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| author | Sven Eisenhauer <sven@sven-eisenhauer.net> | 2023-11-10 15:11:48 +0100 |
|---|---|---|
| committer | Sven Eisenhauer <sven@sven-eisenhauer.net> | 2023-11-10 15:11:48 +0100 |
| commit | 33613a85afc4b1481367fbe92a17ee59c240250b (patch) | |
| tree | 670b842326116b376b505ec2263878912fca97e2 /Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html | |
| download | Studium-master.tar.gz Studium-master.tar.bz2 | |
Diffstat (limited to 'Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html')
| -rw-r--r-- | Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html | 117 |
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diff --git a/Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html b/Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html new file mode 100644 index 0000000..24b126f --- /dev/null +++ b/Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html @@ -0,0 +1,117 @@ +<html>
+
+<head>
+<meta http-equiv="Content-Language" content="en-us">
+<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
+<title>Content</title>
+<base target="_self">
+</head>
+
+<body background="../../images/bg.gif" topmargin="0" leftmargin="0" marginheight="0" marginwidth="0" onLoad="window.focus();" link="#808080" vlink="#808080" alink="#808080">
+
+<table border="0" cellpadding="0" cellspacing="0" width="100%">
+ <tr>
+ <td bgcolor="#336666" width="18" valign="top">
+ <img border="0" src="../../images/content_lines.gif" width="16" height="25">
+ <img border="0" src="../../images/transdot.gif" width="2" height="1"></td>
+ <td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">5</font></b><font face="Arial" size="2" color="#FFFFFF"><b>.1</b></font></td>
+ <td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#336666" width="100%"><b style="mso-bidi-font-weight:normal"><span style="font-family:Arial;mso-bidi-font-family:"Times New Roman""><o:p>
+ <font size="2" color="#FFFFFF">Frame Relay Concepts</font>
+ </o:p>
+ <o:p>
+ </o:p></span></b></td>
+ <td width="9" bgcolor="#336666"> </td>
+ </tr>
+ <tr>
+ <td bgcolor="#669999" height="25" width="18"> </td>
+ <td bgcolor="#669999" height="25"><b>
+ <font face="Arial" size="2" color="#FFFFFF">5.1.4</font></b></td>
+ <td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#669999" height="25" width="100%"><strong>
+ <font face="Arial" size="2" color="#FFFFFF">Frame Relay bandwidth and flow
+ control</font></strong></td>
+ <td bgcolor="#669999" height="25" width="9"> </td>
+ </tr></table>
+
+<table border="0" cellpadding="0" cellspacing="0" width="95%" bordercolor="#111111">
+ <tr>
+ <td width="15"></td>
+ <td>
+
+ <font face="Arial" size="2">The serial connection or access link to
+ the Frame Relay network is normally a leased line. The speed of the
+ line is the access speed or port speed. Port speeds are typically
+ between 64 kbps and 4 Mbps. Some providers offer speeds up to
+ 45 Mbps.</font><p><font face="Arial" size="2">Usually there are
+ several PVCs operating on the access link with each VC having
+ dedicated bandwidth availability. This is called the committed
+ information rate (CIR). The CIR is the rate at which the service
+ provider agrees to accept bits on the VC. </font></p>
+ <p><font face="Arial" size="2">Individual CIRs are normally less than
+ the port speed. However, the sum of the CIRs will normally be greater
+ than the port speed. Sometimes this is a factor of 2 or 3. This statistical
+ multiplexing accomodates the bursty nature of computer communications
+ since channels are unlikely to be at their maximum data rate
+ simultaneously.</font></p>
+ <p><font face="Arial" size="2">While a frame is being transmitted,
+ each bit will be sent at the port speed. For this reason, there must
+ be a gap between frames on a VC if the average bit rate is to be the
+ CIR.</font></p>
+ <p><font face="Arial" size="2">The switch will accept frames from the
+ DTE at rates in excess of the CIR. This effectively provides each
+ channel with bandwidth on demand up to a maximum of the port speed.
+ Some service providers impose a VC maximum that is less than the port
+ speed. The difference between the CIR and the maximum, whether the
+ maximum is port speed or lower, is called the Excess Information Rate
+ (EIR).</font></p>
+ <p><font face="Arial" size="2">The time interval over which the rates
+ are calculated is called the committed time (T<sub>c</sub>).
+ The number of committed bits in T<sub>c</sub> is the committed burst (B<sub>c</sub>). The
+ extra number of bits above the committed burst, up to the maximum
+ speed of the access link, is the excess burst (B<sub>e</sub>).</font></p>
+ <p><font face="Arial" size="2">Although the switch accepts frames in
+ excess of the CIR, each excess frame is marked at the switch by
+ setting the Discard Eligibility (DE) bit in the address field.
+ <img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12">
+ <img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12">
+ <img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></font></p>
+ <p><font face="Arial" size="2">The switch maintains a bit counter for each VC. An incoming frame is
+ marked DE if it puts the counter over B<sub>c</sub>. An incoming frame is
+ discarded if it pushes the counter over B<sub>c</sub> + B<sub>e</sub>. At the end of each T<sub>c</sub>
+ seconds the counter is reduced by B<sub>c</sub>. The counter may not
+ be negative, so idle time cannot be saved up.</font></p>
+ <p><font face="Arial" size="2">Frames arriving at a switch are queued
+ or buffered prior to forwarding. As in any queuing system, it is
+ possible that there will be an excessive buildup of frames at a
+ switch. This causes delays. Delays lead to unnecessary retransmissions
+ that occur when higher-level protocols receive no acknowledgment
+ within a set time. In severe cases this can cause a serious drop in
+ network throughput.</font></p>
+ <p><font face="Arial" size="2">To avoid this problem, frame relay
+ switches incorporate a policy of dropping frames from a queue to keep
+ the queues short. Frames with their DE bit set will be dropped first.</font></p>
+ <p><font face="Arial" size="2">When a switch sees its queue
+ increasing, it tries to reduce the flow of frames to it. It does this
+ by notifying DTEs of the problem by setting the Explicit Congestion
+ Notification (ECN) bits in the frame address field.</font></p>
+ <p><font face="Arial" size="2">The Forward ECN (FECN) bit is set on
+ every frame that the switch receives on the congested link. The
+ Backward ECN (BECN) bit is set on every frame that the switch places
+ onto the congested link. DTEs receiving frames with the ECN bits set are
+ expected to try to reduce the flow of frames until the congestion
+ clears. <img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></font></p>
+ <p><font face="Arial" size="2">If the congestion occurs on an internal
+ trunk, DTEs may receive notification even though they are not the
+ cause of the congestion.</font></p>
+ <p><font face="Arial" size="2">The DE, FECN and BECN bits are part of
+ the address field in the LAPF frame.
+ <img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></font></p>
+ <p><font face="Arial" size="2"> </font>
+ </td>
+ </tr>
+ </table>
+
+</body>
+
+</html>
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