From 33613a85afc4b1481367fbe92a17ee59c240250b Mon Sep 17 00:00:00 2001 From: Sven Eisenhauer Date: Fri, 10 Nov 2023 15:11:48 +0100 Subject: add new repo --- Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/buttons.html | 110 +++++++++++++++++++ Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html | 117 +++++++++++++++++++++ Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/index.html | 29 +++++ Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/media.html | 21 ++++ 4 files changed, 277 insertions(+) create mode 100644 Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/buttons.html create mode 100644 Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/content.html create mode 100644 Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/index.html create mode 100644 Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/media.html (limited to 'Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4') diff --git a/Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/buttons.html b/Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/buttons.html new file mode 100644 index 0000000..f6cd1ff --- /dev/null +++ b/Bachelor/CCNA4/en_CCNA4_v30/ch5/5_1_4/buttons.html @@ -0,0 +1,110 @@ + + + + + + + + + + + +New Page 1 + + + + + + + + + + + + + + + + + + + + + + + + +
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+ + 5.1 + Frame Relay Concepts + + +   
  + 5.1.4 + Frame Relay bandwidth and flow + control 
+ + + + + + +
+ + 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.

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.

+

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.

+

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.

+

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).

+

The time interval over which the rates + are calculated is called the committed time (Tc). + The number of committed bits in Tc is the committed burst (Bc). The + extra number of bits above the committed burst, up to the maximum + speed of the access link, is the excess burst (Be).

+

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. + + +

+

The switch maintains a bit counter for each VC. An incoming frame is + marked DE if it puts the counter over Bc. An incoming frame is + discarded if it pushes the counter over Bc + Be. At the end of each Tc + seconds the counter is reduced by Bc. The counter may not + be negative, so idle time cannot be saved up.

+

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.

+

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.

+

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.

+

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.

+

If the congestion occurs on an internal + trunk, DTEs may receive notification even though they are not the + cause of the congestion.

+

The DE, FECN and BECN bits are part of + the address field in the LAPF frame. +

+

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