Network Infrastructure and its impact on communication latencies
A packet-switched communication network is a collection of nodes, which are interconnected through network switches and/or routers. The underlying network structure involves various technologies, interfaces and protocols along the path. Thus, it is conveniently partitioned into domains: Internet, Data Center and Internet Service Provider (ISP).
Over the 25 years since the ARPANET started; backbone speed increased by 1000 number of hosts by 1 000 000.
The complexity of the Internet belongs at the edges, and the IP layer of the Internet should remain as simple as possible.
Tier 1 is the key to global connectivity is the inter-networking layer. It is built by settlement-free peering. Tier 1 networks usually have only a small number of peers.
Tier 2 do peering with other networks, but who still purchases IP transit to reach some portion of the Internet. Tier 2 networks are motivated to peer with many other Tier 2 and end-user networks. Tier 2 networks with good peering is frequently much ”closer” to most end users or content.
Tier 3 solely purchase IP transit from other networks.
Major latency concern is Propagation and Queue delays.
The figure below shows a simplistic architecture of IP networks.
The data center is a place for computation resources, storage, and other appliances. It facilitates the content or passes it through to end users.
The network design is based on a layered approach. It looks to improve scalability, performance, flexibility and maintenance in reality it is primarily optimized for cost.
Access network where servers physically attach to the network. It provides L2 - L3 topologies.
Aggregation network provides value-added service integration, L2 domain definitions and default gateway. It facilitates multi-tier traffic with fire-walling and load balancing; additionally implements L5-L7 services such as content switch- ing, SSL termination, traffic shaping, etc.
Core network provides the high-speed packet switching for all inbound/outbound traffic flows.
Major latency concern is Processing and Queue delays
Internet Service Provider
Broadband Access Network
- ADSL over copper telephony line (last mile)
- Distributed over short distances less than 4 kilometers
- ADSL2+ down-link 24.0 Mbps uplink 3.3 Mbps
- Fixed terminals
Radio Access Network
- Refers to various cellular technologies: WCDMA, CDMA, WiMAX, LTE
- Radio Cells facilitates wireless communication between user equipment and network.
- Enables terminal mobility
ISP Core Network
- Central part of ISP infrastructure
- Transports large amounts of telephone calls and data traffic over fiber links.
- Services provisioning (e.g. packet-switched service)
- Gateways to access other networks
- Network layer is IP-based
Data-link layer is ATM over SONET/SDH; Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH); Data passing through equipment can be delayed by at most 0.032 ms, compared to a frame rate of 0.125 ms. SONET standard is defined by ANSI; SDH is originally defined by the ETSI.
Data communication is encapsulated either by Asynchronous Transfer Mode (ATM) or Packet over SONET/SDH; ATM encodes data into small fixed-sized cells, a small data cells reduce jitter. ATM 155 Mbps (designed) a typical MTU (1500 byte) transmission takes 0.077ms. In low speed links 1.544 Mbps takes up to 7.8 ms. ATM has cost of segmentation and reassembly, fixed 48-byte cell payload is not suitable for underlying IP. Uses virtual circuits for traffic engineering: QoS, shaping and policing.
Decreasing complexity, CAPEX, OPEX is the major driver for evolution of transport efficiency for IP: IP over ATM over SONET; IP over SONET over WDM IP over WDM; IP over Ethernet.