active barcode excel 2013 download Figure 6-19: The PPP frame format in Software

Maker EAN-13 Supplement 5 in Software Figure 6-19: The PPP frame format

Figure 6-19: The PPP frame format
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This frame contains both useful and redundant fields The redundant fields are the address and control fields, which always carry the same entries The address field contains an all-ones byte, the layer 2 broadcast address (Because PPP only connects two entities together, specific layer 2 addressing is not necessary and individual layer 2 addresses are not assigned) The control field always contains the binary value 00000011, which defines the type of communication used The useful fields are the flag, to indicate the start of a frame, the protocol field, which identifies the layer 3 protocol encapsulated in the frame, and the Frame Check Sequence to ensure no errors were introduced into the frame during transmission
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asynchronous communications, some of which can be considered more art than science We will examine the most common commands and then consider how these commands can be put together in a sample configuration The first command, as always when specifying a link layer protocol, is to define the encapsulation for the chosen interface This is achieved with the following commands: Router1(config)#interface async 1 Router1(config-int)#encapsulation ppp If we are specifying PPP encapsulation, this implies that the asynchronous port will be used for a network connection We therefore should place the port in dedicated mode The mode choice for an async port is either dedicated or interactive Placing the port in interactive mode presents the user with a command prompt and allows the user to manually input user name, passwords, and other connection-related information For security reasons, I prefer to keep the async mode as dedicated, which is achieved with the following command: Router1(config-int)#async mode dedicated Next you will want to enable Van Jacobsen header compression In reality, compressing headers makes comparatively little difference in link bandwidth consumed by the protocol, but with asynchronous communications you should do everything possible to improve throughput Header compression is turned on by default, but it does not hurt to enable it in case it had been previously disabled This is achieved in interface configuration mode: Router1(config-int)#ip tcp header-compression on The next issue is to assign IP addresses to the ports and to computers connecting to those ports You have a choice, either to hard-code IP addresses into computers connecting to the async ports, or have the address assigned to the computer when it connects to the async port If you choose the first option, you must ensure that the IP address assigned to the computer dialing in is in the same subnet as the address range assigned to the async ports themselves My preference is to have the IP address assigned to the computer by the async port upon connection This makes life simpler and does not restrict a computer to being able to dial in to only one location To have the async interface assign an IP address to a computer when it connects, three separate configurations need to take place First the async port must be given an unnumbered address (IP unnumbered is covered more fully in Chap 7) Next, the async port must be configured to deliver a specific IP address to the connecting computer Finally, the connecting computer must have no IP address configured The two entries in the router port configuration, to define IP unnumbered and 193111 as the address assigned to a connecting computer, are as follows: Router1(config)#interface async1 Router1(config-int)#ip unnumbered ethernet 0 Router1(config-int)#async default ip address 193111
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Next we discuss Asynchronous Control Character Maps (ACCMs) Flow control between asynchronous devices can either be of the hardware or the software variety Hardware flow control relies on pin signaling, such as the state of the Data Set Ready (DSR) or Data Terminal Ready (DTR) pins to stop and start transmission Software flow control uses special characters transmitted between asynchronous devices to stop and start transmission When relying on characters transmitted between devices to signal modem actions, there is always a danger that strings within the data transmitted will match these special command strings and be inappropriately interpreted by the modems An ACCM can be configured to tell the port to ignore specified control characters within the data stream The value of ACCM that tells an async port to ignore XON/XOFF (software flow control) characters in the data transmitted is A0000 in hexadecimal This is the default value; if a TCP stack on the computer connecting to the async port does not support ACCM negotiation, however, the port will be forced to use an ACCM of FFFFFFFF In this case, it is useful to manually set the ACCM with the following command: Router1(config-int)#ppp accm match 000a0000 Next, we want to enable CHAP authentication on the link This is done in two stages; first the CHAP user name and password are set in global configuration, then CHAP is enabled on the desired interface This is achieved through the following commands: Router1(config)#username chris password lewis Router1(config)#interface async 1 Router1(config-int)#ppp authentication chap If an asynchronous router is being used to route traffic from a LAN to a dial-up or other slow link, it can be desirable to slow down the speed at which packets are switched from one interface to another If packets are switched from an Ethernet port running at 10 Mbps directly to an async port running at 192 kbps, the async port can quickly get overwhelmed By entering the no ip route-cache command as shown below, the packets are switched at a slower speed Effectively, this command, entered for each async interface in use, stops the router from caching destination addresses and forces a table lookup every time a packet needs to be routed Router1(config-int)#no ip route-cache One aspect of asynchronous communication that causes endless confusion is the DTE rate configured for a port and its meaning in terms of data throughput on an async line The receive and transmit DTE rate of async port 1 is set by the following commands, to 38,400 bits per second Router1(config)#line 1 Router1(config-line)#rxpseed 38400 Router1(config-line)#txspeed 38400
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each packet is sent from the router port to the modem If the modem can only transfer data across a dial-up link at 144 kbps, it will use its flow control procedures to stop more packets from coming out of the router port than it can safely transfer across the link Thus, over the course of 10 or 20 seconds, the amount of data transferred between the router port and the modem port will not be greater than an average of 144 kbps; however, each packet that the router does transmit will be received at a speed of 384 kbps from the device sending async characters These days most modems employ V42bis compression, which will allow a modem to sustain effective throughputs that are higher than the modem-to-modem connection rate V42 compression is generally quoted at providing up to four times the data throughput that the connection rate would suggest For example, with four-to-one compression, a 144 kbps link will support 576 kbps throughput The effective compression ratio is determined by how compressible the data being transported is Compressible data includes things such as ASCII text, although binary file transfers are not normally very compressible In brief, V42bis compression looks for common sequences of bits and the modems agree to assign special characters to represent these often-repeated character sequences By transmitting a special character, the modem may have to transfer only 1 byte of data, rather than the 4 bytes that both modems know it represents Once a receiving modem receives a special character, it will send out the full associated character string on its DTE port Many newcomers to the world of asynchronous communications ask why, even if the DTE rate is set to 115,200 bps, communications across an async link are so slow, often slower than an ISDN link operating at 64 kbps The answer is that you very rarely get sustained throughput of 115,200 on an async link While each packet may be transferred between the router and modem at 115,200 bps, the modem flow control will stop the router port from transmitting continuously at that speed 8 gets into troubleshooting serial communication problems in more depth, but two configuration commands that help asynchronous communications are worth considering here The first is the hold-queue command The hold queue of each interface has a specified size, which is the number of packets waiting to be transmitted that it can hold before the interface starts dropping packets This value can be set for both incoming and outgoing packets For asynchronous interfaces, it is worthwhile increasing the sizes of both the incoming and outgoing hold queues, which in the following example increases both values to 90 Router1(config-int)#hold-queue 90 in Router1(config-int)#hold-queue 90 out If an interface (Async 1, for example) is exceeding its hold queue limits, an increased number of drops will be seen in the show interface async 1 command Drops also can increase if the router buffers for given packet sizes are overflowing The second command we will overview here is the one that sets the number of packet buffers available in the router To view the state of packet buffer use, enter the show buffers command The output will show you the number of small, medium, large, very large, and huge buffers used and available, and the number of occasions on which buffers of a type were needed but a packet was dropped because none were available (shown as failures)
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A point to note is that packets can be dropped even if the maximum number of buffers has not been exceeded This phenomenon occurs if several packets of one particular size arrive at the router very quickly and the router cannot create buffers fast enough If you suspect this may be happening, you can set the number of buffers of a given size to be permanently available The following is an extract from a router configuration that has had its medium-size buffer allocation altered from the default ! buffers medium initial 40 buffers medium min-free 20 buffers medium permanent 50 buffers medium max-free 40 The first entry defines the number of temporary buffers that are to be available after a reload, which is useful for high-traffic environments The second statement forces the router to try to always have 20 medium buffers free, and if a traffic surge reduces the number of free medium buffers to below 20, the router automatically will try to create more The third entry defines 60 permanent buffers, which once created are not retrieved by the IOS for reuse of their memory allocation Finally, the max-free 40 entry ensures that memory is not wasted on unused buffers by returning memory used by more than 40 free medium buffers to the router's general memory pool Synchronous PPP Configurations If a WAN is being built based on point-to-point links between router serial ports, the popular choices for the link-level encapsulation are the default Cisco version of HDLC and PPP operating in synchronous mode PPP is a more modern protocol and offers better link-level authentication than Cisco's HDLC, but there is one compelling reason to build your network based on the Cisco HDLC Consider Fig 6-20
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