Advanced Computer Architecture - Lecture 43: Networks and clusters

CS 704
Advanced Computer Architecture

Lecture 43
Networks and Clusters
(Internetworks and Clusters)

Prof. Dr. M. Ashraf Chughtai


Today’s Topics
Recap:
Internetworks
Cluster
Case Studies
Summary

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Recap:
In our last two lectures on Networks and
Cluster we discussed:
 The formation of generic interconnection
networks and their categorization, the
networks communication model, performance,

media, software, protocols, subnet and
networks topologies
 Here, we noticed that a generic interconnection
network comprises: Computer nodes, H/W and
S/W interface, Links to the interconnection
network and Communication subnet
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Recap
The interconnect communication model
shows that two machines are connected via
two unidirectional wires with a FIFO (queue) at
the end to hold the data
The communication software separates the
header and trailer from the message and
identifies the request, reply, their
acknowledgments and error checking codes
The communication protocols suggest the
sequence of steps to reliable communication
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Recap
We also discussed:
 the properties and performance of interconnect
network media or link – the unshielded twisted
pair (UTP), coaxial cable and fiber optics
 the formation of bus-based and switch-based
communication subnets and introduced the
network topologies
 The bus-based communication subnets share
the common media where arbitration is the
bottleneck
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Recap
 Alternative to sharing media is to use a switch
to provide a dedicated line to all destinations in
order; and facilitates point-to-point
communication much faster than the shared
media
 The switch-based networks are classified as
the centralized and distributed switch networks

Here the routing, to establish interconnection
between two node at a time, depends on the
addressing style: source-based routing and
destination-based routing
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Recap
The performance of a distributed network is
measured in terms of:
– Latency - number of Links between source and
destination
– Bandwidth – number or length of messages
passing per sec.
– Degree - number of links connected to a node
– Diameter - number of nodes between source and
destination; this is in deed the measure of

maximum latency
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Recap
– Bisection - the imaginary line that divides the
interconnect into roughly two equal parts, each
having half the nodes
– Bisection Bandwidth: the volume of
communication allowed between any two
halves of network with equal number of nodes
Last time, we discussed an intermediate class
of network interconnect – Multistage Switch
network
It is built from number of large switch boxes
each containing number of small crossbar
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switches
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Lecture 43 Networks and Clusters (3)


Recap
The performance of Multi-stage switch lies
between performance of non-locking crossbar
and bus-based networks
Following the discussion on centralized switch
topologies we studied the distributed-switch
interconnects; which are categorized as the
fully-connected and partially-connected,
symmetric or asymmetric interconnects

The distributed-switch interconnect topologies,
such as: linear array, ring, 2D mesh/torus and
hypercube were studied
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Recap
We also discussed the relative cost and
performance of these topologies, based on the
bisection bandwidth and number of links for 64
nodes network; which is shown in the
following table
Evaluation
Category

Bus

Ring

2D Torus

Fully
Connected

Performance:

Bisection B/W

1

2

16

1024

5
1

64
128 192

Cost

-Ports/switch
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N/A
3
Total Links
2080

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Today’s Topics: Internetworking
So far we have been talking about the design
styles, topologies and performance of
interconnection networks
Now we are doing to talk about the connection
of two or more interconnection networks,
called Internetworking; the Internet is typical
example of Internetworking
Internetworking deals with the communication
of computers on independent and incompatible
networks reliably and efficiently
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Internetworking
Internetworking relies on the communication
standards to convert information from one kind
of network to another
These standards are composed of hierarchy of
layers, where each layer is responsible for a
portion of overall communication
Each computer, network and switch

implements its layer of standards, called the
Protocol Families or Protocol suites, and
facilitates applications to work with any interconnection
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OSI: 7- Layer Model
The Open Systems Interconnect – OSI
developed a 7-layer model, which describes a
network as the series of layers; with
Application layer at the top (i.e., layer 7) and
Physical layer at the bottom (layer 1) and
presentation, session, transport, network and
data link layers in between the top to bottom
layers, as layer-6 down to layer-2, respectively
The OSI model, layer-7, the Application layer is
used for applications specifically written to run
over the network, e.g., Network File System (NFS) etc.
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OSI Layers
The layer-6, Presentation layer translates from
application to network format and vice versa
The layer-5, Session Layer, establishes
maintains and ends the sessions across the
network
The layer-4, Transport Layer, facilitates
additional connection below the session layer;
the protocol is referred to as the Transmission
Control Protocol - TCP
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OSI Layers
The layer-3, Network Layer, translates the
network address and names to their physical
address; e.g., computer name to Media Access
Control –MAC; the layer-3 protocol is referred
to as Internet Protocol or IP
The layer-2, Data Link layer, turns packets into
raw bits and at the receiving end turns bits into
packets; the example protocol is Ethernet
The layer-1, Physical Layer, transmits raw bitstream over physical cable/media; IEEE 802 is
typical example physical layer protocol

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TCP/IP Families
The protocol family divides the responsibilities
among the layers, with each layer offering
services needed by the layer above
The Transmission Control Protocol/Internet
Protocol - TCP/IP is the most popular
internetworking standard
TCP/IP is the basis of Internet, which connect
the tens of millions of computers around the
world
The protocol at each level is implemented by
adding headers and trailers at the sending ….
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TCP/IP Families
…. layer and removing at the receiving layer

The original message, from the top layer,
includes a header and trailer sent by the lowerlevel protocol
The next-lower protocol in turn adds its own
header (and possibly trailer) to the message
and so on
If the message is too large for a particular
layer, then it is broken into smaller messages;
this division of message and addition of ….
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TCP/IP Families
… header and trailer continues till the message
descends to the physical transmission media
The message is then sent to the destination
Each level of protocol family at the receiving
end, from bottom to the top layer, checks the
message at its level and remove its header and
trailer, and pass it on to the next higher level
The message is rebuilt by putting the pieces
together
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TCP/IP Families
This nesting of protocols layers is referred to
as the Protocol Stack as it reflects the Last-in
First out nature of addition and removal of the
header and trailer
A typical TCP/IP datagram, containing header
and message, is depicted here
Fig. 8.27 pp 835 Text book

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TCP/IP Families
The standard IP and TCP headers are 20 byte
each, stacked as shown
However, the length can optionally be
increased which is specified by the length field
(L)
The length of the whole datagram is identified
by a separate field ‘Length’ in IP header, while
the TCP header includes this information in the

‘sequences number field’
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Internetworking
As the detailed discussion on the TCP / IP is
beyond the scope of this course on Computer
Architecture, therefore
we are leaving this discussion here and are going
to talk about ‘cluster’, the last topic of our study of
the ‘Networks and Cluster’;
rather the last topic of this course on Advance
Computer Architecture
However, the students interested in further study
of Internetworks may consult literature and books
on Computer Networks and Internetworking
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Clusters – System Area Networks

The coordinated use of interconnected
computers in a machine room is referred to as
the cluster or System Area Network
Massively parallel machine providing high
bandwidth can be built from off-the-shelf
components, instead of depending on the
custom machines or networks
A cluster , i.e., a collection or bunch of desktop computer and disk offers low cost
computing infrastructure that could tackle very
large problems and applications, such as: …..
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Clusters

Performance Challenges

databases, file servers, Web servers,
simulation and multiprogramming and batch
processing
The clusters face some performance confront
such as:
 Non-standard connections
 Division of memory
Let us talk about these confronts one by one

Non-Standard Confront: As you know that the
multiprocessors are usually connected ……
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Clusters

Performance Challenges

…… connected memory bus which offers high
bandwidth and low latency; and
Contrary to this, the clusters are connected
using I/O bus of the computer, thus have large
conflicts at high speed
Division of Memory: A large single program
running on a cluster of N machines requires N
independent memory units and N copies of
operating system; on the other hand,
a shared address multiprocessor allows to use
almost all memory in the computer
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Clusters

Performance Advantages

However, contrary to these challenges,
clusters have advantages in respect of
dependability and scalability
The weakness of separate memories for
program size in case of cluster, as discussed
earlier, is indeed a strength in terms of system
availability and expandability (or say the
scalability)
Furthermore, as the cluster consists of
independent computers connected through …
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