Xem mẫu
- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
21
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 companies to extend LANs throughout company office buildings
of
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 extend MAN capabilitiesto include the carriage of telephone and video
to
signalsan
as ‘integrated’
network, this has
but work largely overtaken ATM
been by
(asynchronous transfer mode), that the MAN technologies themselves already obsolescent.
so are
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 DATAINTERFACE
The jibre distributed data interface (FDDI) is a 100 Mbit/s token ring network. It is
defined in IEEE 802.8 and IS0 8802.8. FDDI can be used to interconnect LANs over
an area spanning up to100 km, allowing high speed data transfer. Originally conceived
as a high speed link for the needs of broadband terminal devices, FDDI is now per-
ceived as the optimum backbonetransmission system for campus-wide wiring schemes,
especially where network management and fault recovery are required. In particular,
FDDI became popular in association with the very first optical fibre building cabling
schemes, because it provided oneof 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 itsexpensive 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
- 392 CAMPUS AND METROPOLITAN
(MANS) AREA
NETWORKS
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 stationmanagement ( S M T ) definesamulti-layerednetworkmanagementscheme
which controls MAC, PHY and PMD
The ring of an FDDI 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 CMTs (connec-
tion management mechanisms) of the station immediately on either side the break. To
of
all other stations the ring will appear still to beinitsnormal contra-rotating state
(Figure 21.l(b)).
When configured as a ring, of the stations said to be in dual-attached connection.
each is
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
DAS DAS
'Looped'
[ a ) Failed link-ring
conf igured a s single
( b ) Normal dual contra -
rotating fibre rings
logical loop
Figure 21.1 The fibre distributed data interface (FDDI) fault recovery mechanism for double
attached stations. DAS, double attached station
- TED FIBRE 393
SA S
Net work
connect ion
J /
Bridge and /
multi ort
concenl'rator
SAS
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
(OS1 layer 1) and data-link layer (OS1 layer 2) standard.
essentially only a physical layer
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 lengthof
100 km and a capability to support around stations including telephone and packet
500
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
X 2 5 gateway
PA BX
Public
network
A Bridge
Building 2
Figure 21.3 The fibre distributed data interface-2 (FDDI-2). AU, access unit
- 394 CAMPUS AND METROPOLITAN
(MANS) AREA
NETWORKS
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 eachcycle a certain bandwidthis reserved for circuit-switched traffic
(e.g.voice anddata).Thisguaranteesbandwidthfor establishedconnections 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 queuedual bus (DQDB). DQDB was co-developed by
Telecom Australia, the University Western Australia and
of their joint company, QPSX
CommunicationsLimited. It wasdesigned toprovideabasisforinitialbroadband
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 betweencommunicatingbroadband 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
it +t
frame
generator node 1 node2 node 3 node4 node 5 slave
frame
generator
4
unidirectional bus B
Figure 21.4 Bus structure of DQDB
- MULTIMEGABIT
SWITCHED 395
are 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 maydo 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 intended to ensure fair use of the available bandwidth
is a
of the buses between all the devices wishing to send information.
Before sendinginformation,asendingnodemustknowthe 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 must2
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 Bis 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 totalof the outstanding requests using its
requestcounter. Each time arequest passes on bus B, the requestcounter is incre-
mented, and when a free slot goes by on busA the counteris 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
segment b
4 bvtes
slot segment segmenfof user data (48 bytes)
header header
t
1 byte access control field
Figure 21.5 Slot and segment structure of DQDB
- 3% CAMPUS AND METROPOLITAN AREA NETWORKS (MANS)
header
I user data block trailer
1-
l
segment 3 1
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 layerfor the layer management protocol. Theaccess 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 layer2 address) ofHDLC. This identifies the cells
to the appropriate receiving node.
Data blocks to be carried by DQDB are formatted inthe 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 beup to 9188 bytes in length(192 segments), and the trailer includes
the frame check sequence. Data blocks must be broken downinto individual segments
and then formattedas slots for transmission. If necessary, last segment is filled with
the
padding (Figure 21.6).
FG = frame generator end of bus
Figure 21.7 DQDB or SMDS configured in a looped bus topology
- MULTIMEGABIT
SWITCHED 397
Networks using theDQDB protocol may alsobe configured in alooped-bus topology.
In this case the bus looped so that thetwo frame generators (Figure
is 21.4) are contained
in the same node. This node also contains two ends ofbus devices (Figure 21.7). In real
terms, the networkis still two independent buses, but there maya practical advantage
be
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 being a very simple and purportedly
of
‘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 closeto 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 propagate along bus B until reaching
to
node 1, whence node 1 mustleaveafreeslot 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.
premises network public
customer
FG
I
SNI
SNI = subscriber network interface
FG = frame
generator end of bus
Figure 21.8 SMDS subscribernetworkinterface
- 398 CAMPUS AND METROPOLITAN AREA NETWORKS (MANS)
21.3 THE DEMISE OF MANS
Because of the emergence of A TM (asynchronous transfer mode) a universal network
as
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.
nguon tai.lieu . vn