394 CAMPUS AND METROPOLITAN AREA NETWORKS MANS providing these capabilities, and is now widely available from network and computer equipment manufacturers.. It was designed to provide a
Trang 121 Campus and Metropolitan Area
Networks (MANs)
Metropolitan area networks (MANs) are network technologies similar in nature to local area networks (LANs), but with the capability to extend the reach of the LAN across whole cities or metropolitan areas, rather than being limited to, say, 100-200 metres of cabling MANS have evolved because of the desire of companies to extend LANs throughout company office buildings spread across a campus or a number of different locations in a particular city They provide for high speed data transport (at over lOOMbit/s) and are ideal for the interconnection of LANs There was some effort to extend MAN capabilities to include the carriage of telephone and video signals as an ‘integrated’ network, but this work has largely been overtaken by ATM
(asynchronous transfer mode), so that the MAN technologies themselves are already obsolescent
We review here, but only briefly, the most important MAN techniques, FDDI (fibre distributed data interface), and SMDS (switched multimegabit digital service) which is based on the DQDB (distributed queue dual bus) technique
21.1 FIBRE DISTRIBUTED DATA INTERFACE
The jibre distributed data interface ( F D D I ) is a 100 Mbit/s token ring network It is
defined in IEEE 802.8 an d IS0 8802.8 FDDI can be used to interconnect LANs over
an area spanning up to 100 km, allowing high speed data transfer Originally conceived
as a high speed link for the needs of broadband terminal devices, F DDI is now per- ceived as the optimum backbone transmission system for campus-wide wiring schemes, especially where network management and fault recovery are required In particular, FDD I became popular in association with the very first optical fibre building cabling schemes, because it provided one of the first means to connect LANs on different floors
of a building or in different buildings on a campus via optical fibre Unfortunately, due
to its expensive nature and the rapid development of ATM (asynchronous transfer
mode, see Chapter 26) as well as alternative building cabling schemes, FDDI has fallen into decline, no longer being recommended or further developed by most LAN and
computer manufacturers
391
Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
Trang 2392 CAMPUS AND METROPOLITAN AREA NETWORKS (MANS)
A second generation version of FDDI, FDDI-2, was developed to include a
capability similar to circuit-switching to allow voice and video to be carried reliably in addition to packet data, but these capabilities were never widely used
The FDDI standard is defined in four parts
0 media access control ( M A C ) , like IEEE 802.3 and 802.5 (see Chapter 19) defines the
rules for token passing and packet framing
e physical layer protocol ( P H Y ) defines the data encoding and decoding
e physical media dependent ( P M D ) defines drivers for the fibre optic components
e station management ( S M T ) defines a multi-layered network management scheme
which controls MAC, PHY and PMD
The ring of an FDD I is composed of dual optical fibres interconnecting all stations The dual ring allows for fault recovery even if a link is broken by reversion to a single
ring, as Figure 21.l(a) shows The fault need only be recognized by the C M T s (connec-
tion management mechanisms) of the station immediately on either side of the break To all other stations the ring will appear still to be in its normal contra-rotating state (Figure 21.l(b))
When configured as a ring, each of the stations is said to be in dual-attached connection
Alternatively, a fibre star connection can be formed using single-attached stations with
a multiport concentrator at the hub (Figure 21.2) Single-attachedstations (SASs) do not
share the same capability for fault recovery as double-attached stations (DASs) on a
dual ring
DAS
'Looped'
[ a ) Failed link-ring conf igured a s single logical loop
( b ) Normal dual contra -
rotating fibre rings
Figure 21.1 The fibre distributed data interface (FDDI) fault recovery mechanism for double
attached stations DAS, double attached station
Trang 3Net work
connect ion
S
A S
multi ort concenl'rator
S A S
Figure 21.2 Star configuration of FDDI SAS, single attached station
Like token ring LANs (IEEE 802.5) and ethernet LANs (IEEE 802.3), FDDI is
essentially only a physical layer (OS1 layer 1) and data-link layer (OS1 layer 2) standard
At layers 3 and above, protocols such as X.25, TCP/IP may be used
FDDI-2, the second generation of FDDI (Figure 21.3) has a maximum ring length of
100 km and a capability to support around 500 stations including telephone and packet data terminals Because of this, it was intended to support entire company telecom- munications requirements ATM, however, has proved a more popular prospect for
Building 1
Public
network
X 2 5 gateway
PA BX
Bridge
A
Building 2
Figure 21.3 The fibre distributed data interface-2 (FDDI-2) AU, access unit
Trang 4394 CAMPUS AND METROPOLITAN AREA NETWORKS (MANS)
providing these capabilities, and is now widely available from network and computer equipment manufacturers
The FDDI-2 ring is controlled by one of the stations, called the cycle master The
cycle master maintains a rigid structure of cycles (which are like packets or data slots)
on the ring Within each cycle a certain bandwidth is reserved for circuit-switched traffic (e.g voice and data) This guarantees bandwidth for established connections and ensures adequate delay performance Remaining bandwidth within the cycle is available for packet data use
The voice and video carriage capability of FDDI-2 is possible because of its inter-
working with the integrated voice data (ZVD) LAN standard defined in IEEE 802.9
21.2 SWITCHED MULTIMEGABIT DIGITAL SERVICE (SMDS)
SMDS (switched multimegabit digital service) networks conform to IEEE 802.6 and use
a protocol called distributed queue dual bus ( D Q D B ) DQDB was co-developed by
Telecom Australia, the University of Western Australia and their joint company, QPSX Communications Limited It was designed to provide a basis for initial broadband
metropolitan area interconnection of networks, but also give a possible migration path
to B-ISDN (Chapter 25), for which it is now an optional access protocol As a public
data communication service, the switched multimegabit digital service ( S M D S ) became
available in the United States in 1991
The DQDB protocol uses two slotted buses of bitrates up to 155 Mbit/s to transport
segments of information between communicating broadband devices Segments are
48 byte frames of user data information
Figure 21.4 illustrates the structure of a network using the DQDB protocol Two unidirectional high speed buses run out from master and slave frame generators at
opposite ends of the ribbon topology Each of the devices (nodes) connected to the
network are connected to both buses to send and receive data
The role of the frame generators is to structure the bit stream carried along the buses into 53 byte slots These slots are filled by nodes wishing to send user information and
unidirectional bus A
W
master
frame
generator node 1 node2 node 3 node4 node 5 slave
frame generator
4
unidirectional bus B
Figure 21.4 Bus structure of DQDB
Trang 5are then carried downstream along the bus The relevant receiving node reads informa- tion out of the slot being sent to it, but does not delete the slot contents The slot thus remains on the bus, travelling further downstream until it falls off the end
When a node wishes to send information it may d o so in the first available empty slot, but in doing so must follow the procedure set out in the medium access control ( M A C ) protocol The MAC protocol is intended to ensure a fair use of the available bandwidth
of the buses between all the devices wishing to send information
Before sending information, a sending node must know the relative position of the receiving node on the bus It then sends a request in the opposite direction of the receiving node on the relevant bus For example, say node 2 of Figure 21.4 wished to
transmit to node 5, then it would send a request on bus B This advises the upstream
nodes of bus A (i.e node l in our case) that it requires capacity on bus A Node 2 must then wait until all other previously pending requests from other downstream nodes on
bus A have been cleared Once these are cleared, it may send in any free slot, and may continue to fill slots until a further slot request appears from a downstream node
It is a simple and yet very effective medium access control Requests for use of bus A
are sent on bus B Meanwhile the use of bus B is governed by the requests on bus A The control of the use of the network is decentralized, so that each node may independently determine when it may transmit information, but must be capable of keeping track of the pending requests
When a node is not communicating on one of the buses (say bus A), it monitors the requests for use of the bus, keeping a running total of the outstanding requests using its
request counter Each time a request passes on bus B, the request counter is incre- mented, and when a free slot goes by on bus A the counter is decremented In this way it
can keep track of whether a free slot on bus A is available to it or not The request
counter is never decremented to a value less than zero
Each time a node has a segment it wishes to send on bus A, it generates a waiting
counter The initial value copied into the waiting counter is that currently held in the
request counter The waiting counter is decremented each time a free slot passes on bus A
until the value reaches ‘O’, when the segment may be sent in the next free slot
When transmitted onto one of the buses the 48 byte segment of user information is
supplemented with a 4 byte segment header, a 1 byte access controlfield and a 4 byte
slot header as shown in Figure 21.5, so that the total length of a slot is 57 bytes
I
4 bvtes slot segment header header segmenf of user data (48 bytes)
t
1 byte access control field
Figure 21.5 Slot and segment structure of DQDB
Trang 63% CAMPUS AND METROPOLITAN AREA NETWORKS (MANS)
-1
segment 3 1
l I Figure 21.6 Segmenting a data block for transmission using DQDB
The slot header carries a 2 byte delimiter field and 2 bytes of control information used by the physical layer for the layer management protocol The access controlfield
may be written to by any of the nodes on the bus This is the field in which the slot
requests are transmitted The segment header carries a 20-bit virtual channel identij?er,
like the logical channel number (OS1 layer 2 address) of HDLC This identifies the cells
to the appropriate receiving node
Data blocks to be carried by DQDB are formatted in the standard manner of frame
header, the user data block and the frame trailer The frame header contains the address
of the originating and destination nodes The user data block is the data frame to be carried which may be up to 9188 bytes in length (192 segments), and the trailer includes the frame check sequence Data blocks must be broken down into individual segments and then formatted as slots for transmission If necessary, the last segment is filled with
padding (Figure 21.6)
Figure 21.7 DQDB or SMDS configured in a looped bus topology
Trang 7Networks using the DQDB protocol may also be configured in a looped-bus topology
In this case the bus is looped so that the two frame generators (Figure 21.4) are contained
in the same node This node also contains two ends o f b u s devices (Figure 21.7) In real terms, the network is still two independent buses, but there may be a practical advantage
in not needing two separate frame generator nodes
When offered as a public network service, SMDS is usually configured as shown in Figure 21.8, the public network node acting as the master frame generator and access point for a wide area broadband network which may use a protocol other than DQDB for wide area transport of information In this way SMDS may provide an access network protocol for a broadband network based upon ATM (Chapter 26) As you may note from comparing the two technologies, they have a number of features in
common (cell size of 53 bytes, virtual channel identlJcation of individual channels, etc.)
Although the DQDB protocol has the charm of being a very simple and purportedly
‘fair’ protocol, one of the debates that dogs its wider acceptance is the doubt which exists over its ‘fairness’ The slot request procedure used in the MAC does indeed help
to share out the bandwidth resources between all the competing nodes, but it does not work well when many of the nodes wish to send at a bitrate close to that of the line Let
us return to Figure 21.4 and assume that the network has been idle, but that now both nodes 1 and 4 wish to transmit to node 5, both at the maximum bitrate Node 1 starts sending immediately on bus A in every slot Node 4, meanwhile, must first lodge a slot request on bus B The request takes a little time to propagate along bus B until reaching node 1, whence node 1 must leave a free slot on bus A It then goes on to use all subsequent free slots As node 4 is only allowed to have one outstanding slot request, it
must wait until this request is used up before generating the next one Meanwhile node
1 is hogging all the slots
The ‘fairness’ problem is particularly acute when a very long bus is used, because an entire slot is only about 900 metres long at a bitrate of 155 Mbit/s (57 X 8 [bits per slot] X 3 X 10’ (speed of propagation in m/s/155 X 106 bits/s)) Thus for a lOkm bus there will always be 11 slots between nodes 1 and 4, always with one reserved for use of node 4 and the other ten in use by node 1
public network customer premises
FG
I
SNI
SNI = subscriber network interface
Figure 21.8 SMDS subscriber network interface
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Because of the emergence of A TM (asynchronous transfer mode) as a universal network technology for the carriage of all types of voice, video and data information in local,
metropolitan and wide area networks, the MAN technologies are already obsolescent
This is strong evidence of the rapid pace of development of modern technology, but a chilling reminder of the costs and risks involved in investing in the development of or purchase of new equipment