1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
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>
|
