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Switch Fabric Architectures
Vahid Tabatabaee
Fall 2006
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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References
 Light Reading Report on Switch Fabrics, available online at:
http://www.lightreading.com/document.asp?doc_id=25989
 Title: Network Processors Architectures, Protocols, and Platforms
Author: Panos C. Lekkas
Publisher: McGraw-Hill
 Multi-Gigabit Serdes: The Cornerstome of High Speed Serial
Interconnects, Genesys Logic America, Inc.
 C. Minkenberg, R. P. Luijten, F. Abel, W. Denzel, M. Gusat, Current
issues in packet switch design, ACM SIGCOMM Computer
Communication Review, Volume 33 , Issue 1 (January 2003)
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Two Chip Architecture
 The switch interface (queue manager) chip resides on
the line card
Interface with the traffic manager and/or Network
Processor.
Transmit and receives cells from the switch chip
through the backplane.
Queues and schedules cells in the ingress and
egress side.
 The switching element chip resides on the switch card.
Transmit and switch cells between the line cards.
Temporary buffers cells.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Two Chip Architecture
 The switching element can have buffer (Shared memory, Buffered
Crossbar) or buffer-less (cross-bar)
 Data, flow control and queue info. is sent between chips.
 Generally flow-control from the chip with lower memory to the one with
larger memory is more critical.
Data
From
Traffic Manger
Data
Ingress
Queues
Scheduler
Scheduler
Serdes
Serdes
To
Traffic Manger
Egress
Queues
Data +
Flow Control
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
NxN Switch
(Buffered or
Bufferless)
Arbiter and
Conroller
Data +
Flow Control
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Back Plane
Interface Chip Architecture
Back Plane
TDM 1
Data
Class 1
Data
Class 1
Class 2
TDM T
Class 2
From
Traffic Manger
Class C
Scheduler
Serdes
Serdes
Virtual Output N
Queues
Virtual Output 1
Queues
Class C
Class 1
Multicast Queues
Class M
Class 1
Class 2
To
Traffic Manger
Scheduler
Data +
Flow Control
Class C
Data +
Flow Control
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
Back Plane
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Backpressure Flow Control
 Backpressure flow control is to avoid buffer flow in receiving chips.
 Suppose that the receiving chip does not have any more memory
space.
 It sends the backpressure flow control message to the transmitting
chip to avoid sending anymore cells.
 If there are multiple queues it will desirable to specify which queue
is full.
 We can start sending the backpressure signal before queue is full
to lower the transmission rate (rate-based flow control)
 As the queue gets fuller we send the backpressure signal more
frequently to decrease the transmission rate more and more.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Backpressure Flow Control
 We can have different threshold values for flow-control of different
queues.
 In the switch fabric, multiple line-card are transmitting to a
switching chip.
 It is desirable to back-pressure each one separately.
 Therefore, we need per-input, per-class queueing in the switch
chip.
 For a 32x32 system with 8 classes of service
 How many ingress queues do we need in the interface chip?
 How many do we need in the switching chip?
 Do we need to assign higher threshold value to the high priority
queue or to the low priority queue?
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Latency, Flow Control issues
 The aggregate throughput of the routers are increasing
 The trend is usually increasing number of ports rather than port speed.
 This means that we need to increase line-cards
 The switch fabric can no longer be built in a single rack
 Racks can be tens of meters apart.
 We need to spend more power for signal transmission.
 This yields to higher RTT inside a switch
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
Source: Ref. 4 (Current Issues in Packet Switch Design
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Latency, Flow Control issues
 The latency is time in flight (backplane or cable) and the
pipeline logic and serdes delay.
 The increased RTT means more packets in flight that
should be taken into account for buffering requirements.
Source: Ref. 4 (Current Issues in Packet Switch Design
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Backpressure Flow Control and Latency






We have to be careful about latency in setting the flow control thresholds.
After sending the flow control signal, we can still get up to 2K cells.
Therefore, the threshold could not be larger than buffer size - 2K.
The problem is more serious in multi-rack systems.
We have to take the inside chip hardware delay into account.
This could be a serious problem for buffered-crossbar architectures since they have
limited memory per-queue.
K cells
Line
Interface
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
Switch
Chip
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Shared Memory Switch Fabric
 The switching chip has a large size memory.
 The line interface schedule one of the queues that are not backpressured and send the cell to the swicth chip.
 Switch chip buffers the incoming cells into the memory.
 Since every line port can send a cell we have to write N cells in
every time slot.
 There are N schedulers working in parallel in the switch chip (one
per output port).
 Ideally we need to have 1 queue for each class, input and output
port.
 Practically in many systems we only have per output and class
queue.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Shared Memory Switch Chip Architecture
From Input 1
Shared
Memory
Data
S1
Output 1
Scheduler
ClassC
Class2
Class1
ClassC
Class2
Serdes
Class1
From Input N
SN
Output N
Scheduler
Data +
Flow Control
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Shared Memory Advantages
 The scheduling is always a many-to-one
problem.
 It can efficiently support multi-cast.
 The memory is shared between queues.
 We can achieve high throughput in this system,
since there is no contention as long as we have
enough memory.
 There is no need for transmission
synchronization between ingress ports.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Disadvantages of Shared Memory
 Scalability problems:
Number of queues in the switch chip is NxNxC
N writes and N reads from the memory in one time
slot.
N schedulers in the same chip.
 It is hard to design a cell slice, shared memory switch
fabric (why?)
 The only appropriate redundancy model is 1+1 (why?)
 The flow control mechanism should be sophisticated.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Buffered Crossbar Architectures
 For each input-output pair a separate memory
is used.
 Instead of one big memory we have N2 small
memories.
 Therefore, at every time slot we are writing and
reading at most one cell from each memory.
 There is no contention between memories.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Buffered Crossbar Switch Chip
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Switch Fabric Architectures
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Advantages of Buffered-Crossbar
 We do not need memory speed-up since every memory should
work at line speed.
 There is no contention between input and output ports since each
one is using separate memories.
 We have distinct queues for input-output connections that is
necessary for providing differentiated services between queues.
 We only need to perform many-to-one scheduling.
 Theoretically, we can achieve 100% throughput.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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Disadvantages of Buffered Crossbar
 We need NxNxC queues in a single chip.
 We need NxN distinct memories in a single chip.
 For a 32 port switch, we need 1024 memories.
 Depth of the memories can not be so large.
 This results in Flow Control Problems.
 In order to control number of queues we have to limit number of
Classes.
 We need to take byte-slice approach for speed-up.
 This means we have to synchronize switch chips.
 It seems that 1+1 is the only appropriate redundancy model.
ENTS689L: Packet Processing and Switching
Switch Fabric Architectures
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