MDS213 Datasheet, PDF (60/120 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps + 1Gbps Ethernet Switch

MDS213
Data Sheet
Byte ^{3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 9 8 7 6 5 4 3 2 1 0}
1098765432109876543 210
0
IP1
4
C
P
U
IP0
VLAN Index
VLAN ID
IP3
IP2
8
Next Handle
12
VLAN ID:The VLAN ID this IP Multicast group is located in.
IP[3:0]:IP Multicast Address
VLAN Index:Internal VLAN Index used to identify this IP Multicast group
CPU:1: Switch CPU is part of this IP Multicast group
Next Handle: Pointer to the next entry in a hashed link list.
16.0 Quality of Service (QOS)
Quality of Service (QoS) provides the capability to reserve bandwidth throughout the network. This is particularly
useful for sending voice or video over the switched network. In a switched Ethernet environment, this is only
possible with Resource Reservation Protocol (RSVP), a Layer 3 protocol. In a Layer 2 switch, QoS, referred as
Class of Service (CoS) by the IEEE 802.1Q standard, provides the capability to prioritize certain tasks on the
network. This is done at the application level, where applications can set the priority when the frame is created. The
MDS213 classifying Ethernet frames according to their IEEE 802.1p/Q VLAN priorities. There are three bits in the
VLAN ID reserved to designate the priority of a packet.
Each port stores its transmission jobs into four transmission scheduling queues, one for each priority. Before
transmitting, a port selects a queue from which a transmission job is read. The transmission job points to a frame
stored in memory that is fetched and transmitted. The four queues, representing four classes of traffic, are selected
using a weighted round robin (WRR) strategy. The relative service rates among these queues are programmable
such that bandwidth can be allocated according to classes. This ensures that critical applications get a fair share of
bandwidth, even when the network is overloaded.
The Search Engine recognizes the IEEE 802.1p priority tag and classifies each incoming frame into four internal
priority classes: P0, P1, P2 and P3, in decreasing priority. Since the IEEE 802.1p/Q allows up to eight priorities, a
programmable mapping allows the user to map the 802.1p priority to the internal priority tag via register AVTC.
16.1 Weighted Round Robin Transmission Strategy
Frames of four different priorities are transmitted according to a weighed round robin (WRR) strategy. The WRR is
a modified form of the fair round-robin strategy, in which the server visits the queues in turn. In a fair round-robin
strategy, the server treats all queues equally and visits them with identical frequency. In a WRR, the queues are
weighted, i.e., one queue may be visited more frequently than another. These weighs are programmable via
register AXSC, in which the service rate ratio between two adjacent classes of traffic is set.
In register AXSC, setting QSW0=2, QSW1=QSW2=1 gives the service ratio 8:2:1:1, which is a good start for most
LAN switches. This ratio allocates 67% = 8/12 of bandwidth to P0, 16% = 2/12 of bandwidth to P1, and P2 and P3
each receives 8.3% = 1/12 of bandwidth, assuming all frames have identical frame length.
16.2 Buffer Management Functions
The MDS213 stores frame data in frame buffers. The number of frame buffers in a system is the maximum number
of frames a device can store. When all frame buffers are used, incoming frames cannot enter the memory and are
discarded. Without buffer management, a congested port causes a backlog of frames that eventually occupy all
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Zarlink Semiconductor Inc.