The classical approach to construction of city networks is functional decomposition on levels of access:

• basic network (highway);
• level of distribution/aggregation;
• level of access (client access);

The topological model of a ring is widely applied to maintenance of raised reliability and reservation. Rings usually create at levels: a basic network and access.

Base main technologies for today are: SONET/SDH, ATM, POS (Pocket over Sonet), EoSDH (Ethernet over SDH), DWDM, CWDM, DPT/RPR, Fast/Gigabit/10 Gigabit Ethernet.

The basic advantages of the given technology:

• Standarting
• Scalability
• High reliability (time of restoration of the order 50 msec).

Lacks:

• Orientation to transfer of the voice traffic
• The fixed passband, not dependent on loading channels
• Inefficient use of rings.

SONET/SDH is the most mature and consequently the most widespread technology for construction of the main data links. The basic area of its application - primary networks of operators of communication. The multiplexers incorporated by optical communication lines, form the uniform environment in which digital channels between the equipment of telephone systems or networks of data transmission are laid. Besides technology SONET/SDH can be a transport basis for more modern reports: ATM, POS and MPLS.

In due time with the purpose of creation multiservice (for all existed kinds of the traffic: a voice, video, the Internet) and high-speed technology of data transmission has appeared Asynchronous Transfer Mode - ATM. Universal distribution (despite of the known slogan " ATM everywhere ") it has not turned out (local networks has won Ethernet). But for construction of highways ATM became base technology for many years.

Orientation to high speeds has defined the most preferable physical environment of transfer for ATM - an optical fibre. Often main networks ATM are carried out by imposing on existing infrastructure SONET/SDH. It is initially stipulated by the standard. However the given method is inefficient and concedes to direct connection of the ATM-equipment.

Technology Packet Over Sonet/SDH has been developed for the decision of a problem of an overhead charge in case of transfer of the IP-traffic through networks SONET/SDH with use ATM (POS). It allows insert data in staff SDH. Practically you receive the interface with the IP-address which uses all advantages of transport optical technology, not used any intermediate reports.

Meeting needs of the market on transfer of the Ethernet-traffic on optical networks, there was technology Ethernet over SONET/SDH. And, if only connections of type a point-point in the further there were multidot channels in the beginning were supposed.

Increasing volumes of the traffic demand increase of throughput of optical highways. Except for increase of speeds of transfer there is also other way of the decision of the given problem - condensation of channels. The most developed now is the technology of optical spectral condensation named usually by multiplexing with division on length of a wave - Wavelength Division Multiplexing (WDM).

The principle of its action is very simple: dataflows from separate sources are transferred by light waves of different length (each channel posesses the length) and united by the multiplexer in a uniform multifrequency signal which is transferred on an optical fibre. On the reception party there is a return transformation.

Data are simultaneously transferred on two rings in opposite directions (that effectively using throughput). The Dataflow in each ring includes given in given ring given and control packages directly transported in a given ring for the next ring.

The standard 802.17 (incorporated DPT/RPR) is positioned as high-speed technology of dynamic transfer of IP-packages. It is developed for the decision of problems of construction of new generation of networks Metro. For the given technology the physical optical environment is absolutely transparent (it can be SONET/SDH, WDM, Ethernet, Dark Fiber).

Advantages are obvious:

• The given report meets modern requirements - it пакетно-is focused;
• The additional layer of type ATM for access to the physical optical environment is not required;
• The high level of reservation and fast reducibility in case of failures (50 мс) is incorporated;
• The capacity of optical channels (mixture of control and transferred data) is very effectively used;
• Interfaces of the equipment in rings 802.17 receive IP-addresses and everything, that with it is connected. And if it is required QoS, Traffic Engineering, VPN or it is simple Layer 2 services (for example Ethernet connections) in this case without MPLS to not manage.

Technology Ethernet in the prompt development already for a long time has stepped over a level of local networks. It has got rid of collisions, has received a full duplex and gigabits speeds.

Now overwhelming prevalence of the IP-traffic in consumer networks is marked. For transfer of TDM-signals (for example, for communication between digital automatic telephone exchanges), transparent report TDMoIP emulating TDM-channels is developed for all reports and signal systems. If earlier packages were transported in networks with коммутированием channels now all can change just what isn't needed.

For construction of the developed Ethernet-networks the technology of virtual local networks - Virtual LAN (VLAN) certainly is base. It allows to create independent logic areas in a uniform Ethernet-segment. These areas limit at a channel level limits of distribution of the traffic (including broadcasting). For this purpose (according to standard IEEE 802.1Q) in heading of the Ethernet-frame the additional information on an accessory to VLAN is entered. So it turns out VLAN with the marked staff of data (Tagged VLAN) which are transferred on 802.1Q Trunk. It allows to transfer on one channel data of the several isolated local networks. The further switching occurs in view of a 802.1Q-label. On an output from the switchboard (for example, on the party of client port) a label (Tag) is cleaned (it refers to as ocurrence of port in Untagged VLAN).

The recommended design of a network under the name "eskimo" (" router on stick ") looks as follows: client подсети are isolated from each other by connection to separate VLANs (through ports with Untagged VLAN), and communication between them will be organized by means of a router (Layer 3 OSI) through 802.1Q trunks (containing Tagged VLAN).

In practice use Vlans enables flexibly to change the logic organization of a network irrespective of real physical topology.

The direct decision inherent 802.1Q-VLANs restrictions (for example, their maximal number 4096) was technology Q-in-Q. Its concept is very simple (as well as all ingenious): the camera device receiving the client Ethernet-frame, adds one more 802.1Q-label which takes into consideration at the further switching. So the whole block of labels turns out, and process refers to VLAN stacking (802.1Q stacking). On an output from Hosting Service Providers networks the additional label leaves. It allows to build completely transparent at a channel level (Layer 2 OSI) camera networks of class Metro.

In networks Ethernet switchboards support only treelike, that is not containing loops, communication. And technology Spanning Tree Protocol (STP) allows to create failure-safe topology of a channel level (Layer 2 OSI) type "ring", being absolutely transparent for a higher stack of network reports (IP).

The principle of action STP looks as follows. After activation switchboards exchange special information packages (BPDU) by means of which in the beginning the root bridge (which will be as a result at top of treelike structure) and then the shortest (in sense of throughput) ways from each of switchboards up to root gets out. Finally it is formed logic without loopy topology by blocking some superfluous communications (ports).

Now the increasing recognition receives Rapid Spanning Tree Protocol (RSTP) - considering restrictions and lacks STP the standard.

The report of routing Open Shortest Path First (OSPF) too finds of itself application in networks Metro. It allows to build failure-safe topology of a network level (Layer 3 OSI). Possibly, it is ideologically wrong, but, in case of the organization of several reserve channels between routers. Besides unlike STP, OSPF supposes use of all available communication lines.

The high technology for construction of camera networks is Multiprotocol Label Switching (MPLS), as the most effective architecture for transfer of the IP-traffic.

For promotion of data on network MPLS uses switching software packages to labels. On an input in the MPLS-domain packages receive labels which define routes of their following, and on an output - leave. In a kernel of a network switching on labels that provides the decision of the primary goal - fast transfer of packages is supported only. Besides MPLS supports also other additional services: Traffic Engineering (TE), QoS, VPN, EoMPLS and AToM. Their detailed consideration is beyond the current review.