices runs an earlier version of Cisco IOS.
• You can upgrade the Cisco IOS or set the speed and duplex to auto on both devices in order to resolve this.