network technologies typically used in ip-backbone networks

8/3/2019 Network Technologies Typically Used in IP-Backbone Networks

1/28

Network technologies typically

used in IP-backbone networks


2/28

There are a wide-range of different technologies to choose

from:

point-to-point transmission lines or leaselines;

point-to-multipoint technologies such as frame relay or ATM

(asynchronous transfer mode);

metropolitan area network (MAN) technologies such as FDDIand SMDS/DQDB; and

ethernet (particularly fast ethernet and Gigabit ethernet) LAN

technology.


3/28

Point-to-point transmission line interfaces

up to 155 Mbit/s (STM-1 or OC-3)

A point-to-point line serves as a reserved and private connection

between two neighbouring routers. A number of different interfaces and

bit rates are available (as illustrated in Figure 8.7).

Each interface is typically used with HDLC (high level datalink control) or

PPP (point-to-point protocol) as the layer 2 (datalink ) and IP (Internet

protocol) as the layer 3 (network) protocol. Which particular interface of

Figure 8.7 is the best choice for an given case depends upon the bit rate

of the traffic to be carried, the relative costs of different leaseline types

and the cost of a new interface card for the router (assuming that a spare

port is not already available).


4/28

Figure 8.7 : Point-to-point line interface technologies used for

inter-router trunk and access line connections.


5/28

The line connection of a packet-over-SONET (or packet-over SDH)

interface may be either an optical (i.e. fibre) interface or the alternative

electrical interface. The optical interface allows two routers to be directly

interconnected by fibre cables (so-called dark fibre). The electricalinterface, meanwhile, may be the cheaper alternative, if the STM-1 or OC-

3 connection between the routers is to be multiplexed with other SONET

or SDH connections by collocated SONET or SDH multiplexors (Figure 8.8).

The datalink layer protocol is HDLC andthe network protocol is IP.


6/28

Figure 8.8 : Packet-over-SONET: use of optical and electrical

interface variants of OC-3/STM-1.


7/28

Point-to-point transmission at bit rates

above 155 Mbit/s

Modern transmission technology developed for traditional carriers

telecommunications networks offers rates above 155 Mbit/s (called OC-3

[SONET hierarchy] or STM-1 [SDH hierarchy]) in power-of-4 multiples of

155 Mbit/s, thus:

STM-1 (OC-3) bit rate: 155 Mbit/s

STM-4 (OC-12) bit rate: 622 Mbit/s

STM-16 (OC-48) bit rate: 2.5 Gbit/s

STM-64 (OC-192) bit rate: 10 Gbit/s


8/28

Table 8.1 : Packets-per-second demands of

different router interfaces

Router interface speedPackets received per second

(assuming 576 octet packet size) Packets received per second(assuming 65 535

T1 (1.544 Mbit/s) 2 681 24

E1 (2.048 Mbit/s) 3 556 31

E3 (34 Mbit/s) 59 028 519

T3 (45 Mbit/s) 78 125 687

100baseT (100 Mbit/s) 173 611 1 526

OC-3 or STM-1 (155 Mbit/s) 269 097 2 365

OC-12 or STM-4 (622 Mbit/s) 1 079 861 9 491

1000baseX (1 Gbit/s) 1 736 111 15 259

OC-48 or STM-16 (2.5 Gbit/s) 4 340 278 38 148

OC-192 or STM-64 (10 Gbit/s) 17 361 111 152 590


9/28

Metropolitan trunks: Gigabit ethernet overdark fibre

For very high speed network connections in metropolitan areas, Gigabit

ethernet is becoming the interface of choice (Figure 8.9). As a full uplex

connection, the Gigabit ethernet interface, as we discovered in Chapter 4,

has a range of 3 km. Added to this, Gigabit ethernet cards are cheaper

than other interface cards offering a similar bit rate. And perhaps most

important of all, ethernet is a favoured interface in the data-

communications community.

By using a Gigabit switch like a local exchange a high speed public

metropolitan data networking service can be achieved without a router

and you have ethernet-in-the-first-mile (EFM). A router is used to

interconnect the metropolitan network with the rest of the Internet.

Alternatively, new switch/router technologies are appearing (e.g. fromExtreme Networks and Foundry Systems).


10/28

Figure 8.9 : Gigabit ethernet switch used as a metropolitan area

IP network: ethernet-in-the-first-mile (EFM).


11/28

Introduction to frame relay and ATM

(asynchronous transfer mode) Frame relay connections were widely offered by public

telecommunications carriers during the 1990s as cheaper alternatives to

point-to-point leaseline services. The frame relay service provides for the

transport (relaying) of data frames (i.e. datalink frameslayer2 protocol

frames) across virtual circuits between two UNI (user-network interface)

endpoints (Figure 8.10).

ATM (asynchronous transfer mode) was a further development of frame

relay, undertaken by the public telephone companies under the auspices

of ITU-T and intended to provide both for even higher data connection

speeds (2 Mbit/s up to 34 Mbit/s) but also optimised for efficient and

simultaneous carriage of both voice and data signals. ATM also operatesconnectionoriented switching of virtual channels. Both frame relay and

ATM were revolutionary in their time.


12/28

Figure 8.10 : The frame relay UNI (user-network interface).


13/28

Table 8.2 Maximum waiting time to next opportunity to send

high priority packet

Line bit rate Maximum waiting time for high priority packet or cell (i.e.maximum time required for full packet transmission)

53 octet ATM cell 576 octet standard

IPv4 packet

65 535 octet IP-packet

of maximum

transmission unit

(MTU) size

2 Mbit/s (E1) 207 s 2 ms 256 ms

34 Mbit/s (E3) 12s 136 s 15 ms

45 Mbit/s (T3) 9 s 102 s 12 ms

155 Mbit/s

(STM-1 or OC-3)

3 s 30 s 3 ms


14/28

How to achieve an efficient full-mesh router backbone

By using either frame relay orATM (or nowadays also MPLS

multiprotocol label switching) in the main core of the network, the effect

of a full mesh topology of routers can be achieved, even if each router in

an IP network is only connected to theframe relay (or ATM or MPLS

backbone) switch by a single physical connection.

But why bother, you might say? Why not let the routers sort it all out

automatically using their dynamic routing protocols? The answer could beone of two reasons:

by directly interconnecting each pair of routers, the routing table look-

up and the Ipforwarding process in Figure 8.11 have been limited to a

maximum of two look-ups. This would still be the case even if we

added many more routers to the network; or

each router only requires one (high-speed) physical connection to the

network, so that overall less equipment is required from the router

manufacturer. If the frame relay, ATM or MPLS technology is cheaper,

this has obvious economic benefits.


15/28

Satellite links and other links with long propagation

delays

Figure 8.11 Creating a full router mesh using frame relay (or ATM or MPLS) in the IP

network backbone.


16/28

Summary of backbone network

interfaces used between routers Key: AAL = ATM adaptation

layer; ATM = asynchronoustransfer mode; BNC = bayonetconnector;

FR = frame relay; HDLC =higherlevel datalink control;IEEE 802.2 = LLC = logical linkcontrol;

IP = Internet protocol; IPOFR =Internet protocol over framerelay; MPOA = multiprotocolover ATM;

MPLS = multiprotocol label

switching; PPP = point-to-pointprotocol; STM-1 = synchronoustransport module-1;

UNI = user network interface.


17/28

8. 4 Access network technologies


18/28

That part of an IP network which is intended to provide for the connection

of end-user devices to the nearest backbone router node is commonly

called the access network. Various common access network configurations

are illustrated in Figure 8.13:

dial-in access;

dedicated access; and

xDSL or cable modem access.


19/28

Figure 8.13 Common access network configurations used for IP

network or Internet access.


20/28


21/28

Figure 8.14 ISDN and analogue telephone line interfaces used as dial-in

access connections to IP

backbone networks.

Key: 2w = 2-wire; BNC = bayonet

connector; BRI = basic rate ISDN; ISDN= integrated services digital network;

PRI = primary rate ISDN. The following

are ITU-T recommendations for

interfaces and protocols: G.703, I.430,

I.441, I.451, Q.921, Q.931. RJ-45 is an8-lead connector type.


22/28

Figure 8.15 Datalink aggregation: the need for reverse multiplexing to

overcome different propagation

delays.


23/28

Data link aggregation and reverse multiplexingData link aggregation (Figure 8.15) is a usefulway of providing high bit rate connections. Thus it

is common for basic rate ISDN (BRI) cards to beable to aggregate both of the 64 kbit/s (B-channels) to create the effect of a singleconnection of 128 kbit/s duplex.

Public telephone network configuration for dial-inInternet access Figure 8.16 illustrates typicalconfigurations of a public telephone companysnetwork for dial-in Internet access. Figure 8.16ashows the standard configuration using anetwork access server (NAS a modem pool) asthe interface between the public telephonenetwork partof the connection and the Internetbackbone.


24/28

Figure 8.16 Public telephone network configuration for dial-in Internet

access.

Note : Both shaded networks (Internet and telephone network)are typically operated by the same public telecommunicationscarrier.


25/28

Use of dial-up lines for back-up

service

Use of dial-up lines forback-upservice Before we finally leave the subject

of dial-up lines, we should note that they are commonly used as a means

of back-upa fallback connection set-up on demand should a dedicated

access line (or even an inter-router trunk circuit) fail. Furthermore, by

aggregating (Figure 8.15) different numbers of dial-up lines at different

times of day, connections of variable bit rate can be achieved to carry

data traffic volumes which might fluctuate greatly during a 24-hour cycle.


26/28

Dedicated access

Figure 8.13b illustrates the typical dedicated access configuration used to connectmost business premises to the Internet (or to an enterprise-wide IP-based routerbackbone network). In this configuration, a number of end-user devices at thecustomer premises site share the same high speed connection to the backbonenetwork. These devices are usually connected by means of a LAN (local areanetwork) to an access router on the customers premises. The access routerperforms one or more of the following functions:

forwarding of outgoing packets from the LAN to the default gateway (in this case, the Internetservice providers first backbone router);

filtering of packets allowed to pass into and out of the LAN;

Network address translation (NAT) as necessary to convert local IP network addresses to publicIP-addresses which can be recognised by the public Internet;

selection of connection and bit rate to be used when connecting to external networks; and

keeping track ofreachable destination IP-addresses either bylisteningtoorparticipatingina routing protocol.


27/28

xDSL and cable modem access

At the network end of the access line, the xDSL head-end device separatesthe ISDN and high-speed data connections. The ISDN access line isconnected directly to the collocated public ISDN local exchange.Meanwhile, the data connection is typically backhauled by means of anATM network to the nearest Internet backbone router. Differentmanufacturers and service providers use different marketing names for

their versions of xDSL. The following are examples of a few of the names incommon usage:

ADSLasymmetric digital subscriber linethis is the generic term for deviceswhich offer a higher downstream bit rate than upstream bit rate;

HDSLhigh-speed digital subscriber linethis is a generic term initially used

for devices offering symmetric 2 Mbit/s data carriage; SDSLa proprietary ADSL technique offered by Siemens;

T-DSLthe marketing name used for a 768 kbit/s downstream and 128 kbit/supstream

ADSL service offered by Deutsche Telekom.


28/28

Figure 8.17 Typical network configuration of an xDSL network access

connection.

network technologies typically used in ip-backbone networks

Documents